Identification of a cis-regulatory element that acts in companion cell-specific expression of AtMT2B promoter through the use of Brassica vasculature and gene-gun-mediated transient assay

Regulatory interaction of BcWRKY33A and BcHSFA4A promotes salt tolerance in non-heading Chinese cabbage [Brassica campestris (syn. Brassica rapa) ssp. chinensis]

Salinity is a universal environmental stress that causes yield reduction in plants. WRKY33, which has been extensively studied in plant defense against necrotrophic pathogens, has recently been found to be important in salt-responsive pathways. However, the underlying molecular mechanisms controlling the involvement of WRKY33 in salt tolerance have not been fully characterized. Here, we explored the function of BcWRKY33A in non-heading Chinese cabbage (NHCC). Under salt stress, BcWRKY33A expression is significantly induced in roots. As a nuclear protein, BcWRKY33A has strong transcriptional activation activity. Overexpression of BcWRKY33A confers salt tolerance in Arabidopsis, whereas silencing of BcWRKY33A causes salt sensitivity in NHCC. Furthermore, BcHSFA4A, a protein that interacts with BcWRKY33A, could directly bind to the HSE motif within the promoters of BcZAT12 and BcHSP17.6A, which are involved in the plant response to salt stress. Finally, we found that BcWRKY33A could enhance the transcriptional activity of BcHSFA4A and affect its downstream genes (e.g. BcZAT12 and BcHSP17.6A), and co-overexpression of BcWRKY33A and BcHSFA4A could promote the expression of salt-related genes, suggesting that the regulatory interaction between BcWRKY33A and BcHSFA4A improves salt tolerance in plants. Overall, our results provide insight into the molecular framework of the BcWRKY33A-BcHSFA4A signaling pathway, which also aids in our understanding of the molecular mechanism of salt tolerance in plants.

A comparative genomic and transcriptomic analysis at the level of isolated root hair cells reveals new conserved root hair regulatory elements

KEY MESSAGE

A comparative transcriptomic and genomic analysis between Arabidopsis thaliana and Glycine max root hair genes reveals the evolution of the expression of plant genes after speciation and whole genome duplication. Our understanding of the conservation and divergence of the expression patterns of genes between plant species is limited by the quality of the genomic and transcriptomic resources available. Specifically, the transcriptomes generated from plant organs are the reflection of the contribution of the different cell types composing the samples weighted by their relative abundances in the sample. These contributions can vary between plant species leading to the generation of datasets which are difficult to compare. To gain a deeper understanding of the evolution of gene transcription in and between plant species, we performed a comparative transcriptomic and genomic analysis at the level of one single plant cell type, the root hair cell, and between two model plants: Arabidopsis (Arabidopsis thaliana) and soybean (Glycine max). These two species, which diverged 90 million years ago, were selected as models based on the large amount of genomic and root hair transcriptomic information currently available. Our analysis revealed in detail the transcriptional divergence and conservation between soybean paralogs (i.e., the soybean genome is the product of two successive whole genome duplications) and between Arabidopsis and soybean orthologs in this single plant cell type. Taking advantage of this evolutionary study, we combined bioinformatics, molecular, cellular and microscopic tools to characterize plant promoter sequences and the discovery of two root hair regulatory elements (RHE1 and RHE2) consistently and specifically active in plant root hair cells.

