Five-leaf Stage Research Articles

First Report of Anthracnose Crown Rot of Strawberry Caused by Colletotrichum siamense in Taiwan

In Taiwan, strawberry (Fragaria × ananassa Duch.) is a high-value crop with an average annual cultivated area of ∼500 ha in the last 5 years. Over 90% of strawberry cultivation is in Miaoli County, with ‘Taoyuan No. 1’ as the predominant cultivar for more than 30 years. Anthracnose has become more destructive over the past decade. Although Colletotrichum gloeosporioides, C. dematium, C. fragariae, and C. acutatum were mentioned as the causal agents of strawberry anthracnose in Taiwan (Plant Protection Information System; https://otserv2.tactri.gov.tw/ppm/), we lack information on the isolation, pathogenicity, and morphological or molecular identification of the pathogen. From 2010 to 2016, we surveyed anthracnose in strawberries in Miaoli County; more than 50% of diseased plants showed typical anthracnose crown rot (ACR) symptoms (McInnes et al. 1992). ACR caused up to 30 to 40% plant loss during the seedling stage and ∼20% after transplanting. Infected crown tissue initially showed red and white marbling and then gradually brown rot, followed by rapid wilting of the entire plant. Anthracnose symptoms were observed in other parts of the plant, including leaves, petioles, runners, fruits, and roots. Symptoms appeared as circular black spots on the leaves and withering and girdling on runners. To isolate the causal agent, approximately 0.2 × 0.2-cm fragments of diseased crowns were surface disinfested with 1% sodium hypochlorite, triple rinsed with sterile water, and then placed onto 1.5% water agar. After 2 to 3 days, extended single hyphal tips from tissues were transferred to potato dextrose agar and incubated for 7 days at 25°C under a 12-h/12-h photoperiod. Colonies were initially white, later became somewhat zonate, velvety, light gray on the upper side and gray on the reverse side of plates, with concentric rings of salmon sporodochia. Conidia were 9.68 to 17.95 × 3.88 to 5.84 µm (14.53 ± 0.31 × 4.99 ± 0.08 µm, n = 90), hyaline, oblong to cylindrical, with round obtuse ends. Morphological characteristics of the causal agent resembled species belonging to the C. gloeosporioides species complex (Weir et al. 2012). To confirm the species identification, we extracted genomic DNA from 10 isolates by using the Plant Genomic DNA Extraction Miniprep System (Viogene, Taipei) and polymerase chain reaction–amplified the internal transcribed spacer (ITS) region, chitin synthase (CHS-1), actin (ACT), β-tubulin 2 (TUB2), calmodulin (CAL), and intergenic region of Apn2 and MAT1-2-1 (ApMAT) with published primers (Carbone and Kohn 1999; O’Donnell and Cigelnik 1997; Silva et al. 2012; Weir et al. 2012; White et al. 1990). Sequences were submitted to GenBank (accession nos. MK174223 [ITS], MK174224 [CHS-1], MK174225 [ACT], MK174226 [TUB2], MK174227 [CAL], and MK174228 [ApMAT]). The ITS, CHS-1, ACT, TUB2, CAL, and ApMAT sequences were compared with the GenBank nr database, restricted to type material. Results showed 98 to 99% identity to C. siamense (syn. C. dianesei and C. melanocaulon) (Liu et al. 2016; Prihastuti et al. 2009), which belongs to the C. gloeosporioides species complex, with the corresponding sequences (ITS: NR_144800; CHS-1: KX094094; ApMAT: KX094304 [Lima et al. 2013]; ACT: KX093987; TUB2: KX094290; and CAL: KX094036 [Doyle et al. 2013]). Koch’s postulates were fulfilled for two isolates (ML133 and ML612) by spraying 1 × 10⁶ conidia/ml suspension on seedlings until run-off at the four- to five-leaf stage (two trials per isolate, n = 5 seedlings per trial). Inoculated plants were covered with plastic bags (>90% relative humidity) for 24 h at 30°C and then maintained in a growth chamber at 30°C, 70% relative humidity, under a 12-h/12-h photoperiod. After 7 days, all inoculated plants showed typical necrotic leaf spots and wilt symptoms similar to those in the field. Control plants sprayed with sterile water had no symptoms (n = 5 per trial). Longitudinal sections of the inoculated crown showed reddish-brown and white-marbled necrosis. The fungi were reisolated from lesions of diseased leaves or crowns with 100% frequency (n ≥ 3 isolates per trial), and morphological characteristics and gene sequences were identical to the original isolates. To our knowledge, this is the first report of C. siamense causing ACR of strawberry in Taiwan. The disease has the potential for causing serious losses to the strawberry industry in Taiwan, and research is needed on management strategies to minimize losses.

