WS Plants Research Articles

Differential Aluminum tolerance in Arabidopsis thaliana ecotypes is seemingly related to metabolite changes

Plants are negatively affected by aluminum (Al) in acidic soils (pH ≤ 5.0), which impairs root growth and ultimately plant yield. Organic acids are closely related to Al neutralization improving the metal tolerance with an expansive metabolic cost. Here, we investigated phenotypic, metabolic, and genetic responses of three Arabidopsis thaliana ecotypes, Columbia (Col-0), Wassilewskija (Ws) and Landsberg erecta (Ler) in response to Al. By comparing the respective control plants with Al-exposed plants, the ecotypes Col-0 and Ws displayed stronger reductions in root growth and reproductive yield than Ler. All ecotypes presented high expression of ALMT1, an Al-resistance associated gene. Further analyses revealed a large accumulation of diverse amino acids, carbohydrates and organic acids following Al stress in both shoot and root tissues of Col-0 and Ws plants, but not in Ler. Altogether, our results suggest that a higher capacity for using and translocating reduced carbon molecules seems crucial to overcoming Al stress in the Ler ecotype. In addition, lower expenses in carbon molecules might be linked to a higher capacity to deal with Al stress, supporting a tight relationship between primary metabolism and Al stress responses. Finally, novel insights on the influence of Al over plant growth and primary metabolism in both shoots and roots are described.

Photosynthetic recovery in drought-rehydrated grapevines is associated with high demand from the sinks, maximizing the fruit-oriented performance.

To understand how grapevine sinks compete with each other during water stress and subsequent rehydration, carbon (C) allocation patterns in drought-rehydrated vines (REC) at the beginning of fruit ripening were compared with control vines maintained under drought (WS) or fully irrigated (WW). In the 30 days following rehydration, the quantity and distribution of newly fixed C between leaves, roots and fruits was evaluated through 13 CO2 pulse-labeling and stable isotope ratio mass spectrometry. REC plants diverted the same percentage of fixed C towards the berries as the WS plants, although the percentage was higher than that of WW plants. Net photosynthesis (measured simultaneously with root respiration in a multichamber system for analysis of gas exchange above- and below-ground) was approximately two-fold greater in REC compared to WS treatment, and comparable or even higher than in WW plants. Maximizing C assimilation and delivery in REC plants led to a significantly higher amount of newly fixed C compared to both control treatments, already 2 days after rehydration in root, and 2 days later in the berries, in line with the expression of genes responsible for sugar metabolism. In REC plants, the increase in C assimilation was able to support the requests of the sinks during fruit ripening, without affecting the reserves, as was the case in WS. These mechanisms clarify what is experienced in fruit crops, when occasional rain or irrigation events are more effective in determining sugar delivery towards fruits, rather than constant and satisfactory water availabilities.

Physiological and Biochemical Traits of Two Major Arabidopsis Accessions, Col-0 and Ws, Under Salinity.

Salinity affects plant growth and development as shown with the glycophyte model plant, Arabidopsis thaliana (Arabidopsis). Two Arabidopsis accessions, Wassilewskija (Ws) and Columbia (Col-0), are widely used to generate mutants available from various Arabidopsis seed resources. However, these two ecotypes are known to be salt-sensitive with different degrees of tolerance. In our study, 3-week-old Col-0 and Ws plants were treated with and without 150 mM NaCl for 48, 72, or 96 h, and several physiological and biochemical traits were characterized on shoots to identify any specific traits in their tolerance to salinity. Before salt treatment was carried out, a different phenotype was observed between Col-0 and Ws, whose main inflorescence stem became elongated in contrast to Col-0, which only displayed rosette leaves. Our results showed that Col-0 and Ws were both affected by salt stress with limited growth associated with a reduction in nutrient uptake, a degradation of photosynthetic pigments, an increase in protein degradation, as well as showing changes in carbohydrate metabolism and cell wall composition. These traits were often more pronounced in Col-0 and occurred usually earlier than in Ws. Tandem Mass Tags quantitative proteomics data correlated well with the physiological and biochemical results. The Col-0 response to salt stress was specifically characterized by a greater accumulation of osmoprotectants such as anthocyanin, galactinol, and raffinose; a lower reactive oxygen detoxification capacity; and a transient reduction in galacturonic acid content. Pectin degradation was associated with an overaccumulation of the wall-associated kinase 1, WAK1, which plays a role in cell wall integrity (CWI) upon salt stress exposure. Under control conditions, Ws produced more antioxidant enzymes than Col-0. Fewer specific changes occurred in Ws in response to salt stress apart from a higher number of different fascilin-like arabinogalactan proteins and a greater abundance of expansin-like proteins, which could participate in CWI. Altogether, these data indicate that Col-0 and Ws trigger similar mechanisms to cope with salt stress, and specific changes are more likely related to the developmental stage than to their respective genetic background.

