To synthesize the cofactor thiamin diphosphate (ThDP), plants must first hydrolyze thiamin monophosphate (ThMP) to thiamin, but dedicated enzymes for this hydrolysis step were unknown and widely doubted to exist. The classical thiaminrequiring th2-1 mutation in Arabidopsis thaliana was shown to reduce ThDP levels by half and to increase ThMP levels 5-fold, implying that the THIAMIN REQUIRING2 (TH2) gene product could be a dedicated ThMP phosphatase. Genomic and transcriptomic data indicated that TH2 corresponds to At5g32470, encoding a HAD (haloacid dehalogenase) family phosphatase fused to a TenA (thiamin salvage) family protein. Like the th2-1 mutant, an insertional mutant of At5g32470 accumulated ThMP, and the thiamin requirement of the th2-1 mutant was complemented by wild-type At5g32470. Complementation tests in Escherichia coli and enzyme assays with recombinant proteins confirmed that At5g32470 and its maize (Zea mays) orthologs GRMZM2G148896 and GRMZM2G078283 are ThMP-selective phosphatases whose activity resides in the HAD domain and that the At5g32470 TenA domain has the expected thiamin salvage activity. In vitro and in vivo experiments showed that alternative translation start sites direct the At5g32470 protein to the cytosol and potentially also to mitochondria. Our findings establish that plants have a dedicated ThMP phosphatase and indicate that modest (50%) ThDP depletion can produce severe deficiency symptoms.
Juvenile-to-adult phase transition is an important shift for the acquisition of adult vegetative characteristics and subsequent reproductive competence. We identified a recessive precocious ( pre) mutant exhibiting a long leaf phenotype in rice. The long leaf phenotype is conspicuous in the second to the fourth leaves, which are juvenile and juvenile-to-adult transition leaves. We found that morphological and physiological traits, such as midrib formation, shoot meristem size, photosynthetic rate and plastochron, in juvenile and juvenile-to-adult transition stages of the pre mutant have precociously acquired adult characteristics. In agreement with these results, expression patterns of miR156 and miR172, which are microRNAs regulating phase change, support the accelerated juvenile-to-adult phase change in the pre mutant. The mutated gene encodes an allene oxide synthase (OsAOS1), which is a key enzyme for the biosynthesis of jasmonic acid (JA). The pre mutant showed a low level of JA and enhanced sensitivity to gibberellic acid, which promotes the phase change in some plant species. We also show that prolonged plastochron in the pre mutant is caused by accelerated PLASTOCHRON1 (PLA1) function. The present study reveals a substantial role of JA as a negative regulator of vegetative phase change.
Callose is a plant cell-wall polysaccharide whose deposition is spatiotemporally regulated in various developmental processes and environmental stress responses. Appearance of callose in premeiotic anthers is a prominent histological hallmark for the onset of meiosis in flowering plants; however, the biological role of callose in meiosis remains unknown. Here we show that rice (Oryza sativa) GLUCAN SYNTHASE LIKE5 (OsGSL5), a callose synthase, localizes on the plasma membrane of pollen mother cells (PMCs) and is responsible for biogenesis of callose in anther locules through premeiotic and meiotic stages. In Osgsl5 mutant anthers mostly lacking callose deposition, aberrant PMCs accompanied by aggregated, unpaired or multivalent chromosomes were frequently observed, and furthermore, a considerable number of mutant PMCs had untimely progress into meiosis compared to that of wild-type PMCs. Immunostaining of meiosis-specific protein HOMOLOGOUS PAIRING ABERRATION IN RICE MEIOSIS2 (PAIR2) in premeiotic PMCs revealed precocious meiosis entry in Osgsl5 anthers. These findings provide insights into the function of callose in controlling the timing of male meiosis initiation and progression, in addition to roles in microsporogenesis, in flowering plants.
BackgroundThe rice PLASTOCHRON (PLA) genes PLA1 and PLA2 regulate leaf maturation and the temporal pattern of leaf initiation. Although the function of PLA genes in the leaf initiation process has been analyzed, little is known about how they affect leaf growth. Previously, we suggested that PLA1 and PLA2 function downstream of the gibberellin (GA) signal transduction pathway. In the present study, we examined the phenotype of a double mutant of pla and slender rice 1 (slr1), which is a constitutive GA response mutant. By analyzing these double mutants, we discuss the relationship between PLA-related and GA-dependent pathways and the possible function of PLA genes in leaf growth.FindingsSingle slr1 and pla mutants exhibited elongated and dwarf phenotypes in the vegetative stage, respectively. The stature and leaf size of the pla1/slr1 and pla2/slr1 double mutants were intermediate between those of the pla and slr1 single mutants. However, the effects of slr1 on leaf elongation were markedly suppressed in the pla1 and pla2 mutant backgrounds. On the other hand, the change in cell length in the double mutants was almost the same as that in the single mutants. An expression analysis of genes involved in GA biosynthesis and catabolism indicated that feedback regulation functioned normally in the pla/slr1 double mutants.ConclusionsOur genetic results confirm that PLA genes regulate leaf growth downstream of the GA pathway. Our findings also suggest that PLA1 and PLA2 are partly required for GA-dependent leaf elongation, mainly by affecting cellular proliferation.
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