Application of Loop-Mediated Isothermal Amplification in an Early Warning System for Epidemics of an Externally Sourced Plant Virus.

Restricting Turnip yellows virus (TuYV) spread in canola (Brassica napus) crops often relies upon the application of systemic insecticides to protect young vulnerable plants from wide-scale green-peach aphid (GPA; Myzus persicae) colonization and subsequent virus infection. For these to be applied at the optimal time to ensure they prevent epidemics, growers would need to be forewarned of incoming viruliferous aphid migration and colonization. This study was conducted to field validate a loop-mediated isothermal amplification (LAMP) protocol designed to detect TuYV in aphids caught on traps and develop an early warning system for TuYV epidemics. Double-sided yellow sticky traps were deployed at 30 sites sown with canola over a two-year period in the south-west Australian grainbelt. Using LAMP, the percentage (%) of trap sides with TuYV-carrying aphids was measured and related to TuYV infection incidence in the adjacent crop. When TuYV was detected in aphids on >30% trap sides in a six-week period from pre-emergence to GS15 (five-leaf stage), TuYV reached >60% crop incidence by GS30 (beginning of stem elongation). Whereas, TuYV detection in aphids on ≤15% trap sides during this period was associated with ≤6% TuYV incidence by GS30. Furthermore, when large numbers of aphids, including GPA, were caught during this period but no TuYV was detected in them, minimal TuYV spread (≤5%) occurred in the crop by GS30. Therefore, the LAMP TuYV protocol can be used in an early warning system for TuYV epidemics by providing detection of initial viruliferous aphid migration into a canola crop before they establish colonies throughout the crop and spread virus. This would enable proactive, non-prophylactic, and thereby more effective systemic insecticide applications to minimize seed yield and quality losses due to early season TuYV infection.

First Report of Fusarium commune Causing Stalk Rot on Maize in Liaoning Province, China

Fusarium species are associated with stalk rot disease of maize, which can result in significant yield losses. The typical symptom of the disease is rotting of the plant stem at lower internodes. The color of the plant changes to grayish-green, which later turns tan, and the plant wilts. When the stem is split, the inside of the stalk shows a light pink to tan discoloration and the pith is disintegrated, but the vascular bundles remain intact. During the 2016 crop season, symptomatic maize tissues were collected from four different locations in Dandong city, Liaoning province, China. The disease was found to occur on approximately 5% of the surveyed area. Plant tissues exhibiting typical stalk rot symptoms were cut into small pieces (approximately 5 mm²), placed onto potato dextrose agar (PDA) amended with streptomycin sulfate (150 μg/ml) and kanamycin (150 μg/ml), and incubated at 25°C in darkness for 4 days, as described by Shan et al. (2017). Fungal colonies showing morphological characteristics of Fusarium spp. were subcultured from single conidia onto PDA and carnation leaf agar (CLA) plates for identification and DNA sequence analyses. Microscopic and DNA analyses confirmed the presence of many Fusarium species known to cause ear and stalk rot diseases in maize, such as F. verticillioides, F. graminearum, and F. proliferatum. However, F. commune, a relatively new species described by Skovgaard et al. (2003), was also identified in 12 diseased samples collected from various locations in Dandong city. On PDA plates, colonies of F. commune were white to orange-white in color with densely flocculate to fluffy aerial mycelium and dark violet pigmentation. On CLA plates, macroconidia were slightly curved, hyaline, usually contained three or five septa, with an obvious foot-shaped basal cell and tapering gradually to a point at the tip. Three-septate macroconidia were 39.4 ± 6.2 × 3.7 ± 0.6 μm, whereas five-septate macroconidia were 48.6 ± 4.5 × 4.3 ± 0.3 μm. Microconidia were generally cylindrical, straight to slightly curved, aseptate, and measuring 11.5 ± 3.1 × 3.1 ± 0.7 μm in size. Spherical chlamydospores were 8.6 ± 1.4 × 8.4 ± 1.4 µm in size and were produced singly or in pairs from mycelium. Species identification was also confirmed by partial sequences of the translation elongation factor gene (TEF1-α, EF1 and EF2 primers), the mitochondrial small subunit gene (mtSSU, NMS1 and NMS2 primers), and the second largest subunit of the RNA polymerase gene (RPB2, 5f2 and 7cr primers) (O’Donnell et al. 2010). Partial sequences of the TEF1-α, mtSSU, and RBP2 showed 100, 100, and 99% sequence identity to those of F. commune (GenBank accession nos. KX500397, KY439901, and KU171700, respectively). DNA sequences of partial TEF1-α, mtSSU, and RBP2 from two different isolates were deposited in GenBank (accession nos. MH716808 to MH716813). Pathogenicity tests of two representative isolates were carried out by individually inoculating five maize plants at the five-leaf stage. Maize stalks, at second or third internode, were injected with 20 μl of spore suspension having concentration of 1 × 10⁶ spores/ml as described by Zhang et al. (2016). Five plants inoculated with sterilized water were used as a control. The inoculated plants were maintained at 25 ± 0.5°C in a greenhouse with a 12-h photoperiod. After 10 days, all inoculated plants developed internal dark brown necrotic regions around the inoculated site. The control plants were symptomless. F. commune was reisolated from the symptomatic plants but not from the control. In China, F. commune was also reported to cause root rot diseases in different plant species, for example, sugarcane (Wang et al. 2018) and Gentiana scabra (Guan et al. 2016). To our knowledge, this is the first report of F. commune as a pathogen on maize in China.

