Primary Root Tips Research Articles

Nitrate Controls Root Development through Posttranscriptional Regulation of the NRT1.1/NPF6.3 Transporter/Sensor.

Plants are able to modulate root growth and development to optimize their nitrogen nutrition. In Arabidopsis (Arabidopsis thaliana), the adaptive root response to nitrate (NO3-) depends on the NRT1.1/NPF6.3 transporter/sensor. NRT1.1 represses emergence of lateral root primordia (LRPs) at low concentration or absence of NO3- through its auxin transport activity that lowers auxin accumulation in LR. However, these functional data strongly contrast with the known transcriptional regulation of NRT1.1, which is markedly repressed in LRPs in the absence of NO3- To explain this discrepancy, we investigated in detail the spatiotemporal expression pattern of the NRT1.1 protein during LRP development and combined local transcript analysis with the use of transgenic lines expressing tagged NRT1.1 proteins. Our results show that although NO3- stimulates NRT1.1 transcription and probably mRNA stability both in primary root tissues and in LRPs, it acts differentially on protein accumulation, depending on the tissues considered with stimulation in cortex and epidermis of the primary root and a strong repression in LRPs and to a lower extent at the primary root tip. This demonstrates that NRT1.1 is strongly regulated at the posttranscriptional level by tissue-specific mechanisms. These mechanisms are crucial for controlling the large palette of adaptive responses to NO3- mediated by NRT1.1 as they ensure that the protein is present in the proper tissue under the specific conditions where it plays a signaling role in this particular tissue.

Nitric oxide-cytokinin interplay influences selenite sensitivity in Arabidopsis.

Selenite oppositely modifies cytokinin and nitric oxide metabolism in Arabidopsis organs. A mutually negative interplay between the molecules exists in selenite-exposed roots; and their overproduction causes selenite insensitivity. Selenium-induced phytotoxicity is accompanied by developmental alterations such as primary root (PR) shortening. Growth changes are provoked by the modulation of hormone status and signalling. Cytokinin (CK) cooperates with the nitric oxide (NO) in many aspects of plant development; however, their interaction under abiotic stress has not been examined. Selenite inhibited the growth of Arabidopsis seedlings and reduced root meristem size through cell division arrest. The CK-dependent pARR5::GUS activity revealed the intensification of CK signalling in the PR tip, which may be partly responsible for the root meristem shortening. The selenite-induced alterations in the in situ expressions of cytokinin oxidases (AtCKX4::GUS, AtCKX5::GUS) are associated with selenite-triggered changes of CK signalling. In wild-type (WT) and NO-deficient nia1nia2 root, selenite led to the diminution of NO content, but CK overproducer ipt-161 and -deficient 35S:CKX2 roots did not show NO decrease. Exogenous NO (S-nitroso-N-acetyl-DL-penicillamine, SNAP) reduced the pARR5::GFP and pTCS::GFP expressions. Roots of the 35S:CKX and cyr1 plants suffered more severe selenite-triggered viability loss than the WT, while in ipt-161 and gsnor1-3 no obvious viability decrease was observed. Exogenous NO ameliorated viability loss, but benzyladenine intensified it. Based on the results, selenite impacts development by oppositely modifying CK signalling and NO level. In the root system, CK signalling intensifies which possibly contributes to the nitrate reductase-independent NO diminution. A mutually negative CK-NO interplay exists in selenite-exposed roots; however, overproduction of both molecules worsens selenite sensing. Hereby, we suggest novel regulatory interplay and role for NO and CK in abiotic stress signalling.

