Christophe Reuzeau

Root-specific expression of OsNAC10 improves drought tolerance and grain yield in rice under field drought conditions.

Drought poses a serious threat to the sustainability of rice (Oryza sativa) yields in rain-fed agriculture. Here, we report the results of a functional genomics approach that identified a rice NAC (an acronym for NAM [No Apical Meristem], ATAF1-2, and CUC2 [Cup-Shaped Cotyledon]) domain gene, OsNAC10, which improved performance of transgenic rice plants under field drought conditions. Of the 140 OsNAC genes predicted in rice, 18 were identified to be induced by stress conditions. Phylogenic analysis of the 18 OsNAC genes revealed the presence of three subgroups with distinct signature motifs. A group of OsNAC genes were prescreened for enhanced stress tolerance when overexpressed in rice. OsNAC10, one of the effective members selected from prescreening, is expressed predominantly in roots and panicles and induced by drought, high salinity, and abscisic acid. Overexpression of OsNAC10 in rice under the control of the constitutive promoter GOS2 and the root-specific promoter RCc3 increased the plant tolerance to drought, high salinity, and low temperature at the vegetative stage. More importantly, the RCc3:OsNAC10 plants showed significantly enhanced drought tolerance at the reproductive stage, increasing grain yield by 25% to 42% and by 5% to 14% over controls in the field under drought and normal conditions, respectively. Grain yield of GOS2:OsNAC10 plants in the field, in contrast, remained similar to that of controls under both normal and drought conditions. These differences in performance under field drought conditions reflect the differences in expression of OsNAC10-dependent target genes in roots as well as in leaves of the two transgenic plants, as revealed by microarray analyses. Root diameter of the RCc3:OsNAC10 plants was thicker by 1.25-fold than that of the GOS2:OsNAC10 and nontransgenic plants due to the enlarged stele, cortex, and epidermis. Overall, our results demonstrated that root-specific overexpression of OsNAC10 enlarges roots, enhancing drought tolerance of transgenic plants, which increases grain yield significantly under field drought conditions.

Boosting Crop Yields with Plant Steroids

Plant sterols and steroid hormones brassinosteroids (BRs) are essential for plant growth, reproduction, and plant response to various abiotic and biotic stresses. In this review, we discuss the potential and strategies for enhancing yield in crops by genetically altering sterol and BR metabolism and signaling or by exogenously applying BRs and known inhibitors of their pathways. Plant sterols and steroid hormones, the brassinosteroids (BRs), are compounds that exert a wide range of biological activities. They are essential for plant growth, reproduction, and responses to various abiotic and biotic stresses. Given the importance of sterols and BRs in these processes, engineering their biosynthetic and signaling pathways offers exciting potentials for enhancing crop yield. In this review, we focus on how alterations in components of sterol and BR metabolism and signaling or application of exogenous steroids and steroid inhibitors affect traits of agronomic importance. We also discuss areas for future research and identify the fine-tuning modulation of endogenous BR content as a promising strategy for crop improvement.

The overexpression of OsNAC9 alters the root architecture of rice plants enhancing drought resistance and grain yield under field conditions

Drought conditions limit agricultural production by preventing crops from reaching their genetically predetermined maximum yields. Here, we present the results of field evaluations of rice overexpressing OsNAC9, a member of the rice NAC domain family. Root-specific (RCc3) and constitutive (GOS2) promoters were used to overexpress OsNAC9 and produced the transgenic RCc3:OsNAC9 and GOS2:OsNAC9 plants. Field evaluations over two cultivating seasons showed that grain yields of the RCc3:OsNAC9 and the GOS2:OsNAC9 plants were increased by 13%-18% and 13%-32% under normal conditions, respectively. Under drought conditions, RCc3:OsNAC9 plants showed an increased grain yield of 28%-72%, whilst the GOS2:OsNAC9 plants remained unchanged. Both transgenic lines exhibited altered root architecture involving an enlarged stele and aerenchyma. The aerenchyma of RCc3:OsNAC9 roots was enlarged to a greater extent than those of GOS2:OsNAC9 and non-transgenic (NT) roots, suggesting the importance of this phenotype for enhanced drought resistance. Microarray experiments identified 40 up-regulated genes by more than threefold (P < 0.01) in the roots of both transgenic lines. These included 9-cis-epoxycarotenoid dioxygenase, an ABA biosynthesis gene, calcium-transporting ATPase, a component of the Ca(2+) signalling pathway involved in cortical cell death and aerenchyma formation, cinnamoyl CoA reductase 1, a gene involved in lignin biosynthesis, and wall-associated kinases¸ genes involved in cell elongation and morphogenesis. Interestingly, O-methyltransferase, a gene necessary for barrier formation, was specifically up-regulated only in the RCc3:OsNAC9 roots. Such up-regulated genes that are commonly and specifically up-regulated in OsNAC9 transgenic roots may account for the altered root architecture conferring increased drought resistance phenotype.

From Squalene to Brassinolide: The Steroid Metabolic and Signaling Pathways across the Plant Kingdom

The plant steroid hormones, brassinosteroids (BRs), and their precursors, phytosterols, play major roles in plant growth, development, and stress tolerance. Here, we review the impressive progress made during recent years in elucidating the components of the sterol and BR metabolic and signaling pathways, and in understanding their mechanism of action in both model plants and crops, such as Arabidopsis and rice. We also discuss emerging insights into the regulations of these pathways, their interactions with other hormonal pathways and multiple environmental signals, and the putative nature of sterols as signaling molecules.