Suppression of Annual Ryegrass in Corn with Nicosulfuron

AbstractField studies were conducted to determine the possible rate and timing of nicosulfuron to suppress annual ryegrass (ARG) seeded as a cover crop at the time of corn planting without affecting corn performance near Ridgetown, ON, Canada, in 2016 and 2017. Nicosulfuron was applied at rates from 0.8 to 50 g ai ha–1when the ARG was at the two- to three- or four- to five-leaf stages, or approximately 3 or 4 wk after emergence of both corn and ARG. There were no differences between the two application timings in grain yield responses or ARG suppression. As the rate of nicosulfuron increased from 0.8 to 50 g ai ha–1, ARG was suppressed 6% to 76% and 5% to 96%, at 1 and 4 wk after application (WAA), respectively. At 4 WAA, ARG biomass decreased from 29 to 1 g m–2as the rate of nicosulfuron increased from 0.8 to 50 g ai ha–1, compared to 36 g m–2in the untreated control. Where nicosulfuron was not applied to ARG, grain corn yield was reduced by 6% compared to the ARG-free control; similar effects on corn yield were observed with nicosulfuron at the lowest rate applied at 0.8 g ai ha–1. Grain corn yield was reduced by 2.5% with the application of nicosulfuron at 25 g ai ha–1(label rate for corn) compared to no ARG control, but this was not statistically significant. This study identified rates of nicosulfuron that suppressed ARG when emerged approximately the same day as corn, but there was evidence that grain corn yields were lowered because of interference, possibly during the critical weed control period. Based on this study, an ARG cover crop should not be seeded at the same time as corn unless one is willing to accept a risk for corn grain yield losses for the sake of the cover crop.

The phyllosphere indigenous microbiota of Brassica campestris L. change its diversity in responding to di-n-butyl phthalate pollution.

In this study, the effects of di-n-butyl phthalate (DBP) on the phyllosphere bacterial community of field mustard (Brassica campestris L.) at the five-leaf stage were investigated. The indigenous alpha-diversity of the phyllosphere bacteria was altered after spraying with different concentrations of DBP. Shannon diversity indices were significantly changed on day 5 after treatment at DBP concentrations > 400mgL-1 (P > 0.05). Nevertheless, the difference between treatment and control was not significant on day 9 after DBP treatment (P > 0.05). Exposure to DBP resulted in a decrease in Proteobacteria and Firmicutes, and an increase in Actinobacteria at all sampling intervals. These changes included significant increases in the relative abundance of Paracoccus and Rhodococcus, and significant decreases in that of Pseudomonas, Exiguobacterium, an unclassified genus of Pseudomonadaceae, and an unclassified genus of Enterobacteriaceae. This study provides new evidence for the possibility of using phyllosphere microbiota to remediate DBP contamination.