Efficient regulation of copper homeostasis underlies accession-specific sensitivities to excess copper and cadmium in roots of Arabidopsis thaliana

The commonly used Arabidopsis thaliana natural accessions Columbia (Col-0) and Wassilewskija (Ws) are known to differ in their metal sensitivity, with Col-0 being more sensitive to copper (Cu) and cadmium (Cd) than Ws. As both Cu and Cd are known to affect Cu homeostasis, it was investigated whether this process is part of an accession-specific mechanism underlying their difference in metal sensitivity. As roots are the first contact point during metal exposure, responses were compared between roots of both accessions of hydroponically grown plants exposed to excess Cu or Cd for 24 and 72 h.Root Cu levels increased in both accessions under Cu and Cd exposure. However, under Cu exposure, the downregulation of Cu transporter (COPT) genes in combination with a more pronounced upregulation of metallothionein gene MT2b indicated that Ws plants coped better with the elevated Cu concentrations. The Cd-induced disturbance in Cu homeostasis was more efficiently counteracted in roots of Ws plants than in Col-0 plants. This was indicated by a higher upregulation of the SPL7-mediated pathway, crucial in the regulation of the Cu homeostasis response.In conclusion, maintaining the Cu homeostasis response in roots is key to accession-specific differences in Cu and Cd sensitivity.

Accession-specific life strategies affect responses in leaves of Arabidopsis thaliana plants exposed to excess Cu and Cd

The natural accession Columbia (Col-0) is considered as the reference genome of the model plant Arabidopsis thaliana. Nonetheless, Col-0 plants are more sensitive to excess copper (Cu) and cadmium (Cd) than other widely used accessions such as Wassilewskija (Ws) plants. In the current study, this accession-specific metal sensitivity is further explored by comparing the responses in leaves of Col-0 and Ws plants exposed to excess Cu and Cd.Our results suggest that different life strategies favored by both accessions under physiological conditions affect their response to metal exposure. While Col-0 plants mainly invest in metal detoxification, Ws plants center on nutrient homeostasis. In particular, the higher expression of genes related to Cu homeostasis genes in non-exposed conditions indicates that Ws plants possess a constitutively efficient metal homeostasis. On the other hand, oxidative stress-related MAPK signaling appears to be boosted in leaves of Col-0 plants exposed to excess Cu. Furthermore, the upregulation of the glutathione (GSH) biosynthesis GSH2 gene and the increased GSH concentration after Cd exposure suggest the activation of detoxification mechanisms, such as phytochelatin production, to counteract the more severe Cd-induced oxidative stress in leaves of Col-0 plants. Exposure to Cd also led to a more pronounced ethylene signaling response in leaves of Col-0 as compared to Ws plants, which could be related to Cd-induced GSH metabolism. In conclusion, accession-specific life strategies clearly affect the way in which leaves of A. thaliana plants cope with excess Cu and Cd.

