Abstract
SEPALLATA (SEP) MADS box transcription factors mediate floral development in association with other regulators. Mutants in five rice (Oryza sativa) SEP genes suggest both redundant and unique functions in panicle branching and floret development. leafy hull sterile1/OsMADS1, from a grass-specific subgroup of LOFSEP genes, is required for specifying a single floret on the spikelet meristem and for floret organ development, but its downstream mechanisms are unknown. Here, key pathways and directly modulated targets of OsMADS1 were deduced from expression analysis after its knockdown and induction in developing florets and by studying its chromatin occupancy at downstream genes. The negative regulation of OsMADS34, another LOFSEP gene, and activation of OsMADS55, a SHORT VEGETATIVE PHASE-like floret meristem identity gene, show its role in facilitating the spikelet-to-floret meristem transition. Direct regulation of other transcription factor genes like OsHB4 (a class III homeodomain Leu zipper member), OsBLH1 (a BEL1-like homeodomain member), OsKANADI2, OsKANADI4, and OsETTIN2 show its role in meristem maintenance, determinacy, and lateral organ development. We found that the OsMADS1 targets OsETTIN1 and OsETTIN2 redundantly ensure carpel differentiation. The multiple effects of OsMADS1 in promoting auxin transport, signaling, and auxin-dependent expression and its direct repression of three cytokinin A-type response regulators show its role in balancing meristem growth, lateral organ differentiation, and determinacy. Overall, we show that OsMADS1 integrates transcriptional and signaling pathways to promote rice floret specification and development.
Highlights
SEPALLATA (SEP) MADS box transcription factors mediate floral development in association with other regulators
The largely shared functions of Arabidopsis SEP genes differ from observations that homologs in other plants often have discrete roles in floral development (Malcomber and Kellogg, 2005), but the molecular mechanism underlying their species-specific roles is not well studied
On functional classification of deregulated genes with 3-fold or greater expression level change, we found a significantly high proportion to be implicated in transcription and signaling (Fig. 1, A and B; Supplemental Data Set S2). These data gave a gross overview of genes and pathways downstream to OsMADS1. Primary verification of these data was done by reverse transcription-quantitative PCR (RT-qPCR) analyses of transcript levels for 17 transcription factors (Supplemental Table S1) in RNA samples used in microarray analysis and in an independently generated pool of wild-type and OsMADS1-RNAi panicles (Fig. 1, C and D; Supplemental Fig. S1)
Summary
SEPALLATA (SEP) MADS box transcription factors mediate floral development in association with other regulators. Various studies show that five rice SEP genes have both redundant and nonredundant roles They contribute to panicle morphology, spikelet and FM specification, floral organ differentiation, and meristem determinacy (Jeon et al, 2000; Prasad et al, 2005; Cui et al, 2010; Gao et al, 2010; Kobayashi et al, 2010). Our data show, in instances, inverse gene expression outcomes for some targets common to OsMADS1 and SEP3 (Kaufmann et al, 2009) These findings provide insights on how OsMADS1 complexes modulate other transcription factors and the balance between auxin and cytokinin signaling to promote the determinate development of a rice FM
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