Seed size and seedling ontogenetic stage as modulators of damage tolerance after simulated herbivory in a woody exotic species

Herbivory is one of the most important threats that plants face during early stages of development. Previous studies have indicated that seed size and seedling ontogeny strongly influence the response to herbivory at the seedling stage. However, little is known about their interactive effect. We simulated herbivory on seedlings of the exotic Gleditsia triacanthos L. and evaluated the combined effects of seed size and ontogenetic stage at which herbivory occurred on tolerance to herbivory. A greenhouse experiment was performed, with three non-overlapping levels of seed size. Seedlings from each seed size were clipped at two ontogenetic stages (two and five leaves). After three weeks, seedling survival, height, leaf number and dry biomass were measured. Damaged seedlings were not able to reach the size of the undamaged ones. Seedlings from small seeds showed the lowest values in most of the growth variables. Seedlings cut at two-leaf stage showed a higher compensation capacity than those cut at five-leaf stage. Seedlings from large seeds cut at ontogenetic stage I showed the highest compensation capacity in most of the variables. Partly compensating herbivory at this stage may be an advantage for the expansion of this exotic species to new areas.

Knocking Out the Gene RLS1 Induces Hypersensitivity to Oxidative Stress and Premature Leaf Senescence in Rice

Improving a plant’s level of tolerance to oxidative stress can frequently also enhance its tolerance to several other abiotic stresses. Here, a screen of a japonica type rice T-DNA insertion mutant library identified a highly oxidative stress-sensitive mutant. The line exhibited premature leaf senescence, starting at the three-leaf stage, and the symptoms were particularly severe from the five-leaf stage onwards. The leaves progressively lost chlorophyll, suffered protein degradation and were compromised with respect to their photosynthetic activity; their leaf mesophyll and bulliform cells became shrunken, and several senescence-associated genes (SAGs), senescence-associated transcription factor genes (SATFs) and autophagy-related genes (ATGs) were progressively up-regulated. The product of the gene inactivated by the mutation, identified via positional cloning, was putatively a ubiquitin-conjugating enzyme. The gene was denoted here as RLS1 (reactive oxygen species-sensitive leaf senescence1). The phenotype of plants in which RLS1 was knocked down using RNA interference was comparable to that of the rls1 mutant. A comparative analysis of the knock-out line and the wild type leaves showed that the former accumulated more hydrogen peroxide and more malondialdehyde, expressed a heightened level of superoxide dismutase activity and a decreased level of catalase activity, and exhibited an altered transcriptional profile with respect to several SAGs, SATFs and ATGs, and that these effects were magnified when the plants were exposed to oxidative stress. The product of RLS1 is presumed to be a critical component of the rice oxidative stress response and is involved in ROS (reactive oxygen species)-mediated leaf senescence.

Mortality of Sugarcane Aphid, Melanaphis sacchari (Zehntner, Hemiptera: Aphididae), at Low Temperatures.

We conditioned colonies of Melanaphis sacchari (Zehntner, Hemiptera: Aphididae) to three different temperature regimes over a period of 2 mo: 30°C, 25°C, and a cycle of 18/8°C (L:D), all under a diurnal photoperiod of 12:12 (L:D). Plants of susceptible sorghum, 85Y40, were grown to the five-leaf stage in 4-liter pots, 2 per pot, and then infested with mixtures of second-fourth instar M. sacchari nymphs, either 50 (18/8°C treatment) or 100 (25 and 30°C treatments) aphids per plant. In each run of the experiment, a series of two to four plants of each treatment group were lowered to the desired cold temperature (4.0, 2.0, 0.0, -2.0, and -4.0°C) in a climate-controlled chamber and then evaluated for aphid survival after 12 h in complete darkness. Results revealed substantial survival of aphids from the 18/8 and 25°C conditioned treatments, even at -4.0°C, a temperature sufficient to kill the plants, and >90 and 60% survival, respectively, at 0.0°C. In contrast, aphids conditioned to 30°C experienced about 50% mortality at 4.0°C, and complete mortality at 0.0°C. Our results indicate that M. sacchari is able to survive periods of sub-freezing temperatures quite easily and will not suffer 'cold-shock' mortality unless subjected to a rapid drop in temperature from very warm conditions (e.g., 30°C or higher). Therefore, only temperatures that are cold enough to kill sorghum plants are likely to kill all apterous virginoparae of M. sacchari.

Reactive oxygen species, antioxidant enzyme activity, and gene expression patterns in a pair of nearly isogenic lines of nicosulfuron-exposed waxy maize (Zea mays L.).