Variations of Histone Modification Patterns: Contributions of Inter-plant Variability and Technical Factors.

Inter-individual variability of conspecific plants is governed by differences in their genetically determined growth and development traits, environmental conditions, and adaptive responses under epigenetic control involving histone post-translational modifications. The apparent variability in histone modifications among plants might be increased by technical variation introduced in sample processing during epigenetic analyses. Thus, to detect true variations in epigenetic histone patterns associated with given factors, the basal variability among samples that is not associated with them must be estimated. To improve knowledge of relative contribution of biological and technical variation, mass spectrometry was used to examine histone modification patterns (acetylation and methylation) among Arabidopsis thaliana plants of ecotypes Columbia 0 (Col-0) and Wassilewskija (Ws) homogenized by two techniques (grinding in a cryomill or with a mortar and pestle). We found little difference in histone modification profiles between the ecotypes. However, in comparison of the biological and technical components of variability, we found consistently higher inter-individual variability in histone mark levels among Ws plants than among Col-0 plants (grown from seeds collected either from single plants or sets of plants). Thus, more replicates of Ws would be needed for rigorous analysis of epigenetic marks. Regarding technical variability, the cryomill introduced detectably more heterogeneity in the data than the mortar and pestle treatment, but mass spectrometric analyses had minor apparent effects. Our study shows that it is essential to consider inter-sample variance and estimate suitable numbers of biological replicates for statistical analysis for each studied organism when investigating changes in epigenetic histone profiles.

Aquaporins isoforms in cv. Touriga Nacional grapevine under water stress and recovery—Regulation of expression in leaves and roots

The aim of this work was to advance the knowledge in aquaporins expression and physiological responses of Touriga Nacional grapevine to water stress and recovery. Ungrafted grapevines cv. Touriga Nacional were subjected to two different irrigation treatments: (i) a well watered treatment (WW), in which vines were daily irrigated, being maintained at field capacity; (ii) and a water stress treatment (WS), in which vines were not irrigated along six days, with a later recovery (Rec), with these vines being re-irrigated at field capacity. Measurements started 48h after treatments application (T1, mild stress), and from this point on, every 48h until the maximum water stress point was reached (144h after the beginning of the trial, T2). After this point WS plants were re-watered, and 48h after this recovery, measurements were taken again in both treatments. The expression pattern of different aquaporins (VvPIP2;1–VvTIP1;1–VvTIP2;1–VvTIP2;2 and VvSIP) was studied in roots and leaves in both treatments at T1, T2 and Rec respectively. With the same periodicity, leaf and root specific hydraulic conductivity (Ks-leaf; Ks-root), abscisic acid (ABA) concentration, xylem sieve pH and leaf temperature were assessed. Both VvPIP2;1 and VvTIP2;2 were down regulated in WS roots, which was in line with the decrease observed in Ks-root. However, non-significant effects on Ks-leaf were observed in WS, in parallel with an up-regulation of VvPIP2;1 and VvTIP1;1. An increase of ABA in roots and leaves of WS vines was observed, in line with the results of vine water status, leaf temperature and pH of xylem sieve. Overall, results demonstrate the existence of different regulatory mechanisms occurring in response to water stress, being dependent on stress intensity and plant organ.

Isoprenoids and phenylpropanoids are part of the antioxidant defense orchestrated daily by drought-stressed Platanus × acerifolia plants during Mediterranean summers.

