<Emphasis Type="Italic">ZmFKBP20-1</Emphasis> improves the drought and salt tolerance of transformed Arabidopsis

FK506-binding proteins (FKBPs), which belong to the peptidyl-prolyl cis/trans isomerase superfamily, are involved in plant response to abiotic stresses. A number of FKBP family genes have been isolated in plants, but little has been reported of FKBP genes in maize. In this study, a drought-induced FKBP gene, ZmFKBP20-1, was isolated from maize and was characterized for its role in stress responses using gene expression, protein subcellular localization, transformation in Arabidopsis, expression patterns of the stress-responsive genes, and physiological parameter analysis. During drought and salt stresses, ZmFKBP20-1 transgenic Arabidopsis plants exhibited enhanced tolerance, which was concomitant with the altered expression of stress/ABA-responsive genes, such as COR15a, COR47, ERD10, RD22, KIN1, ABI1, and ABI2. The resistance characteristics of ZmFKBP20-1 overexpression were associated with a significant increase in survival rate. These results suggested that ZmFKBP20-1 plays a positive role in drought and salt stress responses in Arabidopsis and provided new insights into the mechanisms of FKBP in response to abiotic stresses in plants.     

A Leucine-Rich Repeat Receptor-like Kinase from the Antarctic Moss <Emphasis Type="Italic">Pohlia nutans</Emphasis> Confers Salinity and ABA Stress Tolerance

Plant leucine-rich repeats receptor-like kinases (LRR-RLKs) play key roles in plant growth, development, and responses to environmental stresses. However, the functions of LRR-RLKs in bryophytes are still not well documented. Here, a putative LRR-RLK gene, PnLRR-RLK, was cloned and characterized from the Antarctic moss Pohlia nutans. Phylogenetic analysis revealed that PnLRR-RLK protein was clustered with the Arabidopsis thaliana LRR XI family proteins. Subcellular localization analysis of PnLRR-RLK revealed that it was mainly localized on plasma membrane. The expression of PnLRR-RLK was induced by mock high salinity, cold, drought, and exogenously supplied abscisic acid (ABA) and methyl jasmonate (MeJA). Meanwhile, the overexpression of PnLRR-RLK showed an increased tolerance of transgenic Arabidopsis to salt and ABA stresses than that of the wild type (WT) plants. Furthermore, the expression levels of several salt tolerance genes (AtHKT1, AtSOS3, AtP5CS1, and AtADH1) and an ABA negatively regulating gene AtABI1 were significantly increased in transgenic plants. Meanwhile, the expression levels of ABA biosynthesis genes (AtNCED3, AtABA1, and AtAAO3) and ABA early response genes (AtMYB2, AtRD22, AtRD29A, and AtDREB2A) were decreased in transgenic Arabidopsis after salt stress treatment. Therefore, these results suggested that PnLRR-RLK might involve in regulating salt stress-related and ABA-dependent signaling pathway, thereby contribute to the salinity tolerance of the Antarctic moss P. nutans.     

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.     

Functional characterization of chloroplast-targeted RbgA GTPase in higher plants

Key message

Plant RbgA GTPase is targeted to chloroplasts and co-fractionated with chloroplast ribosomes, and plays a role in chloroplast rRNA processing and/or ribosome biogenesis.

Abstract

Ribosome Biogenesis GTPase A (RbgA) homologs are evolutionarily conserved GTPases that are widely distributed in both prokaryotes and eukaryotes. In this study, we investigated functions of chloroplast-targeted RbgA. Nicotiana benthamiana RbgA (NbRbgA) and Arabidopsis thaliana RbgA (AtRbgA) contained a conserved GTP-binding domain and a plant-specific C-terminal domain. NbRbgA and AtRbgA were mainly localized in chloroplasts, and possessed GTPase activity. Since Arabidopsis rbgA null mutants exhibited an embryonic lethal phenotype, virus-induced gene silencing (VIGS) of NbRbgA was performed in N. benthamiana. NbRbgA VIGS resulted in a leaf-yellowing phenotype caused by disrupted chloroplast development. NbRbgA was mainly co-fractionated with 50S/70S ribosomes and interacted with the chloroplast ribosomal proteins cpRPL6 and cpRPL35. NbRbgA deficiency lowered the levels of mature 23S and 16S rRNAs in chloroplasts and caused processing defects. Sucrose density gradient sedimentation revealed that NbRbgA-deficient chloroplasts contained reduced levels of mature 23S and 16S rRNAs and diverse plastid-encoded mRNAs in the polysomal fractions, suggesting decreased protein translation activity in the chloroplasts. Interestingly, NbRbgA protein was highly unstable under high light stress, suggesting its possible involvement in the control of chloroplast ribosome biogenesis under environmental stresses. Collectively, these results suggest a role for RbgA GTPase in chloroplast rRNA processing/ribosome biogenesis, affecting chloroplast protein translation in higher plants.