Nicosulfuron is a post-emergence herbicide used for weed control in maize fields (Zea mays L.). Here, the pair of nearly isogenic inbred lines SN509-R (nicosulfuron resistant) and SN509-S (nicosulfuron sensitive) was used to study the effect of nicosulfuron on growth, oxidative stress, and the activity and gene expression of antioxidant enzymes in waxy maize seedlings. Nicosulfuron treatment was applied at the five-leaf stage and water treatment was used as control. After nicosulfuron treatment, the death of SN509-S might be associated with increased oxidative stress. Compared with SN509-R, higher O2·- and H2O2 accumulations were observed in SN509-S, which can severely damage lipids and proteins, thus reducing membrane stability. The effects were exacerbated with extended exposure time. Both O2·- and H2O2 detoxification is regulated by enzymes. After nicosulfuron treatment, superoxide dismutase (SOD), catalase, ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), glutathione reductase (GR), and glutathione-S-transferase (GST) of SN509-S were significantly lower than those of SN509-R. Compared to SN509-R, ascorbate content (AA), glutathione (GSH) content, GSH to glutathione disulfide ratios, and AA to dehydroascorbate ratios significantly declined with increasing exposure time in SN509-S. Compared to SN509-S, nicosulfuron treatment increased the transcript levels of most of the APX genes except for APX1, and in contrast to Gst1, upregulated the transcription of sod9, MDHAR, DHAR, and GR genes in SN509-R. These results suggest that on a transcription level and in accordance with their responses, detoxifying enzymes play a vital role in the O2·- and H2O2 detoxification of maize seedlings under nicosulfuron exposure.

Transcriptomic analysis reveals the roles of gibberellin-regulated genes and transcription factors in regulating bolting in lettuce (Lactuca sativa L.).

A cool temperature is preferred for lettuce cultivation, as high temperatures cause premature bolting. Accordingly, exploring the mechanism of bolting and preventing premature bolting is important for agriculture. To explore this relationship in depth, morphological, physiological, and transcriptomic analyses of the bolting-sensitive line S39 at the five-leaf stage grown at 37°C were performed in the present study. Based on paraffin section results, we observed that S39 began bolting on the seventh day at 37°C. During bolting in the heat-treated plants, GA3 and GA4 levels in leaves and the indoleacetic acid (IAA) level in the stem reached a maximum on the sixth day, and these high contents were maintained. Additionally, bolting begins in the fifth day after GA3 treatment in S39 plants, GA3 and GA4 increased and then decreased, reaching a maximum on the fourth day in leaves. Similarly, IAA contents reached a maximum in the stem on the fifth day. No bolting was observed in the control group grown at 25°C, and significant changes were not observed in GA3 and GA4 levels in the controls during the observation period. RNA-sequencing data implicated transcription factors (TFs) in regulating bolting in lettuce, suggesting that the high GA contents in the leaves and IAA in the stem promote bolting. TFs possibly modulate the expression of related genes, such as those encoding hormones, potentially regulating bolting in lettuce. Compared to the control group, 258 TFs were identified in the stem of the treatment group, among which 98 and 156 were differentially up- and down-regulated, respectively; in leaves, 202 and 115 TFs were differentially up- and down-regulated, respectively. Significant changes in the treated group were observed for C2H2 zinc finger, AP2-EREBP, and WRKY families, indicating that these TFs may play important roles in regulating bolting.

Spraying Brassinolide improves Sigma Broad tolerance in foxtail millet (Setaria italica L.) through modulation of antioxidant activity and photosynthetic capacity

To explore the role of Brassinolide (BR) in improving the tolerance of Sigma Broad in foxtail millet (Setaria italica L.), effects of 0.1 mg/L of BR foliar application 24 h before 3.37 g/ha of Sigma Broad treatment at five-leaf stage of foxtail millet on growth parameters, antioxidant enzymes, malondialdehyde (MDA), chlorophyll, net photosynthetic rate (PN), chlorophyll fluorescence and P700 parameters were studied 7 and 15 d after herbicide treatment, respectively. Results showed that Sigma Broad significantly decreased plant height, activities of superoxide dismutase (SOD), chlorophyll content, PN, PS II effective quantum yield (Y (II)), PS II electron transport rate (ETR (II)), photochemical quantum yield of PSI(Y (I)) and PS I electron transport rate ETR (I), but significantly increased MDA. Compared to herbicide treatment, BR dramatically increased plant height, activities of SOD, Y (II), ETR (II), Y (I) and ETR (I). This study showed BR pretreatment could improve the tolerance of Sigma Broad in foxtail millet through improving the activity of antioxidant enzymes, keeping electron transport smooth, and enhancing actual photochemical efficiency of PS II and PSI.