Mutations in exocyst complex subunit SEC6 gene impaired polar auxin transport and PIN protein recycling in Arabidopsis primary root

Polar auxin transport, which is critical for land plant pattern formation and directional growth, is largely depended on asymmetric distribution of PIN proteins at the plasma membrane (PM). Endocytosis and recycling processes play important roles in regulating PIN protein distribution and abundance at the PM. Two subunits (SEC8, EXO70A1) of exocyst, an octameric vesicle-tethering complex, have been reported to be involved in PIN protein recycling in Arabidopsis. However, the function of exocyst complex in PIN protein recycling and polar auxin transport remains incompletely understood. In this study, we utilized two SEC6 down-regulation mutants (PRsec6-1 and PRsec6-2) to investigate the role of exocyst subunit SEC6 in the primary root development, polar auxin transport and PIN proteins recycling. We found that in PRsec6 mutants: 1. Primary root growth was retarded, and lateral root initiation were compromised. 2. Primary roots were sensitive to exogenous auxin 1-napthalene acetic acid (NAA) but not 2,4-dichlorophenoxy (2.4-D). 3. Recycling of PIN1 and PIN2 proteins from the Brefeldin A (BFA) compartment to the PM was delayed. 4. Vesicles accumulated in the primary root tip cells, especially accumulated in the cytosol closed to the PM. These results further demonstrated that the exocyst complex plays an important role in PIN protein recycling and polar auxin transport in Arabidopsis primary root.

Serotonin modulates Arabidopsis root growth via changes in reactive oxygen species and jasmonic acid-ethylene signaling.

Serotonin (5-hydroxytryptamine) is a bioactive indoleamine with neurotransmitter function in vertebrates, which represents an emerging signaling molecule in plants, playing key roles in the development and defense. In this study, the role of reactive oxygen species (ROS) and jasmonic acid (JA)-ethylene (Et) signaling in root developmental alterations induced by serotonin was investigated. An Arabidopsis thaliana mutant defective at the RADICAL-INDUCED CELL DEATH1 (RCD1) locus was resistant to paraquat-induced ROS accumulation in primary roots and showed decreased inhibition or root growth in response to serotonin. A suite of JA- and Et-related mutants including coronatine insensitive1, jasmonic acid resistant1 (jar1), etr1, ein2 and ein3 showed tolerance to serotonin in the inhibition of primary root growth and ROS redistribution within the root tip when compared with wild-type (WT) seedlings. Competence assays between serotonin and AgNO3 , a well-known blocker of Et action, showed that primary root growth in medium supplemented with serotonin was normalized by AgNO3 , whereas roots of eto3, an Et overproducer mutant, were oversensitive to serotonin. Comparison of ROS levels in WT, etr1, jar1 and rcd1 primary root tips using the ROS-specific probe 2',7'-dichlorofluorescein diacetate and confocal imaging showed that serotonin inhibition of primary root growth likely occurs independently of its conversion into melatonin. Our results provide compelling evidence that serotonin affects ROS distribution in roots, involving RCD1 and components of the JA-Et signaling pathways.

The effect of spaceflight on the gravity-sensing auxin gradient of roots: GFP reporter gene microscopy on orbit.

Our primary aim was to determine whether gravity has a direct role in establishing the auxin-mediated gravity-sensing system in primary roots. Major plant architectures have long been thought to be guided by gravity, including the directional growth of the primary root via auxin gradients that are then disturbed when roots deviate from the vertical as a gravity sensor. However, experiments on the International Space Station (ISS) now allow physical clarity with regard to any assumptions regarding the role of gravity in establishing fundamental root auxin distributions. We examined the spaceflight green fluorescent protein (GFP)-reporter gene expression in roots of transgenic lines of Arabidopsis thaliana: pDR5r::GFP, pTAA1::TAA1–GFP, pSCR::SCR–GFP to monitor auxin and pARR5::GFP to monitor cytokinin. Plants on the ISS were imaged live with the Light Microscopy Module (LMM), and compared with control plants imaged on the ground. Preserved spaceflight and ground control plants were examined post flight with confocal microscopy. Plants on orbit, growing in the absence of any physical reference to the terrestrial gravity vector, displayed typically “vertical” distribution of auxin in the primary root. This confirms that the establishment of the auxin-gradient system, the primary guide for gravity signaling in the root, is gravity independent. The cytokinin distribution in the root tip differs between spaceflight and the ground controls, suggesting spaceflight-induced features of root growth may be cytokinin related. The distribution of auxin in the gravity-sensing portion of the root is not dependent on gravity. Spaceflight appears benign to auxin and its role in the development of the primary root tip, whereas spaceflight may influence cytokinin-associated processes.