     

Cryptochrome 1b from Sweet Sorghum Regulates Photoperiodic Flowering,Photomorphogenesis, and ABA Response in Transgenic <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis>

Cryptochromes are blue/UV-A light receptors that mediate various aspects of plant growth and development. Here, we report the function and signal mechanism of cryptochrome 1b (SbCRY1b) from sweet sorghum [Sorghum bicolor (L.) Moench], a typical short-day cereal plant, to explore its potential for genetic improvement of sweet sorghum varieties. SbCRY1b mRNA enrichment showed almost 24-h diurnal rhythms in both short-day (SD) and long-day (LD) conditions. Overexpression of SbCRY1b rescued the late-flowering and the long hypocotyl phenotypes of cry1cry2 double mutant in the transgenic Arabidopsis. SbCRY1b mediated Arabidopsis FT mRNA expression in LD and HY5 protein accumulation in response to blue light. SbCRY1b protein was located in both the nucleus and cytoplasm and was degraded by 26S proteasomes in response to blue light. SbCRY1b interacted, respectively, with Arabidopsis suppressor of PHYA-1051 (AtSPA1), E3 ubiquitin ligase constitutive photomorphogenesis 1 (AtCOP1), and a putative COP1 from sweet sorghum (SbCOP1) instead of SbSPA1 in vitro in a blue light-dependent manner. The observations imply SbCRY1b functions as a major regulator of photoperiodic flowering and its function is more similar to that of Arabidopsis CRY2. Moreover, SbCRY1b-overexpressed transgenic Arabidopsis showed oversensitivity to abscisic acid (ABA) during seed germination and root development. The expression of abscisic acid-insensitive 4 (ABI4), ABI5, abscisic acid responsive element-binding 1 (ABF1), (sucrose non-fermenting 1)-related protein kinase (SnRK2.3), RD29A, and EM6 was upregulated in the transgenic Arabidopsis. The results demonstrated that SbCRY1b may integrate blue light and ABA signals to regulate plant development.     

Overexpression of wheat <Emphasis Type="Italic">TaNCED</Emphasis> gene in <Emphasis Type="Italic">Arabidopsis</Emphasis> enhances tolerance to drought stress and delays seed germination

Abscisic acid (ABA) regulates various plant physiological processes, especially participates in the plant responses to harsh environments. The 9-cis-epoxycarotenoid dioxygenase (NCED) is a key enzyme in ABA biosynthesis pathway. Here, a TaNCED with an 1 887-bp open reading frame was cloned from wheat, which encodes a peptide of 628 amino acids. A chloroplast transit peptide sequence was found at the N-terminus of the TaNCED protein. Multiple sequence alignments indicate that the TaNCED protein shared high similarities with other NCEDs from different species. Real-time quantitative PCR analysis shows that expression of TaNCED was strongly up-regulated by treatments with ABA, polyethylene glycol, and drought stress, and it was down-regulated during germination of the wheat seeds. Ectopic overexpression of the TaNCED gene in Arabidopsis resulted in an increase of endogenous ABA and free proline content. A lower water loss rate and stomatal conductance of leaves were found in the transgenic plants in comparison with the wild type. Subsequently, the transgenic plants displayed an enhanced tolerance to drought stress but delayed seed germination. These data provide evidence that the TaNCED might play a primary role in regulation of ABA content during water stress and seed dormancy.     

