NFX1-LIKE2 (NFXL2) Suppresses Abscisic Acid Accumulation and Stomatal Closure in Arabidopsis thaliana

The NFX1-LIKE1 (NFXL1) and NFXL2 genes were identified as regulators of salt stress responses. The NFXL1 protein is a nuclear factor that positively affects adaptation to salt stress. The nfxl1-1 loss-of-function mutant displayed reduced survival rates under salt and high light stress. In contrast, the nfxl2-1 mutant, defective in the NFXL2 gene, and NFXL2-antisense plants exhibited enhanced survival under these conditions. We show here that the loss of NFXL2 function results in abscisic acid (ABA) overaccumulation, reduced stomatal conductance, and enhanced survival under drought stress. The nfxl2-1 mutant displayed reduced stomatal aperture under all conditions tested. Fusicoccin treatment, exposition to increasing light intensities, and supply of decreasing CO2 concentrations demonstrated full opening capacity of nfxl2-1 stomata. Reduced stomatal opening presumably is a consequence of elevated ABA levels. Furthermore, seedling growth, root growth, and stomatal closure were hypersensitive to exogenous ABA. The enhanced ABA responses may contribute to the improved drought stress resistance of the mutant. Three NFXL2 splice variants were cloned and named NFXL2-78, NFXL2-97, and NFXL2-100 according to the molecular weight of the putative proteins. Translational fusions to the green fluorescent protein suggest nuclear localisation of the NFXL2 proteins. Stable expression of the NFXL2-78 splice variant in nfxl2-1 plants largely complemented the mutant phenotype. Our data show that NFXL2 controls ABA levels and suppresses ABA responses. NFXL2 may prevent unnecessary and costly stress adaptation under favourable conditions

The extracellular EXO protein mediates cell expansion in Arabidopsis leaves

<p>Abstract</p> <p>Background</p> <p>The <it>EXO </it>(<it>EXORDIUM</it>) gene was identified as a potential mediator of brassinosteroid (BR)-promoted growth. It is part of a gene family with eight members in Arabidopsis. <it>EXO </it>gene expression is under control of BR, and <it>EXO </it>overexpression promotes shoot and root growth. In this study, the consequences of loss of <it>EXO </it>function are described.</p> <p>Results</p> <p>The <it>exo </it>loss of function mutant showed diminished leaf and root growth and reduced biomass production. Light and scanning electron microscopy analyses revealed that impaired leaf growth is due to reduced cell expansion. Epidermis, palisade, and spongy parenchyma cells were smaller in comparison to the wild-type. The <it>exo </it>mutant showed reduced brassinolide-induced cotyledon and hypocotyl growth. In contrast, <it>exo </it>roots were significantly more sensitive to the inhibitory effect of synthetic brassinolide. Apart from reduced growth, <it>exo </it>did not show severe morphological abnormalities. Gene expression analyses of leaf material identified genes that showed robust EXO-dependent expression. Growth-related genes such as <it>WAK1</it>, <it>EXP5</it>, and <it>KCS1</it>, and genes involved in primary and secondary metabolism showed weaker expression in <it>exo </it>than in wild-type plants. However, the vast majority of BR-regulated genes were normally expressed in <it>exo</it>. HA- and GFP-tagged EXO proteins were targeted to the apoplast.</p> <p>Conclusion</p> <p>The <it>EXO </it>gene is essential for cell expansion in leaves. Gene expression patterns and growth assays suggest that EXO mediates BR-induced leaf growth. However, EXO does not control BR-levels or BR-sensitivity in the shoot. EXO presumably is involved in a signalling process which coordinates BR-responses with environmental or developmental signals. The hypersensitivity of <it>exo </it>roots to BR suggests that EXO plays a diverse role in the control of BR responses in the root.</p

