An isoform of the plastid RNA polymerase-associated protein FSD3 negatively regulates chloroplast development.

BACKGROUND: Plastid-encoded RNA polymerase (PEP) plays an essential role in chloroplast development by governing the expression of genes involved in photosynthesis. At least 12 PEP-associated proteins (PAPs), including FSD3/PAP4, regulate PEP activity and chloroplast development by modulating formation of the PEP complex. RESULTS: In this study, we identified FSD3S, a splicing variant of FSD3; the FSD3 and FSD3S transcripts encode proteins with identical N-termini, but different C-termini. Characterization of FSD3 and FSD3S proteins showed that the C-terminal region of FSD3S contains a transmembrane domain, which promotes FSD3S localization to the chloroplast membrane but not to nucleoids, in contrast to FSD3, which localizes to the chloroplast nucleoid. We also found that overexpression of FSD3S negatively affects photosynthetic activity and chloroplast development by reducing expression of genes involved in photosynthesis. In addition, FSD3S failed to complement the chloroplast developmental defects in the fsd3 mutant. CONCLUSION: These results suggest FSD3 and FSD3S, with their distinct localization patterns, have different functions in chloroplast development, and FSD3S negatively regulates expression of PEP-dependent chloroplast genes, and development of chloroplasts.

pTAC10, a Key Subunit of Plastid-Encoded RNA Polymerase, Promotes Chloroplast Development.

Regulation of photosynthetic gene expression by plastid-encoded RNA polymerase (PEP) is essential for chloroplast development. The activity of PEP largely relies on at least 12 PEP-associated proteins (PAPs) encoded in the nuclear genome of plant cells. A recent model proposed that these PAPs regulate the establishment of the PEP complex through broad PAP-PEP or PAP-PAP interactions. In this study, we identified the Arabidopsis (Arabidopsis thaliana) seedling-lethal mutant ptac10-1, which has defects in chloroplast development, and found that the mutant phenotype is caused by the suppression of PLASTID S1 RNA-BINDING DOMAIN PROTEIN (pTAC10/PAP3). Analysis of the heterozygous mutant and pTAC10-overexpressing transgenic plants indicated that the expression level of pTAC10 is tightly linked to chloroplast development. Characterization of the interaction of pTAC10 with PAPs revealed that pTAC10 interacts with other PAPs, such as FSD2, FSD3, TrxZ, pTAC7, and pTAC14, but it does not interact with PEP core enzymes, such as rpoA and rpoB. Analysis of pTAC10 interactions using truncated pTAC10 proteins showed that the pTAC10 carboxyl-terminal region downstream of the S1 domain is involved in the pTAC10-PAP interaction. Furthermore, overexpression of truncated pTAC10s lacking the C-terminal regions downstream of the S1 domain could not rescue the ptac10-1 mutant phenotype and induced an abnormal whitening phenotype in Columbia-0 plants. Our observations suggested that these pTAC10-PAP interactions are essential for the formation of the PEP complex and chloroplast development.

Overexpression of the OsERF71 Transcription Factor Alters Rice Root Structure and Drought Resistance.

Plant responses to drought stress require the regulation of transcriptional networks via drought-responsive transcription factors, which mediate a range of morphological and physiological changes. AP2/ERF transcription factors are known to act as key regulators of drought resistance transcriptional networks; however, little is known about the associated molecular mechanisms that give rise to specific morphological and physiological adaptations. In this study, we functionally characterized the rice (Oryza sativa) drought-responsive AP2/ERF transcription factor OsERF71, which is expressed predominantly in the root meristem, pericycle, and endodermis. Overexpression of OsERF71, either throughout the entire plant or specifically in roots, resulted in a drought resistance phenotype at the vegetative growth stage, indicating that overexpression in roots was sufficient to confer drought resistance. The root-specific overexpression was more effective in conferring drought resistance at the reproductive stage, such that grain yield was increased by 23% to 42% over wild-type plants or whole-body overexpressing transgenic lines under drought conditions. OsERF71 overexpression in roots elevated the expression levels of genes related to cell wall loosening and lignin biosynthetic genes, which correlated with changes in root structure, the formation of enlarged aerenchyma, and high lignification levels. Furthermore, OsERF71 was found to directly bind to the promoter of OsCINNAMOYL-COENZYME A REDUCTASE1, a key gene in lignin biosynthesis. These results indicate that the OsERF71-mediated drought resistance pathway recruits factors involved in cell wall modification to enable root morphological adaptations, thereby providing a mechanism for enhancing drought resistance.

Posttranscriptional control of photosynthetic mRNA decay under stress conditions requires 3' and 5' untranslated regions and correlates with differential polysome association in rice.