Analysis of Microtubule-Associated-Proteins during IBA-Mediated Adventitious Root Induction Reveals KATANIN Dependent and Independent Alterations of Expression Patterns

Adventitious roots (AR) are post embryonic lateral organs that differentiate from non-root tissues. The understanding of the molecular mechanism which underlies their differentiation is important because of their central role in vegetative plant propagation. Here it was studied how the expression of different microtubule (MT)-associated proteins (MAPs) is affected during AR induction, and whether expression differences are dependent on MT organization itself. To examine AR formation when MTs are disturbed we used two mutants in the MT severing protein KATANIN. It was found that rate and number of AR primordium formed following IBA induction for three days was reduced in bot1-1 and bot1-7 plants. The reduced capacity to form ARs in bot1-1 was associated with altered expression of MAP-encoding genes along AR induction. While the expression of MAP65-4, MAP65-3, AURORA1, AURORA2 and TANGLED, increased in wild-type but not in bot1-1 plants, the expression of MAP65-8 and MDP25 decreased in wild type plants but not in the bot1-1 plant after two days of IBA-treatment. The expression of MOR1 was increased two days after AR induction in wild type and bot1-1 plants. To examine its expression specifically in AR primordium, MOR1 upstream regulatory sequence was isolated and cloned to regulate GFP. Expression of GFP was induced in the primary root tips and lateral roots, in the pericycle of the hypocotyls and in all stages of AR primordium formation. It is concluded that the expression of MAPs is regulated along AR induction and that reduction in KATANIN expression inhibits AR formation and indirectly influences the specific expression of some MAPs.

Aluminum-Dependent Terminal Differentiation of the Arabidopsis Root Tip Is Mediated through an ATR-, ALT2-, and SOG1-Regulated Transcriptional Response.

By screening for suppressors of the aluminum (Al) hypersensitive Arabidopsis thaliana mutant als3-1, it was found that mutational loss of the Arabidopsis DNA damage response transcription factor SUPPRESSOR OF GAMMA RESPONSE1 (SOG1) confers increased Al tolerance similar to the loss-of-function mutants for the cell cycle checkpoint genes ATAXIA TELANGIECTASIA AND RAD3 RELATED (ATR) and ALUMINUM TOLERANT2 (ALT2). This suggests that Al-dependent terminal differentiation of the root tip is an active process resulting from activation of the DNA damage checkpoint by an ATR-regulated pathway, which functions at least in part through SOG1. Consistent with this, ATR can phosphorylate SOG1 in vitro. Analysis of SOG1's role in Al-dependent root growth inhibition shows that sog1-7 prevents Al-dependent quiescent center differentiation and endoreduplication in the primary root tip. Following Al exposure, SOG1 increases expression of several genes previously associated with DNA damage, including BRCA1 and PARP2, with gel-shift analysis showing that SOG1 can physically associate with the BRCA1 promoter in vitro. Al-responsive expression of these SOG1-regulated genes requires ATR and ALT2, but not ATAXIA TELANGIECTASIA MUTATED, thus demonstrating that in response to chronic Al exposure, ATR, ALT2, and SOG1 function together to halt root growth and promote terminal differentiation at least in part in a transcription-dependent manner.

Calcium alleviates cadmium-induced inhibition on root growth by maintaining auxin homeostasis in Arabidopsis seedlings.

Cadmium (Cd) toxicity has been widely studied in different plant species. However, the mechanism involved in its toxicity and the cell response to Cd has not been well established. In the present study, we investigated the possible mechanism of calcium (Ca) in protecting Arabidopsis from Cd toxicity. The results showed that 50 μM Cd significantly inhibited the seedling growth and decreased the chlorophyll content in Arabidopsis. Specifically, the primary root (PR) length was decreased but the lateral root (LR) number was increased under Cd stress. Furthermore, Cd enhanced the hydrogen peroxide (H2O2) content and lipid peroxidation as indicated by malondialdehyde (MDA) accumulation. Cd also altered the level and the distribution of auxin in PR tips (as evidenced by DR5::GUS and PIN:GFP reporter expression) and the expression of several putative auxin biosynthetic, catabolic, and transport pathway-related genes. Application of 3 mM Ca alleviated the inhibition of Cd on the root growth. Ca application not only led to reducing oxidative injuries but also restoring the normal auxin transport and distribution in Arabidopsis root under Cd stress. Taken together, these results suggest that Ca alleviates the root growth inhibition caused by Cd through maintaining auxin homeostasis in Arabidopsis seedlings.