The hypothesis was tested that isoprenoids and phenylpropanoids play a prominent role in countering photooxidative stress, following the depletion of antioxidant enzyme activity in plants exposed to severe drought stress under high solar irradiance and high temperatures. Platanus × acerifolia, a high isoprene-emitting species, was drought-stressed during summer (WS) and compared with unstressed controls (WW). Water relations and photosynthetic parameters were measured under mild, moderate, and severe drought stress conditions. Volatile and nonvolatile isoprenoids, antioxidant enzymes, and phenylpropanoids were measured with the same time course, but in four different periods of the day. Drought severely inhibited photosynthesis, whereas it did not markedly affect the photochemical machinery. Isoprene emission and zeaxanthin concentration were higher in WS than in WW leaves, particularly at mild and moderate stresses, and during the hottest hours of the day. The activities of catalase and ascorbate peroxidase steeply declined during the day, while the activity of guaiacol peroxidase and the concentration of quercetin increased during the day, peaking in the hottest hours in both WW and WS plants. Our experiment reveals a sequence of antioxidants that were used daily by plants to orchestrate defense against oxidative stress induced by drought and associated high light and high temperature. Secondary metabolites seem valuable complements of antioxidant enzymes to counter oxidative stress during the hottest daily hours.

Water use efficiency in Sangiovese grapes (Vitis vinifera L.) subjected to water stress before veraison: different levels of assessment lead to different conclusions.

Several recent papers have shown that in grapevine (Vitis vinifera L.), interpretation of responses to drought can differ depending upon the parameter chosen to express water use efficiency (WUE). In the present paper, a series of WUE expressions, including physiological and agronomical, were compared in potted grapevines (Vitis vinifera L. cv. Sangiovese) that were either well-watered (WW) or subjected to progressive drought before veraison (WS) by supplying decreasing fractions (i.e. 70%, 50% and 30% of daily vine transpiration (Trd) determined gravimetrically before vines were fully rewatered. Although single-leaf intrinsic and instantaneous WUE increased with water stress severity, seasonal and whole-canopy WUE were similar to that before stress, at 70% Trd and upon rewatering, but dropped during severe water stress. WUE calculated as mass of DW stored in annual biomass (leaves, canes and bunches) per litre of water used did not differ on a seasonal basis, whereas WS plants showed lower must soluble solids at harvest, and unchanged colour and phenolic concentration in spite of smaller berries with higher relative skin growth. Results confirm that whole-canopy WUE is a much better index than any single-leaf based WUE parameter for extrapolation to agronomic WUE and actual grape composition. In our specific case study, it can be recommended that water supply to drought-stressed Sangiovese grapevines before veraison should not be lower than 70% of daily vine water use.

Effects of elevated CO2 concentration and water deficit on fructan metabolism in Viguiera discolor Baker

Elevated [CO2 ] is suggested to mitigate the negative effects of water stress in plants; however responses vary among species. Fructans are recognised as protective compounds against drought and other stresses, as well as having a role as reserve carbohydrates. We analysed the combined effects of elevated [CO2 ] and water deficit on fructan metabolism in the Cerrado species Viguiera discolor Baker. Plants were cultivated for 18 days in open-top chambers (OTC) under ambient (∼380 ppm), and high (∼760 ppm) [CO2 ]. In each OTC, plants were submitted to three treatments: (i) daily watering (control), (ii) withholding water (WS) for 18 days and (iii) re-watering (RW) on day 11. Analyses were performed at time 0 and days 5, 8, 11, 15 and 18. High [CO2 ] increased photosynthesis in control plants and increased water use efficiency in WS plants. The decline in soil water content was more distinct in WS 760 (WS under 760 ppm), although the leaf and tuberous root water status was similar to WS 380 plants (WS under 380 ppm). Regarding fructan active enzymes, 1-SST activity decreased in WS plants in both CO2 concentrations, a result consistent with the decline in photosynthesis and, consequently, in substrate availability. Under WS and both [CO2 ] treatments, 1-FFT and 1-FEH seemed to act in combination to generate osmotically active compounds and thus overcome water deficit. The proportion of hexoses to sucrose, 1-kestose and nystose (SKN) was higher in WS plants. In WS 760, this increase was higher than in WS 380, and was not accompanied by decreases in SKN at the beginning of the treatment, as observed in WS 380 plants. These results suggest that the higher [CO2 ] in the atmosphere contributed to maintain, for a longer period, the pool of hexoses and of low DP fructans, favouring the maintenance of the water status and plant survival under drought.

