Antagonistic Regulation of Arabidopsis Growth by Brassinosteroids and Abiotic Stresses

To withstand ever-changing environmental stresses, plants are equipped with phytohormone-mediated stress resistance mechanisms. Salt stress triggers abscisic acid (ABA) signaling, which enhances stress tolerance at the expense of growth. ABA is thought to inhibit the action of growth-promoting hormones, including brassinosteroids (BRs). However, the regulatory mechanisms that coordinate ABA and BR activity remain to be discovered. We noticed that ABA-treated seedlings exhibited small, round leaves and short roots, a phenotype that is characteristic of the BR signaling mutant, brassinosteroid insensitive1-9 (bri1-9). To identify genes that are antagonistically regulated by ABA and BRs, we examined published Arabidopsis microarray data sets. Of the list of genes identified, those upregulated by ABA but downregulated by BRs were enriched with a BRRE motif in their promoter sequences. After validating the microarray data using quantitative RT-PCR, we focused on RD26, which is induced by salt stress. Histochemical analysis of transgenic Arabidopsis plants expressing RD26pro:GUS revealed that the induction of GUS expression after NaCl treatment was suppressed by co-treatment with BRs, but enhanced by co-treatment with propiconazole, a BR biosynthetic inhibitor. Similarly, treatment with bikinin, an inhibitor of BIN2 kinase, not only inhibited RD26 expression, but also reduced the survival rate of the plant following exposure to salt stress. Our results suggest that ABA and BRs act antagonistically on their target genes at or after the BIN2 step in BR signaling pathways, and suggest a mechanism by which plants fine-tune their growth, particularly when stress responses and growth compete for resources.     

Role of PP2C-mediated ABA signaling in the moss Physcomitrella patens

Plant hormone abscisic acid (ABA) is found in a wide range of land plants, from mosses to angiosperms. However, our knowledge concerning the function of ABA is limited to some angiosperm plant species. We have shown that the basal land plant Physcomitrella patens and the model plant Arabidopsis thaliana share a conserved abscisic acid (ABA) signaling pathway mediated through ABI1-related type 2C protein phosphatases (PP2Cs). Ectopic expression of Arabidopsis abi1-1, a dominant allele of ABI1 that functions as a negative regulator of ABA signaling, or targeted disruption of Physcomitrella ABI1-related gene (PpABI1A) resulted in altered ABA sensitivity and abiotic stress tolerance of Physcomitrella, as demonstrated by osmostress and freezing stress. Moreover, transgenic Physcomitrella overexpressing abi1-1 showed altered morphogenesis. These trangenic plants had longer stem lengths compared to the wild type, and continuous growth of archegonia (female organ) with few sporophytes under non-stress conditions. Our results suggest that PP2C-mediated ABA signaling is involved in both the abiotic stress responses and developmental regulation of Physcomitrella.Key words: ABA, ABI1, Physcomitrella patens, PP2C, signaling     

Characterization of a maize ERF gene, <Emphasis Type="Italic">ZmERF1,</Emphasis> in hormone and stress responses

ERFs are downstream component in ethylene signaling pathway and involved in plant’s abiotic stress response. The specific role of ERFs under stress and the molecular mechanism underlying the signaling cross talk still need to be elucidated. This study describes the isolation and characterization of ZmERF1 promoter. There were many cis-regulatory elements related to stress responses in the ZmERF1 promoter sequence. ZmERF1 could be highly induced by ABA and ethylene treatment in maize, suggesting that it might be at the crossroads of multiple hormone signaling pathways. Furthermore, ZmERF1 transgenic Arabidopsis lines (35S::ZmERF1) showed higher salt-tolerant, drought- and heat resistance. Consistently, tolerance-related genes were up-regulated in 35S::ZmERF1 lines compared with the WT plants in Arabidopsis. Overall, ZmERF1 might play an important role in plant resistance to a coercive environment by mediating various physiological processes via ethylene and ABA signaling pathways.     