A novel approach to dissect the abscission process in Arabidopsis

Abscission is the consequence of a specialized layer of cells undergoing a complex series of molecular and biochemical events. Analysis of the specific molecular changes associated with abscission is hampered by contamination from neighboring nonseparating tissues. Moreover, studies of abscission frequently involve the examination of events that take place in isolated segments of tissue exposed to nonphysiological concentrations of ethylene or indole-3-acetic acid for protracted periods (more than 24 h) of time. To resolve these problems, we have adopted the use of a transgenic line of Arabidopsis (Arabidopsis thaliana) where the promoter of an abscission-specific polygalacturonase gene (At2g41850/ARABIDOPSIS DEHISCENCE ZONE POLYGALACTURONASE2) has been fused to a green fluorescent protein reporter. RNA was extracted from green fluorescent protein-tagged cells, released from abscising floral organs, and used to generate a complementary DNA library. This library was used to probe a microarray, and a population of abscission-related transcripts was studied in detail. Seven novel abscission-related genes were identified, four of which encode proteins of unknown function. Reverse transcription-polymerase chain reaction analyses and promoter fusions to the β-glucuronidase reporter gene confirmed the expression of these genes in the abscission zone and revealed other places of expression during seedling development. Three of these genes were studied further by crossing reporter lines to the abscission mutants inflorescence deficient in abscission (ida) and blade-on-petiole1 (bop1)/bop2 and an IDA-overexpressing line. Phenotypic analysis of an At3g14380 transfer DNA insertion line indicates that this gene plays a functional role in floral organ shedding. This strategy has enabled us to uncover new genes involved in abscission, and their possible contribution to the process is discussed.

Multiple cis-regulatory elements are involved in the complex regulation of the sieve element-specific MtSEO-F1 promoter from Medicago truncatula

The sieve element occlusion (SEO) gene family includes several members that are expressed specifically in immature sieve elements (SEs) in the developing phloem of dicotyledonous plants. To determine how this restricted expression profile is achieved, we analysed the SE-specific Medicago truncatula SEO-F1 promoter (PMtSEO-F1) by constructing deletion, substitution and hybrid constructs and testing them in transgenic tobacco plants using green fluorescent protein as a reporter. This revealed four promoter regions, each containing cis-regulatory elements that activate transcription in SEs. One of these segments also contained sufficient information to suppress PMtSEO-F1 transcription in the phloem companion cells (CCs). Subsequent in silico analysis revealed several candidate cis-regulatory elements that PMtSEO-F1 shares with other SEO promoters. These putative sieve element boxes (PSE boxes) are promising candidates for cis-regulatory elements controlling the SE-specific expression of PMtSEO-F1.

The Arabidopsis CORI3 promoter contains two cis-acting regulatory regions required for transcriptional activity in companion cells

Companion cells are metabolically active and functionally specialized cells that behave as terminals for the transport of materials between phloem and the surrounding tissue. Although previous research has clarified the distinct function of companion cells, it is still largely unknown how plants establish and maintain the special identity of these cells. To shed further light on this issue, we carried out expressed sequence tag (EST) analysis. To minimize the difficulty of dissociating and gathering intact companion cells, vascular strings with an abundant content of companion cells were excised from the petioles of Brassica napus. By random sequencing with a string-specific cDNA library derived by suppression subtractive hybridization between the string itself and the petiole from which it had been removed, we identified 377 ESTs and assembled them into 247 EST groups. The most frequent EST was ExBdl-102 (15 of 377 ESTs), which showed the highest sequence similarity to the Arabidopsis CORI3 (CORONATINE INDUCED 3) gene. The CORI3 promoter:GUS showed predominant expression in the vascular tissue of Arabidopsis. Through transient expression assay using Brassica vasculature and gene-gun-mediated transient assay, we found two integrated cis-regulatory regions of the CORI3 promoter. This work has provided not only string-specific EST information and shown that two novel cis-regulatory regions sustain transcriptional activity in companion cells, but also a series of procedures for efficiently examining the transcriptional framework of companion cells by exploiting the histochemical advantage of B. napus as an experimental material.

Plant metallothioneins--metal chelators with ROS scavenging activity?

Metallothioneins (MTs) are ubiquitous cysteine-rich proteins present in plants, animals, fungi and cyanobacteria. In plants, MTs are suggested to be involved in metal tolerance or homeostasis, as they are able to bind metal ions through the thiol groups of their cysteine residues. Recent reports show that MTs are also involved in the scavenging of reactive oxygen species (ROS). The interplay between these roles is not entirely clear. Plants have many MT isoforms with overlapping expression patterns, and no specific role for any of them has been assigned. This review is focused on recent findings on plant MTs.