A role for jasmonates in the release of dormancy by cold stratification in wheat

Hydration at low temperatures, commonly referred to as cold stratification, is widely used for releasing dormancy and triggering germination in a wide range of species including wheat. However, the molecular mechanism that underlies its effect on germination has largely remained unknown. Our previous studies showed that methyl-jasmonate, a derivative of jasmonic acid (JA), promotes dormancy release in wheat. In this study, we found that cold-stimulated germination of dormant grains correlated with a transient increase in JA content and expression of JA biosynthesis genes in the dormant embryos after transfer to 20 (o)C. The induction of JA production was dependent on the extent of cold imbibition and precedes germination. Blocking JA biosynthesis with acetylsalicylic acid (ASA) inhibited the cold-stimulated germination in a dose-dependent manner. In addition, we have explored the relationship between JA and abscisic acid (ABA), a well-known dormancy promoter, in cold regulation of dormancy. We found an inverse relationship between JA and ABA content in dormant wheat embryos following stratification. ABA content decreased rapidly in response to stratification, and the decrease was reversed by addition of ASA. Our results indicate that the action of JA on cold-stratified grains is mediated by suppression of two key ABA biosynthesis genes, TaNCED1 and TaNCED2.This project was funded by a CSIRO Office of the Chief Executive PDF scheme

MicroR159 regulation of most conserved targets in Arabidopsis has negligible phenotypic effects

BACKGROUND A current challenge of microRNA (miRNA) research is the identification of biologically relevant miRNA:target gene relationships. In plants, high miRNA:target gene complementarity has enabled accurate target predictions, and slicing of target mRNAs has facilitated target validation through rapid amplification of 5' cDNA ends (5'-RACE) analysis. Together, these approaches have identified more than 20 targets potentially regulated by the deeply conserved miR159 family in Arabidopsis, including eight MYB genes with highly conserved miR159 target sites. However, genetic analysis has revealed the functional specificity of the major family members, miR159a and miR159b is limited to only two targets, MYB33 and MYB65. Here, we examine the functional role of miR159 regulation for the other potential MYB target genes. RESULTS For these target genes, functional analysis failed to identify miR159 regulation that resulted in any major phenotypic impact, either at the morphological or molecular level. This appears to be mainly due to the quiescent nature of the remaining family member, MIR159c. Although its expression overlaps in a temporal and spatial cell-specific manner with a subset of these targets in anthers, the abundance of miR159c is extremely low and concomitantly a mir159c mutant displays no anther defects. Examination of potential miR159c targets with conserved miR159 binding sites found neither their spatial or temporal expression domains appeared miR159 regulated, despite the detection of miR159-guided cleavage products by 5'-RACE. Moreover, expression of a miR159-resistant target (mMYB101) resulted predominantly in plants that are indistinguishable from wild type. Plants that displayed altered morphological phenotypes were found to be ectopically expressing the mMYB101 transgene, and hence were misrepresentative of the in vivo functional role of miR159. CONCLUSIONS This study presents a novel explanation for a paradox common to plant and animal miRNA systems, where among many potential miRNA-target relationships usually only a few appear physiologically relevant. The identification of a quiescent miR159c:target gene regulatory module in anthers provides a likely rationale for the presence of conserved miR159 binding sites in many targets for which miR159 regulation has no obvious functional role. Remnants from the demise of such modules may lead to an overestimation of miRNA regulatory complexity when investigated using bioinformatic, 5'-RACE or transgenic approaches.RSA was funded by an ANU postgraduate scholarship and by a CSIRO Emerging Science Initiative. JL is the recipient of an ANU international student postgraduate scholarship. This research was supported by an Australian Research Council grant DP0773270

Over-expression of miR172 causes loss of spikelet determinacy and floral organ abnormalities in rice (Oryza sativa)

<p>Abstract</p> <p>Background</p> <p>Regulation of gene expression by microRNAs (miRNAs) plays a crucial role in many developmental and physiological processes in plants. miRNAs act to repress expression of their target genes via mRNA cleavage or translational repression. Dozens of miRNA families have been identified in rice, 21 of which are conserved between rice and Arabidopsis. miR172 is a conserved miRNA family which has been shown to regulate expression of <it>APETALA2 </it>(<it>AP2</it>)-like transcription factors in Arabidopsis and maize. The rice genome encodes five <it>AP2</it>-like genes predicted to be targets of miR172. To determine whether these rice <it>AP2</it>-like genes are regulated by miR172 and investigate the function of the target genes, we studied the effect of over-expressing two members of the miR172 family on rice plant development.</p> <p>Results</p> <p>Analysis of miR172 expression showed that it is most highly expressed in late vegetative stages and developing panicles. Analyses of expression of three miR172 targets showed that <it>SUPERNUMERARY BRACT </it>(<it>SNB</it>) and <it>Os03g60430 </it>have high expression in developing panicles. Expression of miR172 was not inversely correlated with expression of its targets although miR172-mediated cleavage of <it>SNB </it>was detected by 5' rapid amplification of cDNA ends (RACE). Over-expression of miR172b in rice delayed the transition from spikelet meristem to floral meristem, and resulted in floral and seed developmental defects, including changes to the number and identity of floral organs, lower fertility and reduced seed weight. Plants over-expressing miR172b not only phenocopied the T-DNA insertion mutant of <it>SNB </it>but showed additional defects in floret development not seen in the <it>snb </it>mutant. However <it>SNB </it>expression was not reduced in the miR172b over-expression plants.</p> <p>Conclusions</p> <p>The phenotypes resulting from over-expression of miR172b suggests it represses <it>SNB </it>and at least one of the other miR172 targets, most likely <it>Os03g60430</it>, indicating roles for other <it>AP2</it>-like genes in rice floret development. miR172 and the <it>AP2</it>-like genes had overlapping expression patterns in rice and their expression did not show an obvious negative correlation. There was not a uniform decrease in the expression of the <it>AP2</it>-like miR172 target mRNAs in the miR172b over-expression plants. These observations are consistent with miR172 functioning via translational repression or with expression of the <it>AP2</it>-like genes being regulated by a negative feedback loop.</p