Effect of drought and re-watering on fructan metabolism in Vernonia herbacea (Vell.) Rusby

Vernonia herbacea (Vell.) Rusby, a native species from the Brazilian Cerrado, accumulates about 80% of fructans in the rhizophores, the underground reserve organs. Besides their role as reserve, fructans have been recognized as protective compounds against drought. This physiological function attributed to fructans seems consistent with the wide occurrence of these carbohydrates in the cerrado, a biome that undergoes seasonal drought. The aim of this work was to analyze fructan composition and the activities of the enzymes involved in fructan synthesis, sucrose:sucrose 1-frutosyltransferase (1-SST) and fructan:fructan 1-frutosyltransferase (1-FFT), and depolymerization, fructan 1-exohydrolase (1-FEH) in plants submitted to water suppression. The plants were divided into 3 groups receiving 3 treatments: daily watering (control), water suppression for 23 days (WS) and re-watering after 15 days (RW). Samples were taken at the beginning of the experiment (Time 0) and after 3, 7, 11, 15, 17 and 23 days of water suppression. 1-SST and 1-FFT activities increased at the beginning of the water restriction period, coinciding with a decrease in 1-FEH activity, the onset of the reduction in soil water potential and in leaf water potential. Increases in 1-FEH and invertase activities led to a high yield of reducing sugars at the 23rd day after water suppression, and together with 1-FFT, 1-FEH also seemed to act in the redistribution of fructan molecules after re-watering. The increase in reducing sugars and in the fructo-oligo:fructo-polysaccharide ratio were associated to the maintenance of rhizophore turgor. Considering that WS plants showed changes in fructan metabolism that favored water retention and absorption after re-watering, the occurrence of osmotic adjustment mechanisms is suggested, reinforcing the hypothesis of fructans as protective agents against abiotic stresses, such as drought.

An assessment of morphogenetic fluctuation during reproductive phase change in Arabidopsis

Reproductive phase change in Arabidopsis thaliana is characterized by two transitions in phytomer identity, the differentiation of the first elongate internode (bolting transition) and of the first flower (floral transition). An evaluation of the dynamics of these transitions was sought by examining the precision of the corresponding phytomer identity changes. The length of the first elongate internode and the frequency of chimeric inflorescence structures, e.g. paraclades not subtended by a leaf (no-leaf/paraclades) and flowers subtended by a bract (bract/flowers), were measured in the Wassilewskija (Ws) accession and 47 early flowering mutants under a wide range of photoperiods. The impact of photoperiodic perturbations applied to Ws plants at different times of development was also evaluated. In Ws, both types of characters were remarkably constant across photoperiods in spite of a high degree of interindividual variability. Bract/flowers were not normally produced in Ws, but they were observed in conditions that suggest enhanced light signalling, e.g. in response to continuous light perturbations and in mutants with reduced hypocotyl elongation. In contrast, no-leaf/paraclades were normally present in approx. 20 % of Ws plants, and their frequency was increased in conditions that suggest reduced light signalling, e.g. in mutants with altered specification of long-day responses. The length of the first elongate internode was unrelated to the rate of stem elongation and to the regulation of reproductive phase change. Bract/flowers and no-leaf/paraclades corresponded to opposite effects on the floral transition that reflected different dynamics of progression to flowering. In contrast, the length of the first elongate internode was only indirectly related to the regulation of reproductive phase change and was mainly dependent on global morphogenetic constraints. This paper proposes that morphogenetic variability could be used to identify critical phases of development and characterize the canalization of developmental patterns.

An early transient water deficit reduces flower number and pod production but increases seed size in chickpea (Cicer arietinum L.)