The Arabidopsis MCM2 gene is essential to embryo development and its over‐expression alters root meristem function

* Minichromosome maintenance (MCM) proteins are subunits of the pre-replication complex that probably function as DNA helicases during the S phase of the cell cycle. Here, we investigated the function of AtMCM2 in Arabidopsis. * To gain an insight into the function of AtMCM2, we combined loss- and gain-of-function approaches. To this end, we analysed two null alleles of AtMCM2, and generated transgenic plants expressing AtMCM2 downstream of the constitutive 35S promoter. * Disruption of AtMCM2 is lethal at a very early stage of embryogenesis, whereas its over-expression results in reduced growth and inhibition of endoreduplication. In addition, over-expression of AtMCM2 induces the formation of additional initials in the columella root cap. In the plt1,2 mutant, defective for root apical meristem maintenance, over-expression of AtMCM2 induces lateral root initiation close to the root tip, a phenotype not reported in the wild-type or in plt1,2 mutants, even when cell cycle regulators, such as AtCYCD3;1, were over-expressed. * Taken together, our results provide evidence for the involvement of AtMCM2 in DNA replication, and suggest that it plays a crucial role in root meristem function.

Traitmill™: a functional genomics platform for the phenotypic analysis of cereals

The improvement of quality and quantitative traits in industrial crops is among the most important goals in plant breeding. Many traits of interest are controlled by multiple genes and improvements have so far only been obtained through conventional breeding. The use of biotechnological tools to modify quantitative traits is highly challenging. CropDesign has developed TraitMill™, an automated plant evaluation platform allowing high-throughput testing of the effect of plant-based transgenes on agronomically valuable traits in crop plants. The focus of the platform is currently on rice, a good model for other important cereals such as maize and wheat. TraitMill™ offers a high-throughput prediction of gene function. Genes of validated function that confer trait improvement can then be transferred to other cereal crop species such as maize, but also to dicots, trees and ornamentals. TraitMill™ involves the following key components: (i) selection of candidate trait improvement genes among genes involved in signal transduction, cell cycle control, transcription, nutrient metabolism, etc.; (ii) a suite of validated constitutive or tissue-specific promoters from rice allowing for the selection of the most appropriate promoter–gene combination in view of the desired trait improvement; (iii) an industrialized plant transformation system generating tens of thousands of transgenic plants annually; and (iv) a robotized trait evaluation set-up for plant evaluation, proprietary image analysis software for measuring plant performance parameters and statistical analysis of results.

Complementary and dose-dependent action of AtCCS52A isoforms in endoreduplication and plant size control

· The dimension of organs depends on the number and the size of their component cells. Formation of polyploid cells by endoreduplication cycles is predominantly associated with increases in the cell size and implicated in organ growth. In plants, the CCS52A proteins play a major role in the switch from mitotic to endoreduplication cycles controlling thus the number of mitotic cells and the endoreduplication events in the differentiating cells. · Arabidopsis has two CCS52A isoforms; AtCCS52A1 and AtCCS52A2. Here we focused on their roles in endoreduplication and cell size control during plant development. We demonstrate their complementary and dose-dependent actions that are dependent on their expression patterns. Moreover, the impact of CCS52A overexpression on organ size in transgenic plants was dependent on the expression level; while enhanced expression of the CCS52A genes positively correlated with the ploidy levels, organ sizes were negatively affected by strong overexpression whereas milder overexpression resulted in a significant increase in the organ sizes. · Taken together, these finding support both complementary and dose-dependent actions for the Arabidopsis CCS52A isoforms in plant development and demonstrate that elevated ectopic CCS52A expression positively correlates with organ size, opening a route to higher biomass production.

Evolutionary trails of plant steroid genes

Plant steroids - brassinosteroids (BRs) and their precursors, phytosterols - play a major role in plant growth, development, stress tolerance, and have high potential for agricultural applications. Currently, this prospect is limited by a lack of information about their evolution and expression dynamics (spatial and temporal) across plant species. The increasing number of sequenced genomes offers an opportunity for evolutionary studies that might help to prioritize functional analyses with the aim to improve crop yield and stress tolerance. In this review we provide a glimpse of the origin, evolution, and functional conservation of phytosterol and BR genes in the green plant lineage using comparative sequence and expression analyses of publicly available datasets.

The Arabidopsis Locus RCB Mediates Upstream Regulation of Mitotic Gene Expression

Transcriptional regulation of cell cycle regulatory genes, such as B-type cyclins, is tightly linked with the mitotic activity in the meristems. To study the regulation of a B-type cyclin gene, a targeted genetic approach was undertaken. An Arabidopsis line containing a fusion construct between the CYCB1;1 promoter and a bacterial β-glucuronidase marker gene (uidA) was used in ethyl methanesulfonate mutagenesis. The mutants were screened for altered CYCB1;1::uidA expression patterns. In a reduced CYCB1;1 expression mutant (rcb), the CYCB1;1::uidA expression was severely affected, being excluded from the shoot and root apical meristems and leaf primordia and shifted to cells associated with root cap and stomata. In addition to the overall reduction of the endogenous CYCB1;1 transcript levels, other G2-to-M phase-specific genes were also down-regulated by the mutation. In the mutant plants, the inflorescence stem growth was reduced, indicating low meristem activity. Based on the altered CYCB1;1::uidA expression patterns in rcb root meristem, a model is proposed for RCB that mediates the tissue specificity of CYCB1;1 promoter activity.