Chromate induces adventitious root formation via auxin signalling and SOLITARY-ROOT/IAA14 gene function in Arabidopsis thaliana.

Morphological root plasticity optimizes nutrient and water uptake by plants and is a promising target to improve tolerance to metal toxicity. Exposure to sublethal chromate [Cr(VI)] concentrations inhibits root growth, decreases photosynthesis and compromises plant development and productivity. Despite the increasing environmental problem that Cr(VI) represents, to date, the Cr tolerance mechanisms of plants are not well understood, and it remains to be investigated whether root architecture remodelling is important for plant adaptation to Cr(VI) stress. In this report, we analysed the growth response of Arabidopsis thaliana seedlings to concentrations of Cr(VI) that strongly repress primary and lateral root growth. Interestingly, adventitious roots started developing, branched and allowed seedlings to grow under highly growth-repressing Cr(VI) concentrations. Cr(VI) negatively regulates auxin transport and response gene expression in the primary root tip, as evidenced by decreased expression of auxin-related reporters DR5::GFP, DR5::uidA and PIN1::PIN1::GFP, and then, another auxin maximum is established at the site of adventitious root initiation that drives adventitious root organogenesis. Both primary root growth inhibition and adventitious root formation induced by high Cr(VI) levels are blocked by a gain-of-function mutation in the SOLITARY-ROOT/IAA14 gene of Arabidopsis. These data provide evidence that suggests a critical role for auxin transport and signalling via IAA14/SLR1 in the developmental program linking Cr(VI) to root architecture remodelling.

The anthocyanin reduced tomato mutant demonstrates the role of flavonols in tomato lateral root and root hair development.

This study utilized tomato (Solanum lycopersicum) mutants with altered flavonoid biosynthesis to understand the impact of these metabolites on root development. The mutant anthocyanin reduced (are) has a mutation in the gene encoding FLAVONOID 3-HYDROXYLASE (F3H), the first step in flavonol synthesis, and accumulates higher concentrations of the F3H substrate, naringenin, and lower levels of the downstream products kaempferol, quercetin, myricetin, and anthocyanins, than the wild type. Complementation of are with the p35S:F3H transgene reduced naringenin and increased flavonols to wild-type levels. The initiation of lateral roots is reduced in are, and p35S:F3H complementation restores wild-type root formation. The flavonoid mutant anthocyanin without has a defect in the gene encoding DIHYDROFLAVONOL REDUCTASE, resulting in elevated flavonols and the absence of anthocyanins and displays increased lateral root formation. These results are consistent with a positive role of flavonols in lateral root formation. The are mutant has increased indole-3-acetic acid transport and greater sensitivity to the inhibitory effect of the auxin transport inhibitor naphthylphthalamic acid on lateral root formation. Expression of the auxin-induced reporter (DR5-β-glucuronidase) is reduced in initiating lateral roots and increased in primary root tips of are. Levels of reactive oxygen species are elevated in are root epidermal tissues and root hairs, and are forms more root hairs, consistent with a role of flavonols as antioxidants that modulate root hair formation. Together, these experiments identify positive roles of flavonols in the formation of lateral roots and negative roles in the formation of root hairs through the modulation of auxin transport and reactive oxygen species, respectively.

The S-Domain Receptor Kinase Arabidopsis Receptor Kinase2 and the U Box/Armadillo Repeat-Containing E3 Ubiquitin Ligase9 Module Mediates Lateral Root Development under Phosphate Starvation in Arabidopsis.