Regulatory function of AMP1 in ABA biosynthesis and drought resistance in arabidopsis

In this study we reported the isolation of a mutant in which the reporter pVP14-LUC was highly expressed in Arabidopsis. The gene expression of maize VP14 is closely correlated with the endogenous ABA levels, and the Arabidopsis homolog of VP14, AtNCED1, encoding an enzyme of ABA biosynthesis, was up-regulated, and high ABA level was detected in the mutant. Map-based cloning revealed that the mutated gene is a novel allele of the AMP1 (Altered Meristem Program 1) which encodes a glutamate carboxypeptidase that plays an important role in shoot apical meristem development and phytohormone homeostasis. We found that the mutant displayed obvious drought tolerance, being with more lateral roots, high seed germination under mannitol, increased ABA accumulation, and highly induced gene expression of RD29A. Using the approaches of artificial microRNA gene silencing in transgenic plants, three AMP1 down-regulated lines were obtained. The AMP1 down-regulated plants exhibited a low rate of water loss, decreased stomatal aperture, and enhanced drought tolerance. These results provide evidence demonstrating the regulatory function of AMP1 in plant drought tolerance and stress responsive gene expression.     

A novel protein kinase involved in Na+ exclusion revealed from positional cloning

Salinity is a major abiotic stress which affects crop plants around the world, resulting in substantial loss of yield and millions of dollars of lost revenue. High levels of Na⁺ in shoot tissue have many adverse effects and, crucially, yield in cereals is commonly inversely proportional to the extent of shoot Na⁺ accumulation. We therefore need to identify genes, resistant plant cultivars and cellular processes that are involved in salinity tolerance, with the goal of introducing these factors into commercially available crops. Through the use of an Arabidopsis thaliana mapping population, we have identified a highly significant quantitative trait locus (QTL) linked to Na⁺ exclusion. Fine mapping of this QTL identified a protein kinase (AtCIPK16), related to AtSOS2, that was significantly up‐regulated under salt stress. Greater Na⁺ exclusion was associated with significantly higher root expression of AtCIPK16, which is due to differences in the gene's promoter. Constitutive overexpression of the gene in Arabidopsis leads to plants with significant reduction in shoot Na⁺ and greater salinity tolerance. amiRNA knock‐downs of AtCIPK16 in Arabidopsis show a negative correlation between the expression levels of the gene and the amount of shoot Na⁺. Transgenic barley lines overexpressing AtCIPK16 show increased salinity tolerance.     

Function of the wheat TaSIP gene in enhancing drought and salt tolerance in transgenic Arabidopsis and rice

Microarray analysis of a salt-tolerant wheat mutant identified a gene of unknown function that was induced by exposure to high levels of salt and subsequently denoted TaSIP (Triticum aestivum salt-induced protein). Quantitative PCR analysis revealed that TaSIP expression was induced not only by salt, but also by drought, abscisic acid (ABA), and other environmental stress factors. Transgenic rice plants that expressed an RNA interference construct specific for a rice gene homologous to TaSIP was more susceptible to salt stress than wild-type rice plants. Subcellular localization studies showed that the TaSIP localized to the cell membrane. Under conditions of salt and drought stress, transgenic Arabidopsis plants that overexpressed TaSIP showed superior physiological properties compared with control plants, including lower Na⁺ content and upregulation of several stress resistance genes. Staining of transgenic tissues with β-glucuronidase (GUS) failed to indicate tissue-specific activity of the full-length TaSIP promoter. Quantitative analysis of GUS fluorescence in transgenic plants treated with ABA or salt stress revealed that the region 1,176–1,410 bp from the start codon contained an ABA-responsive element and that the region 579–1,176 bp from the start codon upstream of the exon contained a salt-stress-responsive element. Based on these results, we conclude that the key part of the TaSIP gene is the region of its promoter involved in salt tolerance.