Investigation of effective microorganisms application method on alleviation of salt stress effects on root morphology of sweet corn

Root morphology can be affected by many factors such as microorganisms. To determine the effect of effective microorganisms (EM) on the root morphology of sweet corn under salt stress, a factorial experiment was carried out in a randomized complete block design with three replications in Islamic Azad University of Sabzevar in 2013. Examined factors included application method of EM (soil application, foliar application and soil+foliar application) and intensity of salinity (0, 25, 50, 75 mM). Commercial solution of EM was applied at 30 liters per hectare for soil application and foliar application during five-leaf stage. The highest root dry weight, root density and membrane stability were observed in soil application while the highest root volume, root length, root and shoot dry weight occurred when soil and foliar application were performed together. With increasing intensity of salinity, all traits decreased and the highest traits were observed in the control treatment. Soil application of EM in comparison with other methods alleviates effects of salinity under saline conditions.

Anatomical features of an African sorghum landrace adapted to flooded conditions

This research aims to evaluate adaptation of two contrasting sorghum genotypes to flooding: a landrace from Cameroon described as a “floating sorghum” (Wulaga) and a typical dryland improved variety (Cirad437) from Burkina Faso. Anatomical and morphological features of the basal internodes were quantified in control conditions (drained pots) and long-term flooded conditions from the five-leaf stage for the entire cropping cycle. This study revealed that the Wulaga variety expressed aerenchyma in basal internodes in both control and flooded treatments (constitutive aerenchyma) whereas Cirad437 only developed aerenchyma in flooded conditions (inducible aerenchyma). The level of lignification and epidermis thickness of basal internodes was identical for the two varieties and the two treatments, suggesting that lignin does not contribute to the enhancement of a tight barrier against O2 losses. However, Cirad437 showed shorter and larger internodes in flooded conditions while Wulaga did not show any difference between the two conditions. Results indicate the existence of diversity in adaptive response to flooded conditions for the two varieties. Further studies on phenotypic traits associated with phenology, leaf traits, root traits and yield components, are needed to confirm these preliminary results and to explore a broader range of genetic diversity that could be used for future sorghum breeding programs designed for flood-prone environments. (Resume d'auteur)

The competitive ability of Chenopodium album and Echinochloa crus-galli in maize crops depending on the time of their occurrence or removal

Small-plot experiments for two levels of infestation (20 and 40 plants per m<sup>2</sup>) with <em>Chenopodium album</em> L. and <em>Echinochloa crus-galli</em> (L.) P. Beav were conducted in the 2009–2011 seasons. In the first variant, the effect of time of weed removal on maize was investigated. Weeds were removed in the following growth stages of maize: one, three, five, and seven leaves, the beginning of stem elongation, and the stage from the fourth to sixth node. The second variant concerned the effect of time of weed emergence on maize and included the same maize growth stages as mentioned above. In both treatments, weed competition was compared to the control – the plot completely free from weeds, as well as to the plot fully covered with weeds. <em>Echinochloa crus-galli</em> in the amount of 20 plants per m<sup>2</sup>, which were not removed until the five-leaf stage or which emerged immediately after the seven-leaf stage of maize, did not show any competitive effect on the growth and development of maize. <em>Chenopodium album</em> was characterized by a similar effect at the same level of weed infestation severity and when not removed until the five-leaf stage as well as in the case of plants that emerged after the seven-leaf stage of maize. Both species present in an amount of 40 plants per m<sup>2</sup> needed to be removed no later than at the three-leaf stage of maize.

Exogenous nitric oxide mediates alleviation of mercury toxicity by promoting auxin transport in roots or preventing oxidative stress in leaves of rice seedlings

Nitric oxide (NO), a multifunctional gaseous molecule, mediates a variety of responses to biotic and abiotic stresses. The effects of exogenous NO on rice (Oryza sativa cv. ‘Zhonghua 11’) growth under mercuric chloride (HgCl2) stress were investigated. The results showed that 60 μM Hg significantly inhibited the root elongation of rice plantlets after seed germination. While 100 μM or 200 μM sodium nitroprusside (SNP, a donor of NO) could increase the root length by attenuating the effects of 2,3,5-triiodobenzoic acid (TIBA) and Hg, which indicated the role of NO in auxin transport-promoting in roots. On the other hand, SNP decreased the absorption and transportation of Hg in roots and shoots of rice seedlings at five-leaf stage. Moreover, the levels of superoxide radical (O2·−) and hydrogen peroxide (H2O2) in leaves were also decreased significantly. However, the activities of antioxidant enzymes were not enhanced by SNP. Moreover, NO promoted the growth of rice plantlets under Hg stress even when superoxide dismutase (SOD, EC 1.15.1.1) or catalase (CAT, 1.11.1.6) activity was inhibited by diethyldithiocarbamate (DDC, an inhibitor of SOD) or 3-amino-1,2,4-triazole (AT, an inhibitor of catalase), respectively. These results confirmed that NO could act as the direct quencher of O2·− and then prevent the oxidative damage caused by Hg ion in leaves.