When plants encounter nutrient-limiting conditions in the soil, the root architecture is redesigned to generate numerous lateral roots (LRs) that increase the surface area of roots, promoting efficient uptake of these deficient nutrients. Of the many essential nutrients, reduced availability of inorganic phosphate has a major impact on plant growth because of the requirement of inorganic phosphate for synthesis of organic molecules, such as nucleic acids, ATP, and phospholipids, that function in various crucial metabolic activities. In our screens to identify a potential role for the S-domain receptor kinase1-6 and its interacting downstream signaling partner, the Arabidopsis (Arabidopsis thaliana) plant U box/armadillo repeat-containing E3 ligase9 (AtPUB9), we identified a role for this module in regulating LR development under phosphate-starved conditions. Our results show that Arabidopsis double mutant plants lacking AtPUB9 and Arabidopsis Receptor Kinase2 (AtARK2; ark2-1/pub9-1) display severely reduced LRs when grown under phosphate-starved conditions. Under these starvation conditions, these plants accumulated very low to no auxin in their primary root and LR tips as observed through expression of the auxin reporter DR5::uidA transgene. Exogenous auxin was sufficient to rescue the LR developmental defects in the ark2-1/pub9-1 lines, indicating a requirement of auxin accumulation for this process. Our subcellular localization studies with tobacco (Nicotiana tabacum) suspension-cultured cells indicate that interaction between ARK2 and AtPUB9 results in accumulation of AtPUB9 in the autophagosomes. Inhibition of autophagy in wild-type plants resulted in reduction of LR development and auxin accumulation under phosphate-starved conditions, suggesting a role for autophagy in regulating LR development. Thus, our study has uncovered a previously unknown signaling module (ARK2-PUB9) that is required for auxin-mediated LR development under phosphate-starved conditions.

Research on Chromosome Karyotype Analysis of <i>Plumbago auriculata</i>

In this study, the chromosome karyotype of Plumbago auriculata was analyzed by using a chromosome mounting technique with the primary root tip of seed germination. The aim was to find out the most suitable method for Plumbago auriculata by comparing the effect of different pre-treatment and dissociation time on chromosome. The results showed that the best pretreatment was 0.05% colchicine solution under 0℃ - 4℃ or 2 h and then 1 mol/L HCL under 60℃ water for 6 min. Karyotype analysis of Plumbago auriculata showed that the chromosome number was 12 and the karyotype formula was 2n = 2x = 12 = 8 m (4 sat) + 4 sm. The karyotype belonged to 2B and the karyotype asymmetry coefficient was 59.41%. The absolute average length of chromosome was 0.40 μm and the average arm ratio was 1.54. The chromosome of Plumbago auriculata belonged to minichromosome, including a couple of SAT-chromosomes.

Infections with the vascular pathogens Verticillium longisporum and Verticillium dahliae induce distinct disease symptoms and differentially affect drought stress tolerance of Arabidopsis thaliana

Verticillium longisporum and Verticillium dahliae are soil borne vascular pathogens. Under standard laboratory growth conditions both fungi infect Arabidopsis thaliana ecotype Col-0 plants via primary and lateral (secondary) root tips and colonize the vascular cylinder including proto- and metaxylem. After entry into the hypocotyl both Verticillium species spread into petioles and leaves, but with different efficiency and distinct disease symptom patterns. V. longisporum typically induces early senescence whereas V. dahliae infections cause wilting. In both cases, these symptoms coincide with the switch from a biotrophic to a necrotrophic life style. Fungal proliferation analyses monitored by quantitative real-time PCR and anatomical studies show that V. dahliae is able to colonize aerial plant parts more efficiently compared to V. longisporum. Moreover, V. longisporum-infection triggers hyperplastic xylem formation and cellular transdifferentiation in Arabidopsis, whereas V. dahliae-infection induces enhanced xylem lignification. We have recently shown that V. longisporum induced de novo xylem formation results in enhanced drought stress tolerance, suggesting a conditionally mutualistic interaction. Here we show that on the contrary V. dahliae infections do not provide protection of the plant against drought. Together, our results demonstrate that V. dahliae and V. longisporum induce distinct and species-specific developmental and structural alterations in Arabidopsis, which differently affect plant fitness under concomitant abiotic drought stress conditions.

