A role for TRIANGULAR HULL1 in fine-tuning spikelet morphogenesis in rice.

The lemma and palea, which enclose the pistil, stamens, and lodicules, are the most conspicuous organs in the rice spikelet. We isolated a mutant line (ng6569) in which the lemma and palea were narrower than those of the wild type, and found that the mutant had a defect in TRIANGULAR HULL1 (TH1), which encodes a nuclear protein with an ALOG domain. Detailed morphological analysis indicated that the th1 mutation caused a reduction in the size of tubercles, which are convex structures on the surface of the lemma and palea. This reduction was more pronounced in the apical region of the lemma than in the basal region, resulting in the formation of a beak-like spikelet. By contrast, the number of tubercle rows and their spatial distribution on the lemma were not affected in the th1 mutant. Thus, the TH1 gene seems to be involved in fine-tuning the morphogenesis of the lemma and palea. In situ hybridization analysis revealed that TH1 was highly expressed in the primordia of the lemma and palea, but only weakly expressed in the primordia of the sterile lemma and rudimentary glume. We then examined the effect of th1 mutation on the lemma-like structure formed in the long sterile lemma/glume1 (g1) and extra glume1 (eg1) mutants. The result showed that the th1 mutation strongly affected the morphology of the extra lemma of eg1, but had no significant effect on the transformed lemma of g1.