ANNUAL GRASSES CONTROL WITH TOPRAMEZONE IN MIXTURE WITH ALS-INHIBITING HERBICIDES

ABSTRACTPanicoid grasses are major weeds of maize and sugarcane as well as of several other important grains, including sorghum, pearl millet, and foxtail millet. Pot trials were conducted to study the activity and potential interactions of topramezone in mixture with recommended rates of rimsulfuron or nicosulfuron on three annual panicoid grasses (i.e. Echinochloa oryzoides,E.phyllopogon, and Panicum miliaceum). Target weeds were treated at the four- to five-leaf growth stage. On the basis of fresh weight reduction, topramezone alone provided 78% control of E.oryzoides, 68% control of E.phyllopogon, and 99% control of P.miliaceum. Topramezone plus rimsulfuron or nicosulfuron provided decreased control of both Echinochloa spp. compared with topramezone alone. The decreased control of E.oryzoidesand E.phyllopogon was more pronounced with rimsulfuron as a companion herbicide in the mixtures. Slightly decreased control of P.milaceum was observed with topramezone plus rimsulfuron compared with topramezone alone, but this was not the case for topramezone plus nicosulfuron. Increased topramezone rates mixed with rimsulfuron or nicosulfuron did not improve control of E.oryzoides and E.phyllopogon compared with the lowest topramezone rate. Also, increased topramezone rates mixed with rimsulfuron or nicosulfuron showed decreased control of both Echinochloa spp. when compared with either rimsulfuron or nicosulfuron alone, suggesting a two-way interaction between topramezone and the ALS-inhibiting herbicides. The above-mentioned interaction was not observed in P.miliaceum, probably related with species sensitivity to the herbicides tested. Newly introduced or naturalized panicoid grasses in maize fields may complicate selection of companion herbicides and rates for effective weed control.

Early vigour improves phosphate uptake in wheat.

Quantitative trait loci (QTLs) for shoot biomass were identified in wheat grown on a soil high in total phosphorus (P) but low in plant-available P. The two populations screened included recombinant inbred lines (RILs) from Chuan-Mai 18/Vigour 18 and doubled-haploid lines from Kukri/Janz. Glasshouse-grown plants were harvested at the five-leaf stage. Seven QTLs for shoot biomass were identified in the RILs, with the largest on chromosome 7A accounting for 7.4% of the phenotypic variance. RILs from the upper tail had larger embryos than RILs from the lower tail. Tail lines were then grown in non-limiting P and the results indicated that early vigour and the capacity to access P contributed to the initial distribution. The influence of early vigour on P nutrition was examined further with advanced vigour lines (AVLs). The AVLs accumulated more shoot biomass, maintained lower shoot P concentrations, and showed greater P-acquisition efficiency than Vigour 18. Nine QTLs for shoot biomass were identified in the Kukri/Janz population. Two on chromosomes 4B and 4D accounted for 24.8% of the variance. Candidates underlying these QTLs are the Rht genes. We confirmed the influence of these genes using near-isogenic lines with different Rht alleles. The dwarf and semi-dwarf alleles affected shoot and root biomass at high and low P but not the efficiency of P acquisition. We conclude that early vigour contributed to the distributions in both populations. Early vigour can increase plant growth at suboptimal P and some sources can also improve the efficiency of P acquisition.

Genome-wide identification of microRNAs and their targets in wild type and phyB mutant provides a key link between microRNAs and the phyB-mediated light signaling pathway in rice.