The extracellular EXO protein mediates cell expansion in Arabidopsis leaves

<p>Abstract</p> <p>Background</p> <p>The <it>EXO </it>(<it>EXORDIUM</it>) gene was identified as a potential mediator of brassinosteroid (BR)-promoted growth. It is part of a gene family with eight members in Arabidopsis. <it>EXO </it>gene expression is under control of BR, and <it>EXO </it>overexpression promotes shoot and root growth. In this study, the consequences of loss of <it>EXO </it>function are described.</p> <p>Results</p> <p>The <it>exo </it>loss of function mutant showed diminished leaf and root growth and reduced biomass production. Light and scanning electron microscopy analyses revealed that impaired leaf growth is due to reduced cell expansion. Epidermis, palisade, and spongy parenchyma cells were smaller in comparison to the wild-type. The <it>exo </it>mutant showed reduced brassinolide-induced cotyledon and hypocotyl growth. In contrast, <it>exo </it>roots were significantly more sensitive to the inhibitory effect of synthetic brassinolide. Apart from reduced growth, <it>exo </it>did not show severe morphological abnormalities. Gene expression analyses of leaf material identified genes that showed robust EXO-dependent expression. Growth-related genes such as <it>WAK1</it>, <it>EXP5</it>, and <it>KCS1</it>, and genes involved in primary and secondary metabolism showed weaker expression in <it>exo </it>than in wild-type plants. However, the vast majority of BR-regulated genes were normally expressed in <it>exo</it>. HA- and GFP-tagged EXO proteins were targeted to the apoplast.</p> <p>Conclusion</p> <p>The <it>EXO </it>gene is essential for cell expansion in leaves. Gene expression patterns and growth assays suggest that EXO mediates BR-induced leaf growth. However, EXO does not control BR-levels or BR-sensitivity in the shoot. EXO presumably is involved in a signalling process which coordinates BR-responses with environmental or developmental signals. The hypersensitivity of <it>exo </it>roots to BR suggests that EXO plays a diverse role in the control of BR responses in the root.</p

Identification of brassinosteroid-related genes by means of transcript co-response analyses

The comprehensive systems-biology database (CSB.DB) was used to reveal brassinosteroid (BR)-related genes from expression profiles based on co-response analyses. Genes exhibiting simultaneous changes in transcript levels are candidates of common transcriptional regulation. Combining numerous different experiments in data matrices allows ruling out outliers and conditional changes of transcript levels. CSB.DB was queried for transcriptional co-responses with the BR-signalling components BRI1 and BAK1: 301 out of 9694 genes represented in the nasc0271 database showed co-responses with both genes. As expected, these genes comprised pathway-involved genes (e.g. 72 BR-induced genes), because the BRI1 and BAK1 proteins are required for BR-responses. But transcript co-response takes the analysis a step further compared with direct approaches because BR-related non BR-responsive genes were identified. Insights into networks and the functional context of genes are provided, because factors determining expression patterns are reflected in correlations. Our findings demonstrate that transcript co-response analysis presents a valuable resource to uncover common regulatory patterns of genes. Different data matrices in CSB.DB allow examination of specific biological questions. All matrices are publicly available through CSB.DB. This work presents one possible roadmap to use the CSB.DB resources

Expression pattern and putative function of EXL1 and homologous genes in Arabidopsis

The Arabidopsis EXORDIUM-LIKE1 (EXL1) gene (At1g35140) is required for adaptation to carbon (C)- and energy-limiting growth conditions. An exl1 loss of function mutant showed diminished biomass production in a low total irradiance growth regime, impaired survival during extended night, and impaired survival of anoxia stress. We show here additional expression data and discuss the putative roles of EXL1. We hypothesize that EXL1 suppresses brassinosteroid-dependent growth and controls C allocation in the cell. In-depth expression analysis of homologous genes suggests that the EXL2 (At5g64260) and EXL4 (At5g09440) genes play similar roles

EXORDIUM-LIKE1 Promotes Growth during Low Carbon Availability in Arabidopsis1[C][W]

Little is known about genes that control growth and development under low carbon (C) availability. The Arabidopsis (Arabidopsis thaliana) EXORDIUM-LIKE1 (EXL1) gene (At1g35140) was identified as a brassinosteroid-regulated gene in a previous study. We show here that the EXL1 protein is required for adaptation to C- and energy-limiting growth conditions. In-depth analysis of EXL1 transcript levels under various environmental conditions indicated that EXL1 expression is controlled by the C and energy status. Sugar starvation, extended night, and anoxia stress induced EXL1 gene expression. The C status also determined EXL1 protein levels. These results suggested that EXL1 is involved in the C-starvation response. Phenotypic changes of an exl1 loss-of-function mutant became evident only under corresponding experimental conditions. The mutant showed diminished biomass production in a short-day/low-light growth regime, impaired survival during extended night, and impaired survival of anoxia stress. Basic metabolic processes and signaling pathways are presumed to be barely impaired in exl1, because the mutant showed wild-type levels of major sugars, and transcript levels of only a few genes such as QUA-QUINE STARCH were altered. Our data suggest that EXL1 is part of a regulatory pathway that controls growth and development when C and energy supply is poor