Cadmium interferes with maintenance of auxin homeostasis in Arabidopsis seedlings

Auxin and its homeostasis play key roles in many aspects of plant growth and development. Cadmium (Cd) is a phytotoxic heavy metal and its inhibitory effects on plant growth and development have been extensively studied. However, the underlying molecular mechanism of the effects of Cd stress on auxin homeostasis is still unclear. In the present study, we found that the root elongation, shoot weight, hypocotyl length and chlorophyll content in wild-type (WT) Arabidopsis seedlings were significantly reduced after exposure to Cd stress. However, the lateral root (LR) formation was markedly promoted by Cd stress. The level and distribution of auxin were both greatly altered in primary root tips and cotyledons of Cd-treated plants. The results also showed that after Cd treatment, the IAA content was significantly decreased, which was accompanied by increases in the activity of the IAA oxidase and alteration in the expression of several putative auxin biosynthetic and catabolic genes. Application of the auxin transport inhibitor, 1-naphthylphthalamic acid (NPA) and 1-naphthoxyacetic acid (1-NOA), reversed the effects of Cd on LR formation. Additionally, there was less promotion of LR formation by Cd treatment in aux1-7 and pin2 mutants than that in the WT. Meanwhile, Cd stress also altered the expression of PINs and AUX1 in Arabidopsis roots, implying that the auxin transport pathway is required for Cd-modulated LR development. Taken together, these findings suggest that Cd stress disturbs auxin homeostasis through affecting auxin level, distribution, metabolism, and transport in Arabidopsis seedling.

The involvement of auxin in root architecture plasticity in Arabidopsis induced by heterogeneous phosphorus availability

Homogeneous low phosphorus availability was reported to regulate root architecture in Arabidopsis via auxin, but the roles of auxin in root architecture plasticity to heterogeneous P availability remain unclear. In this study, we employed auxin biosynthesis-, transport- and signalling-related mutants. Firstly, we found that in contrast to low P (LP) content in the whole medium, primary root (PR) growth of Arabidopsis was partially rescued in the medium divided into two parts: upper with LP and lower with high P (HP) content or in the reverse arrangement. The down part LP was more effective to arrest PR growth as well as to decrease density of lateral roots (DLR) than the upper LP, and effects were dependent on polar auxin transport. Secondly, we verified that auxin receptor TIR1 was involved in the responses of PR growth and lateral root (LR) development to P supply and loss of function of TIR1 inhibited LR development. Thirdly, effects of heterogeneous P on LRD in the upper part of PR was dependent on PIN2 and PIN4, and in the down part on PIN3 and PIN4, whereas density of total LRs was dependent on auxin transporters PIN2 and PIN7. Finally, heterogeneous P availability altered the accumulation of auxin in PR tip and the expression of auxin biosynthesisrelated genes TAA1, YUC1, YUC2, and YUC4. Taken together, we provided evidences for the involvement of auxin in root architecture plasticity in response to heterogeneous phosphorus availability in Arabidopsis.

OsORC3 is required for lateral root development in rice

The origin recognition complex (ORC) is a pivotal element in DNA replication, heterochromatin assembly, checkpoint regulation and chromosome assembly. Although the functions of the ORC have been determined in yeast and model animals, they remain largely unknown in the plant kingdom. In this study, Oryza sativa Origin Recognition Complex subunit3 (OsORC3) was cloned using map-based cloning procedures, and functionally characterized using a rice (Oryza sativa) orc3 mutant. The mutant showed a temperature-dependent defect in lateral root (LR) development. Map-based cloning showed that a G→A mutation in the 9th exon of OsORC3 was responsible for the mutant phenotype. OsORC3 was strongly expressed in regions of active cell proliferation, including the primary root tip, stem base, lateral root primordium, emerged lateral root primordium, lateral root tip, young shoot, anther and ovary. OsORC3 knockdown plants lacked lateral roots and had a dwarf phenotype. The root meristematic zone of ORC3 knockdown plants exhibited increased cell death and reduced vital activity compared to the wild-type. CYCB1;1::GUS activity and methylene blue staining showed that lateral root primordia initiated normally in the orc3 mutant, but stopped growing before formation of the stele and ground tissue. Our results indicate that OsORC3 plays a crucial role in the emergence of lateral root primordia.

