Evidence that exogenous urea acts as a potent cue to alleviate ammonium-inhibition of root system growth of cotton plant (Gossypium hirsutum).

Many plants grown with low-millimolar concentration of NH4 + as a sole nitrogen source develop NH4 + -toxicity symptoms. To date, crucial molecular identities and a practical approach involved in the improvement of plant NH4 + -tolerance remain largely unknown. By phenotyping of upland cotton grown on varied nitrogen forms, we came across a phenomenon that caused sub-millimolar concentrations of urea (e.g., up 50 µM) to repress the growth inhibition of roots and whole plant cultivated in a NH4 + -containing nutrient solution. A growth-recovery assay revealed that the relief in NH4 + -inhibited growth required only a short-term exposure (≧12 h) of the roots to urea, implying that urea could elicit an internal signaling and be involved in antagonizing NH4 + -sensitivity. Intriguingly, split-root experiments demonstrated that low urea occurrence in one root-half could efficaciously stimulate not only supplied root but also the root-half grown in NH4 + -solution without urea, indicating the existence of urea-triggered local and systemic long-distance signaling. In the split-root experiment we also observed high arginase activity, strong arginine reduction and remarkable upregulation of polyamine biosynthesis-related genes (ADC1/2, SPDS and SPMS). Therefore, we suggest that external urea might serve as an effective cue (signal molecule) in an arginine-/polyamine-related process for ameliorating NH4 + -suppressed root growth, providing a novel aspect for deeper exploring and understanding plant NH4 + -tolerance.

Molecular identification of a root apical cell-specific and stress-responsive enhancer from an Arabidopsis enhancer trap line.

BACKGROUND: Plant root apex is the major part to direct the root growth and development by responding to various signals/cues from internal and soil environments. To study and understand root system biology particularly at a molecular and cellular level, an Arabidopsis T-DNA insertional enhancer trap line J3411 expressing reporters (GFP) only in the root tip was adopted in this study to isolate a DNA fragment. RESULTS: Using nested PCR, DNA sequencing and sequence homology search, the T-DNA insertion site(s) and its flanking genes were characterised in J3411 line. Subsequently, a 2000 bp plant DNA-fragment (Ertip1) upstream of the insert position of the coding T-DNA was in silico analysed, revealing certain putative promoter/enhancer cis-regulatory elements. Cloning and transformation of this DNA fragment and its truncated segments tagged with or without 35S minimal promoter (35Smini), all of which were fused with a GFP or GUS reporter, allowed to detect GFP and GUS expression mediated only by Ertip1 + 35mini (PErtip1+35Smini) specifically in the Arabidopsis root tip region. The PErtip1+35Smini activity was further tested to be strong and stable under many different growth conditions but suppressed by cold, salt, alkaline pH and higher ammonium and phosphorus. CONCLUSION: This work describes a promising strategy to isolate a tissue-/cell-specific enhancer sequence from the enhancer trap lines, which are publically available. The reported synthetic promoter i.e. PErtip1+35Smini may provide a valuable and potent molecular-tool for comprehensive investigation of a gene function related to root growth and development as well as molecular engineering of root-architectural formation aiming to improve plant growth.

[Effects of brassinolide on leaf physiological characteristics and differential gene expression profiles of NaCl-stressed cotton].

This study analyzed the effects of brassinolide (BL) on Na⁺ accumulation, leaf physiological characteristics and differentially expressed genes (DEGs) of cotton leaves under NaCl stress. The results showed that NaCl stress increased the Na⁺, proline and MDA content in the leaves of Sumian 12 and Sumian 22, and changed the expression level of genes in cotton leaves. The application of BL counteracted the NaCl stress-induced growth inhibition in the two tested cotton cultivars. It reduced the accumulation of Na⁺, enhanced proline content, and resulted in a decrease in the MDA content of NaCl-stressed leaves, and the influence of BL on salt-stressed Sumian 12 plants was more pronounced than that on Sumian 22. The digital gene expression analysis in Sumian 12 indicated that BL application significantly influenced the gene expression in NaCl-stressed cotton leaves, the gene expression pattern as a result of the root applied BL on NaCl-stressed cotton treatment (BL+NaCl) was similar to the normal cotton plants (CK). Our results indicated that brassinolide alleviated NaCl stress on cotton through improving leaf physiological characteristics and gene expression, and resulted in an increase in biomass of NaCl-stressed cotton.

Rice DUR3 mediates high-affinity urea transport and plays an effective role in improvement of urea acquisition and utilization when expressed in Arabidopsis.

• Despite the great agricultural and ecological importance of efficient use of urea-containing nitrogen fertilizers by crops, molecular and physiological identities of urea transport in higher plants have been investigated only in Arabidopsis. • We performed short-time urea-influx assays which have identified a low-affinity and high-affinity (K(m) of 7.55 µM) transport system for urea-uptake by rice roots (Oryza sativa). • A high-affinity urea transporter OsDUR3 from rice was functionally characterized here for the first time among crops. OsDUR3 encodes an integral membrane-protein with 721 amino acid residues and 15 predicted transmembrane domains. Heterologous expression demonstrated that OsDUR3 restored yeast dur3-mutant growth on urea and facilitated urea import with a K(m) of c. 10 µM in Xenopus oocytes. • Quantitative reverse-transcription polymerase chain reaction (qPCR) analysis revealed upregulation of OsDUR3 in rice roots under nitrogen-deficiency and urea-resupply after nitrogen-starvation. Importantly, overexpression of OsDUR3 complemented the Arabidopsis atdur3-1 mutant, improving growth on low urea and increasing root urea-uptake markedly. Together with its plasma membrane localization detected by green fluorescent protein (GFP)-tagging and with findings that disruption of OsDUR3 by T-DNA reduces rice growth on urea and urea uptake, we suggest that OsDUR3 is an active urea transporter that plays a significant role in effective urea acquisition and utilisation in rice.