Terminal drought is known to decrease flower production, increase flower and pod abortion, and decrease yield of chickpea (Cicer arietinum L.), but the effects of early-season drought have not been evaluated. The influence of an early transient water deficit on flower and pod production and abortion, and seed yield and its components was evaluated in two chickpea cultivars, Rupali, a desi type, and Almaz, a kabuli type. Thirty-six-day-old plants were subjected to: (i) a transient water deficit by withholding water for 35 days, and then rewatered (WS), and (ii) kept well watered (WW) throughout. In the WS treatment the soil water content, leaf relative water content and leaf photosynthetic rate decreased after water was withheld and, following rewatering, recovered to the WW level. Despite the WS treatment being imposed at different phenological stages in the two cultivars, WS reduced flower number per plant by ~50% in Rupali and Almaz, respectively, compared with the WW plants. In WW plants, ~15% of flowers aborted in both cultivars, and 42 and 67% of the pods aborted in Rupali and Almaz, respectively, whereas in WS plants, 18 and 23% of flowers aborted and 27 and 67% of pods aborted in Rupali and Almaz, respectively. While seed growth in WS plants of Rupali and Almaz occurred primarily after the plants were rewatered, the duration of seed growth decreased by 17 and 36 days, the maximum rate of seed filling increased by 3 times and 5 times, and seed size increased by 26 and 16%, respectively, compared with the WW plants. Seed yield per plant in WS plants decreased by 31% in Rupali and 38% in Almaz compared with the WW controls. The early transient water deficit decreased flower production, but improved flower and pod development; increased the rate of seed growth and increased final seed size; and had a smaller effect on seed yield compared with chickpea subjected to terminal drought.

Flower numbers, pod production, pollen viability, and pistil function are reduced and flower and pod abortion increased in chickpea (Cicer arietinum L.) under terminal drought

Terminal drought during the reproductive stage is a major constraint to yield of chickpea in many regions of the world. Termination of watering (WS) during podding in a small-seeded desi chickpea (Cicer arietinum L.) cultivar, Rupali, and a large-seeded kabuli chickpea cultivar, Almaz, induced a decrease in predawn leaf water potential (LWP), in the rate of photosynthesis, and in stomatal conductance. Compared to well-watered (WW) controls, the WS treatment reduced flower production by about two-thirds. In the WW treatment, about 15% of the flowers aborted and 42% (Rupali) and 67% (Almaz) of the pods aborted, whereas in the WS treatment 37% and 56% of the flowers aborted and 54% and 73% of the pods aborted, resulting in seed yields of 33% and 15% of the yields in WW plants in Rupali and Almaz, respectively. In vitro pollen viability and germination in Rupali decreased by 50% and 89% in the WS treatment, and pollen germination decreased by 80% in vivo when pollen from a WS plant was placed on a stigma of a WW plant. While about 37% of the germinated pollen tubes from WW plants and 22% from the WS plants reached the ovary in the WW plants, less than 3% of pollen grains reached the ovary when pollen from either WS or WW plants was placed on a stigma of a WS plant. It is concluded that, in addition to pod abortion, flower abortion is an important factor limiting yield in chickpea exposed to terminal drought and that water deficit impaired the function of the pistil/style more than the pollen.

Isozyme-Specific Modes of Activation of CTP:Phosphorylcholine Cytidylyltransferase in Arabidopsis thaliana at Low Temperature