Phytochrome B (phyB), a member of the phytochrome family in rice, plays important roles in regulating a range of developmental processes and stress responses. However, little information about the mechanisms involved in the phyB-mediated light signaling pathway has been reported in rice. MicroRNAs (miRNAs) also perform important roles in plant development and stress responses. Thus, it is intriguing to explore the role of miRNAs in the phyB-mediated light signaling pathway in rice. In this study, comparative high-throughput sequencing and degradome analysis were used to identify candidate miRNAs and their targets that participate in the phyB-mediated light signaling pathway. A total of 720 known miRNAs, 704 novel miRNAs and 1957 target genes were identified from the fourth leaves of wild-type (WT) and phyB mutant rice at the five-leaf stage. Among them, 135 miRNAs showed differential expression, suggesting that the expression of these miRNAs is directly or indirectly under the control of phyB. In addition, 32 out of the 135 differentially expressed miRNAs were found to slice 70 genes in the rice genome. Analysis of these target genes showed that members of various transcription factor families constituted the largest proportion, indicating miRNAs are probably involved in the phyB-mediated light signaling pathway mainly by regulating the expression of transcription factors. Our results provide new clues for functional characterization of miRNAs in the phyB-mediated light signaling pathway, which should be helpful in comprehensively uncovering the molecular mechanisms of phytochrome-mediated photomorphogenesis and stress responses in plants.

Physiological Responses of Hard Red Winter Wheat to Infection by Wheat streak mosaic virus.

Wheat streak mosaic virus (WSMV) causes significant yield loss in hard red winter wheat in the U.S. Southern High Plains. Despite the prevalence of this pathogen, little is known about the physiological response of wheat to WSMV infection. A 2-year study was initiated to (i) investigate the effect of WSMV, inoculated at different development stages, on shoot and root growth, water use, water use efficiency (WUE), and photosynthesis and (ii) understand the relationships between yield and photosynthetic parameters during WSMV infection. Two greenhouse experiments were conducted with two wheat cultivars mechanically inoculated with WSMV at different developmental stages, from three-leaf to booting. WSMV inoculated early, at three- to five-leaf stage, resulted in a significant reduction in shoot biomass, root dry weight, and yield compared with wheat infected at the jointing and booting stages. However, even when inoculated as late as jointing, WSMV still reduced grain yield by at least 53%. Reduced tillers, shoot biomass, root dry weight, water use, and WUE contributed to yield loss under WSMV infection. However, infection by WSMV did not affect rooting depth and the number of seminal roots but reduced the number of nodal roots. Leaf photosynthetic parameters (chlorophyll [SPAD], net photosynthetic rate [Pn], stomatal conductance [Gs], intercellular CO2 concentration [Ci], and transpiration rate [Tr]) were reduced when infected by WSMV, and early infection reduced parameters more than late infection. Photosynthetic parameters had a linear relationship with grain yield and shoot biomass. The reduced Pn under WSMV infection was mainly in response to decreased Gs, Ci, and SPAD. The results of this study indicated that leaf chlorophyll and gas exchange parameters can be used to quantify WSMV effects on biomass and grain yield in wheat.

Aminocyclopyrachlor Absorption and Translocation in Three Aquatic Weeds

Studies were conducted to evaluate 14C-aminocyclopyrachlor absorption and translocation in alligatorweed, waterhyacinth, and waterlettuce. Alligatorweed plants were treated at the seven-node stage, waterhyacinth was treated at the five-leaf stage, and waterlettuce was treated at the eight-leaf stage. All plants were pretreated with nonlabeled aminocyclopyrachlor at 0.14 kg ai ha−1 with 1% (v/v) methylated seed oil (MSO). 14C-aminocyclopyrachlor was then applied to a protected leaf, and plants were harvested at 1, 2, 4, 12, 24, and 96 h after treatment (HAT). Radioactivity was determined in the treated leaf, shoots above treated leaf, shoots below treated leaf, roots, and growing solution. Absorption was rapid in all species and reached a maximum of 73, 72, and 73% of applied radioactivity for alligatorweed, waterhyacinth, and waterlettuce, respectively. In alligatorweed at 96 HAT, 43% of absorbed carbon-14 (14C) was translocated to shoots above the treated leaf and 17% was translocated to lower shoot tissue. In waterhyacinth at 96 HAT, 56% of absorbed 14C remained in the treated leaf, whereas 14 and 13% were found in parts above and below the treated leaf, respectively. In waterlettuce at 96 HAT, 50 and 33% of absorbed radioactivity was located above the treated leaf and in the growing solution, respectively. The low recovery of aminocyclopyrachlor in alligatorweed roots and growing solution might explain regrowth potential after herbicide treatment. These results also indicate that the lack of waterlettuce control with aminocyclopyrachlor is not due to reduced absorption or translocation.