Abiotic stress, including drought, salinity, and temperature extremes, regulates gene expression at the transcriptional and posttranscriptional levels. Expression profiling of total messenger RNAs (mRNAs) from rice (Oryza sativa) leaves grown under stress conditions revealed that the transcript levels of photosynthetic genes are reduced more rapidly than others, a phenomenon referred to as stress-induced mRNA decay (SMD). By comparing RNA polymerase II engagement with the steady-state mRNA level, we show here that SMD is a posttranscriptional event. The SMD of photosynthetic genes was further verified by measuring the half-lives of the small subunit of Rubisco (RbcS1) and Chlorophyll a/b-Binding Protein1 (Cab1) mRNAs during stress conditions in the presence of the transcription inhibitor cordycepin. To discern any correlation between SMD and the process of translation, changes in total and polysome-associated mRNA levels after stress were measured. Total and polysome-associated mRNA levels of two photosynthetic (RbcS1 and Cab1) and two stress-inducible (Dehydration Stress-Inducible Protein1 and Salt-Induced Protein) genes were found to be markedly similar. This demonstrated the importance of polysome association for transcript stability under stress conditions. Microarray experiments performed on total and polysomal mRNAs indicate that approximately half of all mRNAs that undergo SMD remain polysome associated during stress treatments. To delineate the functional determinant(s) of mRNAs responsible for SMD, the RbcS1 and Cab1 transcripts were dissected into several components. The expressions of different combinations of the mRNA components were analyzed under stress conditions, revealing that both 3' and 5' untranslated regions are necessary for SMD. Our results, therefore, suggest that the posttranscriptional control of photosynthetic mRNA decay under stress conditions requires both 3' and 5' untranslated regions and correlates with differential polysome association.

RAV-Like1 maintains brassinosteroid homeostasis via the coordinated activation of BRI1 and biosynthetic genes in rice.

Temporal and spatial variation in the levels of and sensitivity to hormones are essential for the development of higher organisms. Traditionally, end-product feedback regulation has been considered as the key mechanism for the achievement of cellular homeostasis. Brassinosteroids (BRs) are plant steroid hormones that are perceived by the cell surface receptor kinase Brassinosteroid Insensitive1. Binding of these hormones to the receptor activates BR signaling and eventually suppresses BR synthesis. This report shows that RAVL1 regulates the expression of the BR receptor. Furthermore, RAVL1 is also required for the expression of the BR biosynthetic genes D2, D11, and BRD1 that are subject to BR negative feedback. Activation by RAVL1 was coordinated via E-box cis-elements in the promoters of the receptor and biosynthetic genes. Also, RAVL1 is necessary for the response of these genes to changes in cellular BR homeostasis. Genetic evidence is presented to strengthen the observation that the primary action of RAVL1 mediates the expression of genes involved in BR signaling and biosynthesis. This study thus describes a regulatory circuit modulating the homeostasis of BR in which RAVL1 ensures the basal activity of both the signaling and the biosynthetic pathways.

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.

Oryza sativa COI homologues restore jasmonate signal transduction in Arabidopsis coi1-1 mutants.

CORONATINE INSENSITIVE 1 (COI1) encodes an E3 ubiquitin ligase complex component that interacts with JAZ proteins and targets them for degradation in response to JA signaling. The Arabidopsis genome has a single copy of COI1, but the Oryza sativa genome has three closely related COI homologs. To examine the functions of the three OsCOIs, we used yeast two-hybrid assays to examine their interactions with JAZ proteins and found that OsCOIs interacted with OsJAZs and with JAZs, in a coronatine dependent manner. We also tested whether OsCOI1a and OsCOI1b could complement Arabidopsis coi1-1 mutants and found that overexpression of either gene in the coi1-1 mutant resulted in restoration of JA signal transduction and production of seeds, indicating successful complementation. Although OsCOI2 interacted with a few OsJAZs, we were not able to successfully complement the coi1-1 mutant with OsCOI2. Molecular modeling revealed that the three OsCOIs adopt 3D structures similar to COI1. Structural differences resulting from amino acid variations, especially among amino acid residues involved in the interaction with coronatine and JAZ proteins, were tested by mutation analysis. When His-391 in OsCOI2 was substituted with Tyr-391, OsCOI2 interacted with a wider range of JAZ proteins, including OsJAZ1, 2, 5∼9 and 11, and complemented coi1-1 mutants at a higher frequency than the other OsCOIs and COI1. These results indicate that the three OsCOIs are orthologues of COI1 and play key roles in JA signaling.

Identification of a novel jasmonate-responsive element in the AtJMT promoter and its binding protein for AtJMT repression.

Jasmonates (JAs) are important regulators of plant biotic and abiotic stress responses and development. AtJMT in Arabidopsis thaliana and BcNTR1 in Brassica campestris encode jasmonic acid carboxyl methyltransferases, which catalyze methyl jasmonate (MeJA) biosynthesis and are involved in JA signaling. Their expression is induced by MeJA application. To understand its regulatory mechanism, here we define a novel JA-responsive cis-element (JARE), G(C)TCCTGA, in the AtJMT and BcNTR1 promoters, by promoter deletion analysis and Yeast 1-Hybrid (Y1H) assays; the JARE is distinct from other JA-responsive cis-elements previously reported. We also used Y1H screening to identify a trans-acting factor, AtBBD1, which binds to the JARE and interacts with AtJAZ1 and AtJAZ4. Knockout and overexpression analyses showed that AtBBD1 and its close homologue AtBBD2 are functionally redundant and act as negative regulators of AtJMT expression. However, AtBBD1 positively regulated the JA-responsive expression of JR2. Chromatin immunoprecipitation from knockout and overexpression plants revealed that repression of AtJMT is associated with reduced histone acetylation in the promoter region containing the JARE. These results show that AtBBD1 interacts with JAZ proteins, binds to the JARE and represses AtJMT expression.