Arabidopsis thaliana increases cellular phosphatidylcholine (PC) content during cold acclimation by up-regulating PC biosynthesis. The A. thaliana genes CCT1 and CCT2 encode CTP:phosphorylcholine cytidylyltransferases (CCTs; EC 2.7.7.15), which regulate PC biosynthesis via the CDP-choline pathway. We isolated the T-DNA-tagged knockout mutants cct1 and cct2 of A. thaliana (Wassilevskaja; WS). CCT activity in cct1 and cct2 plants accounted for 29 and 79% to the cellular CCT activity of WS plants, respectively. When plants were exposed to 2 degrees C for 7 d, CCT activity increased in both cct1 and cct2 plants, and immunoblot analyses revealed that cct1 contained an increased level of CCT2 protein whereas cct2 exhibited little increase in CCT1 level. For each mutant grown at 23 degrees C, CCT activity was mainly enriched in the particulate (15,000 x g pellet) and microsomal (150,000 x g pellet) fractions from rosette leaf homogenates. After exposure to cold, the particulate and microsomal fractions of cct1 plants had higher total CCT activity due to increased levels of CCT2; in contrast, the levels of CCT1 in cct2 plants remained unchanged in particulate and microsomal fractions despite a significant increase in the total CCT activity. We conclude that the CDP-choline pathway of A. thaliana is up-regulated at low temperature via isogene-specific modes: enhanced expression of CCT2 and post-translational activation/inactivation of CCT1 in membranes. PC levels were similarly maintained in both mutants and WS plants after 14 d at 2 degrees C, suggesting that either of the CCT genes is sufficient for PC biosynthesis at low temperature.

Photosynthetic responses of Populus przewalski subjected to drought stress

Cuttings of P. przewalski were exposed to two different watering regimes which were watered to 100 and 25 % of field capacity (WW and WS, respectively). Drought stress not only significantly decreased net photosynthetic rate (P N), transpiration rate (E), stomatal conductance (g s), efficiency of photosystem 2 (PS2) (Fv/Fm and yield), and increased intrinsic water use efficiency (WUEi) under controlled optimal conditions, but also altered the diurnal changes of gas exchange, chlorophyll fluorescence, and WUEi. On the other hand, WS also affected the P N-photosynthetically active radiation (PAR) response curve. Under drought stress, P N peak appeared earlier (at about 10:30 of local time) than under WW condition (at about 12:30). At midday, there was a depression in P N for WS plants, but not for WW plants, and it could be caused by the whole microclimate, especially high temperature, low relative humidity, and high PAR. There were stomatal and non-stomatal limitations to photosynthesis. Stomatal limitation dominated in the morning, and low P N at midday was caused by both stomatal and non-stomatal limitations, whereas non-stomatal limitation dominated in the afternoon. In addition, drought stress also increased compensation irradiance and dark respiration rate, and decreased saturation irradiance and maximum net photosynthetic rate. Thus drought stress decreased plant assimilation and increased dissimilation through affected gas exchange, the diurnal pattern of gas exchange, and photosynthesis-PAR response curve, thereby reducing plant growth and productivity.

A central role for the nitrate transporter NRT2.1 in the integrated morphological and physiological responses of the root system to nitrogen limitation in Arabidopsis.

Up-regulation of the high-affinity transport system (HATS) for NO(3)(-) and stimulation of lateral root (LR) growth are two important adaptive responses of the root system to nitrogen limitation. Up-regulation of the NO(3)(-) HATS by nitrogen starvation is suppressed in the atnrt2.1-1 mutant of Arabidopsis (Arabidopsis thaliana), deleted for both NRT2.1 and NRT2.2 nitrate transporter genes. We then used this mutant to determine whether lack of HATS stimulation affected the response of the root system architecture (RSA) to low NO(3)(-) availability. In Wassilewskija (Ws) wild-type plants, transfer from high to low NO(3)(-) medium resulted in contrasting responses of RSA, depending on the level of nitrogen limitation. Moderate nitrogen limitation (transfer from 10 mm to 1 or 0.5 mm NO(3)(-)) mostly led to an increase in the number of visible laterals, while severe nitrogen stress (transfer from 10 mm to 0.1 or 0.05 mm NO(3)(-)) promoted mean LR length. The RSA response of the atnrt2.1-1 mutant to low NO(3)(-) was markedly different. After transfer from 10 to 0.5 mm NO(3)(-), the stimulated appearance of LRs was abolished in atnrt2.1-1 plants, whereas the increase in mean LR length was much more pronounced than in Ws. These modifications of RSA mimicked those of Ws plants subjected to severe nitrogen stress and could be fully explained by the lowered NO(3)(-) uptake measured in the mutant. This suggests that the uptake rate of NO(3)(-), rather than its external concentration, is the key factor triggering the observed changes in RSA. However, the mutation of NRT2.1 was also found to inhibit initiation of LR primordia in plants subjected to nitrogen limitation independently of the rate of NO(3)(-) uptake by the whole root system and even of the presence of added NO(3)(-) in the external medium. This indicates a direct stimulatory role for NRT2.1 in this particular step of LR development. Thus, it is concluded that NRT2.1 has a key dual function in coordinating root development with external NO(3)(-) availability, both indirectly through its role as a major NO(3)(-) uptake system that determines the nitrogen uptake-dependent RSA responses, and directly through a specific action on LR initiation under nitrogen-limited conditions.