Development: CLAVATA1 Joins the Club of Root Stem Cell Regulators

Stem cell maintenance and daughter cell differentiation is essential for root and shoot development. Genetic and physical interaction of the receptor-like kinases ACR4 and CLV1 bring a new player to the field of distal stem cell control in the primary root tip.

Soybean Root Development Relative to Vegetative and Reproductive Phenology

Knowledge of soybean [Glycine max (L.) Merr.] primary, secondary, and tertiary root tip locations in the soil vs. seasonal time would enhance modeling of soybean development. The seasonal progression of root tip development and shoot phenology was evaluated in situ using an imaging device inserted into minirhizotron tubes installed in the soil at an in‐row 30° angle. Primary root tip extension was linear (i.e., 1.5 and 1.2 cm d−1 each year) until the full‐seed stage. Emergent 5‐mm secondary roots were routinely detected about 10‐cm above the primary root tip, and thus present in a soil layer 11 d after the primary root tip had passed through that layer. Secondary roots followed a similar temporal pattern. Primary root tip location in the soil paralleled a 17°C soil temperature isoline. The 3.7‐d phyllochron of main‐stem node accrual between first node and seed fill may be a calibratable proxy for inferring correspondent root tip depths.

Effectiveness of tissue culture media components on the growth and development of cauliflower (Brassica oleracea var. Botrytis) seedling explants in vitro

A key factor in the application of in vitro techniques to cauliflower improvement is the development of efficient protocols for regeneration of plants from tissue for use in breeding programs for the selection of the desirable genotypes under biotic and abiotic stress. Experiments were conducted to study the effect of different media components (agar or agar + sucrose or agar + Murashige and Skoog (MS) salts or agar + sucrose + MS) on callus induction and regeneration from five explants types (cotyledon, hypocotyls, shoot apex, primary root and root tip) and also the effects of auxin and cytokinin were carried out using one F 1 hybrid cauliflower cv. Medallion. The results show that cotyledons, mid roots and root apices grown on agar + MS + sugar were the most developed compared with explants on the other media. This medium was the most productive in terms of lateral root number and root length. The presence of 2,4-dichlorophenoxyacetic acid increased callus production compared to 6-benzylamino purine. Within the selected explants, a significant difference was indicated between different explants under different treatments. Liquid culture was more successful at producing viable plantlets than solid culture. Key words: Cauliflower, explants, in vitro , growth regulator, growth characters, solid and liquid culture.

BcNRT1, a plasma membrane-localized nitrate transporter from non-heading Chinese cabbage

A nitrate transporter, BcNRT1, was isolated from non-heading Chinese cabbage (Brassica campestris ssp. chinensis Makino) cultivar 'Suzhouqing'. The full-length cDNA was obtained using the rapid amplification of cDNA ends technique and contains an open reading frame of 1,770bp that predicts a protein of 589 acid residues that possesses 12 putative transmembrane domains. Using the GUS marker gene driven by the BcNRT1 promoter, we found BcNRT1 expression to be concentrated in primary and lateral root tips and in shoots of transgenic Arabidopsis plants. The YFP fused to BcNRT1 and transformed into cabbage protoplasts indicated that BcNRT1 was localized to the plasma membrane. The expression of BcNRT1 in roots was induced by exposure to 25mM nitrate, and the BcNRT1 cRNA heterologously expressed in Xenopus laevis oocytes showed nitrate conductance when nitrate was included in the medium. Moreover, mutant chl1-5 plants harboring 35S::BcNRT1 showed sensitivity to chlorate treatment and exhibited restored nitrate uptake. In conclusion, the results indicate that BcNRT1 functions as a low affinity nitrate transporter in non-heading Chinese cabbage.