Abstract
As a basic unit of rice inflorescence, spikelet has profound influence on grain size, weight and yield. The molecular mechanism underlying spikelet development has not been fully elucidated. Here, we identified four allelic rice mutants, s2-89, xd151, xd281 and xd425, which exhibited reduced width of spikelet, especially in the apical region. Map-based cloning revealed that all these mutants had missense mutation in the TRIANGULAR HULL1 (TH1) gene, encoding an ALOG family protein. TH1 has been shown to regulate the lateral development of spikelet, but its mode of action remains unclear. Microscopic analysis revealed that the reduction in spikelet width was caused by decreased cell size rather than cell division. The TH1 protein was shown to localize in the nucleus and possess transcriptional repression activity. TH1 could form a homodimer and point mutation in the s2-89, xd281 and xd425 mutant inhibited homodimerization. The transcriptional repression activity of TH1 was partially relieved by the His129Tyr substitution in the s2-89 mutant. Fusion of an exogenous EAR transcription suppression domain to the mutant protein TH1s2-89 could largely complemented the narrow spikelet phenotype. These results indicate that TH1 functions as a transcription repressor and regulates cell expansion during the lateral development of spikelet.
Highlights
Rice (Oryza sativa L.), a model monocot with the smallest genome of major cereals, provides the staple food for over half of the world’s population
Lemma and palea play a critical role in determining the grain shape, size and yield, because the space enclosed by the lemma and palea determines the size of spikelet hull
Nipponbare mutagenized by the ethyl methane sulphonate (EMS)
Summary
Rice (Oryza sativa L.), a model monocot with the smallest genome of major cereals, provides the staple food for over half of the world’s population. GRAIN WIDTH 5 (GW5), a major QTL underlying rice width and weight, encodes a novel nuclear protein which likely acts in the ubiquitin-proteasome pathway to regulate cell division during seed development[6]. A spikelet is composed of two rudimentary glumes, two empty glumes (or sterile lemmas) and a floret which consists of one lemma, one palea, two lodicules, six stamens and one carpel[13,14,15,16] Among these floral organs, lemma and palea play a critical role in determining the grain shape, size and yield, because the space enclosed by the lemma and palea determines the size of spikelet hull. The TH1 gene encodes a nuclear protein with a conserved ALOG domain of unknown function[32,33] It is still unclear how TH1 regulates the lateral development of the lemma and palea
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