Chloroplast ribonucleoprotein-like proteins of the moss <Emphasis Type="Italic">Physcomitrella patens</Emphasis> are not involved in RNA stability and RNA editing

Many RNA recognition motif (RRM)-containing proteins are known to exist in chloroplasts. Major members of the RRM protein family, which are chloroplast ribonucleoproteins (cpRNPs), have been investigated in seed plants, including tobacco and Arabidopsis thaliana, but never in early land plants, such as bryophytes. In this study, we surveyed RRM proteins encoded in the moss Physcomitrella patens genome and predicted 25 putative chloroplast RRM proteins. Among them, two RRM-containing proteins, PpRBP2a and PpRBP2b, resembled cpRNPs and were thus referred to as cpRNP-like proteins. However, knockout mutants of either one or two PpRBP2 genes exhibited a wild type-like phenotype. Unlike Arabidopsis cpRNPs, the levels of mRNA accumulation in chloroplasts were not affected in the PpRBP2 knockout mutants. In addition, the efficiency of RNA editing was also not altered in the mutants. This suggests that PpRBP2a and 2b may be functionally distinct from Arabidopsis cpRNPs.     

UV-B stress-induced ABA production in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> mutants defective in ethylene signal transduction pathway

In this study, we examined the influence of UV-B radiation (280–320 nm) on ABA accumulation in 14-day-old Arabidopsis thaliana (L.) Heynh plants of wild type (WT), ethylene receptor mutant (etr1-1), and mutant with a constitutively active ethylene signal transduction pathway (ctr1-1). ABA content in nonirradiated WT plants was twice higher than in each mutant. UV-B irradiation caused dose-dependent ABA accumulation in WT plants. In the etr1-1 mutant, the amount of accumulated ABA was significantly less. In the ctr1-1 mutant, ABA content didn’t increase after UV-B irradiation. These data suggest that start of stress-induced ABA formation requires the adjustable ethylene signal pathway. In the ctr1-1 mutant, a constitutively active (nonadjustable) ethylene signal pathway blocks stress-induced ABA accumulation.     

Chloroplast ultrastructure in leaves of tobacco plants with the introduced gene for the acyl-lipid Δ9-desaturase from <Emphasis Type="Italic">Synechococcus vulcanus</Emphasis> at normal and low temperature

Ultrastructural changes in chloroplasts of tobacco plants (Nicotiana tabacum L.) with the introduced desC gene for the acyl-lipid Δ9-desaturase from the thermophilic cyanobacterium Synechococcus vulcanus were investigated during plant acclimation to cold. Control plants were transformed with an empty pGA482 binary vector. At optimum growth temperature, a decreased number of grana and thylakoids and an increased number of plastoglobules and their larger area were observed in transgenic plants when compared to control ones. In control plants, acclimation to cold (6 days at 10°C) resulted in the larger areas of chloroplasts and grana. These changes indicated starting cold-induced injuries manifested in swelling of the stroma and a slight decrease in the total number of thylakoids in the chloroplast. In contrast, transgenic plants responded to cold by reducing the chloroplast, granal, and plastoglobule areas. Meantime, the number of thylakoids per granum increased noticeably. The total number of thylakoids in the chloroplast increased from 123 to 203. It was concluded that expression of the acyl-lipid Δ9-desaturase gene in tobacco plants provided for the formation of the cell ultrastructure similar to one characteristic of cold-tolerant plants.     

A Novel Little Membrane Protein Confers Salt Tolerance in Rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.)

Plasma membrane proteins play critical roles in sensing and responding abiotic and biotic stresses in plants. In the present study, we characterized a previously unknown gene stress associated little protein 1 (SALP1) encoding a plasma membrane protein. SALP1, a small and plant-specific membrane protein, contains only 74 amino acid residues. SALP1 was constitutively expressed in various rice tissues while highly expressed in roots, leaf blade, and immature panicles. Expression analysis indicated that SALP1 was induced by various abiotic stresses and abscisic acid (ABA). Subcellular localization assay indicated that SALP1 was localized on plasma membrane in rice protoplast cells. Overexpressing of SALP1 in rice improved salt tolerance through increasing free proline contents and the expression level of OsP5CS gene, and balancing ion contents under salt stress. Moreover, SALP1 transgenic rice showed reduced sensitivity to ABA treatment, and expression level of SALP1 is not altered by ABI5-like 1 protein. Conclusively, SALP1, a novel membrane protein, is involved in salt tolerance through an ABA-independent signaling pathway in rice.