ALTERED MERISTEM PROGRAM 1 Is involved in Development of Seed Dormancy in Arabidopsis

Mutants in the rice PLASTOCHRON 3 and maize VIVIPAROUS 8 genes have been shown to have reduced dormancy and ABA levels. In this study we used several mutants in the orthologous gene ALTERED MERISTEM PROGRAM 1 (AMP1) to determine its role in seed dormancy in Arabidopsis. Here we report that there are accession-specific effects of mutations in AMP1. In one accession, amp1 mutants produce seeds with higher dormancy, while those in two other accessions produce seeds of lower dormancy. These accession-specific effects of mutating AMP1 were shown to extend to ABA levels. We assayed global gene transcription differences in seeds of wild-type and mutant from two accessions demonstrating opposing phenotypes. The transcript changes observed indicate that the amp1 mutation shifts the seed transcriptome from a dormant into an after-ripened state. Specific changes in gene expression in the mutants give insight into the direct and indirect effects that may be contributing to the opposing dormancy phenotypes observed, and reveal a role for AMP1 in the acquisition and/or maintenance of seed dormancy in Arabidopsis

Dormancy in cereals (not too much, not so little): about the mechanisms behind this trait

As in other cultivated species, dormancy can be seen as a problem in cereal production, either due to its short duration or to its long persistence. Indeed, cereal crops lacking enough dormancy at harvest can be exposed to pre-harvest sprouting damage, while a long-lasting dormancy can interfere with processes that rely on rapid germination, such as malting or the emergence of a uniform crop. Because the ancestors of cereal species evolved under very diverse environments worldwide, different mechanisms have arisen as a way of sensing an appropriate germination environment (a crucial factor for winter or summer annuals such as cereals). In addition, different species (and even different varieties within the same species) display diverse grain morphology, allowing some structures to impose dormancy in some cereals but not in others. As in seeds from many other species, the antagonism between the plant hormones abscisic acid and gibberellins is instrumental in cereal grains for the inception, expression, release and re-induction of dormancy. However, the way in which this antagonism operates is different for the various species and involves different molecular steps as regulatory sites. Environmental signals (i.e. temperature, light quality and quantity, oxygen levels) can modulate this hormonal control of dormancy differently, depending on the species. The practical implications of knowledge accumulated in this field are discussed.Fil: Rodríguez, María Verónica. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas A la Agricultura; ArgentinaFil: Barrero, J. M.. CSIRO Agriculture Flagship; AustraliaFil: Corbineau, Francoise. Universite Pierre et Marie Curie; FranciaFil: Gubler, Frank. CSIRO Agriculture Flagship; AustraliaFil: Benech-arnold, Roberto Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas A la Agricultura; Argentin

The barley grain thioredoxin system - an update.

Thioredoxin reduces disulfide bonds and play numerous important functions in plants. In cereal seeds, cytosolic h-type thioredoxin facilitates the release of energy reserves during the germination process and is recycled by NADPH-dependent thioredoxin reductase. This review presents a summary of the research conducted during the last ten years to elucidate the structure and function of the barley seed thioredoxin system at the molecular level combined with proteomic approaches to identify target proteins

Mitochondrial Protein Import

The role of nucleoside triphosphates (NTPs) in mitochondrial protein import was investigated with the precursors of N. crassa ADP/ATP carrier, F1-ATPase subunit β, F0-ATPase subunit 9, and fusion proteins between subunit 9 and mouse dihydrofolate reductase. NTPs were necessary for the initial interaction of precursors with the mitochondria and for the completion of translocation of precursors from the mitochondrial surface into the mitochondria. Higher levels of NTPs were required for the latter reactions as compared with the early stages of import. Import of precursors having identical presequences but different mature protein parts required different levels of NTPs. The sensitivity of precursors in reticulocyte lysate to proteases was decreased by removal of NTPs and increased by their readdition. We suggest that the hydrolysis of NTPs is involved in modulating the folding state of precursors in the cytosol, thereby conferring import competence