Contrasting effects of ozone under different water supplies in two Mediterranean tree species

Abstract The effects of ozone (O3) exposure under different water availabilities were studied in two Mediterranean tree species: Quercus ilex and Ceratonia siliqua. Plants were exposed to different O3 concentrations in open top chambers (charcoal-filtered air (CF), non-filtered air (NF)) and non-filtered air plus 40 ppbv of O3 ((7:00–17:00 solar time) (NF+)) during 2 years, and to different water regimes (IR, sample irrigation, and WS, reduced water dose to 50%) through the last of those 2 years. AOT40 in the NF+ treatment was 59265 ppbv h (from March 1999 to August 1999) while in the NF treatment, the AOT40 was 6727 ppbv h for the same period. AOT40 was always 0 in the CF treatment. WS plants presented lower stomatal conductances and net photosynthetic rates, and higher foliar N concentrations than IR plants in both species. The irrigation treatment did not change the response trends to ozone in Q. ilex, the most sensitive species to O3 ambient concentrations, but it changed those of C. siliqua, the least sensitive species, since its ozone-fumigated WS plants did not decrease their net photosynthetic rates nor their biomass accumulation as it happened to its ozone-fumigated IR plants. These results show interspecific variations in O3 sensitivity under different water availabilities.

Abscisic acid and cytokinins in the root exudates and leaves and their relationship to senescence and remobilization of carbon reserves in rice subjected to water stress during grain filling.

The possible regulation of senescence-initiated remobilization of carbon reserves in rice (Oryza sativa L.) by abscisic acid (ABA) and cytokinins was studied using two rice cultivars with high lodging resistance and slow remobilization. The plants were grown in pots and either well-watered (WW, soil water potential = 0 MPa) or water-stressed (WS, soil water potential = -0.05 MPa) from 9 days after anthesis until they reached maturity. Leaf water potentials of both cultivars markedly decreased at midday as a result of water stress but completely recovered by early morning. Chlorophyll (Chl) and photosynthetic rate (Pr) of the flag leaves declined faster in WS plants than in WW plants, indicating that the water deficit enhanced senescence. Water stress accelerated starch remobilization in the stems, promoted the re-allocation of pre-fixed (14)C from the stems to grains, shortened the grain-filling period and increased the grain-filling rate. Sucrose phosphate synthase (SPS, EC 2.4.1.14) activity was enhanced by water stress and positively correlated with sucrose accumulation in both the stem and leaves. Water stress substantially increased ABA but reduced zeatin (Z) + zeatin riboside (ZR) concentrations in the root exudates and leaves. ABA significantly and negatively, while Z+ZR positively, correlated with Pr and Chl of the flag leaves. ABA, not Z+ZR, was positively and significantly correlated with SPS activity and remobilization of pre-stored carbon. Spraying ABA reduced Chl in the flag leaves, and enhanced SPS activity and remobilization of carbon reserves. Spraying kinetin had the opposite effect. The results suggest that both ABA and cytokinins are involved in controlling plant senescence, and an enhanced carbon remobilization is attributed to an elevated ABA level in rice plants subjected to water stress.