A role for <i>TRIANGULAR HULL1</i> in fine-tuning spikelet morphogenesis in rice

Abstract

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.

The rice TRIANGULAR HULL1 protein acts as a transcriptional repressor in regulating lateral development of spikelet

“…Meanwhile, in the th1 mutants, the marginal region of palea (mrp) looks normal and the alterations are due to shape changes of the lemma and the rest of palea (body of palea, bop) 36 . In addition, the lodicules of th1 florets were elongated and green 34 , 36 , 37 . Further observations showed that the surface of elongated lodicules was rough, crumpled and possessed some protrusions that were similar with those in the wild type hulls 36 .…”

“…In our study, the th1 mutation only resulted in a slight change in the shape of the lemma and palea. SEM observation of early stages of floret development in several allelic mutants of TH1 , such as bls1-1 and afd1 , showed that TH1 does not function at stages of floral-organ initiation and patterning but is required for lateral development of the lemma and palea in the final stages 34 – 36 . Our observation that TH1 controls cell size during the lateral development of lemma and palea is consistent with previous finding that TH1 is not associated with meristem function.…”

“…Thus, mutations in the genes mentioned above caused dramatic abnormality in the spikelet. By contrast, in the triangular hull1 ( th1 ) mutant, the floral organ identity and patterning are normal, but the lemma and palea become narrower especially in the apical region, forming a beak-like spikelet 33 , 34 . Several allelic mutants of th1 named beak like spikelet1 ( bls1 ), beak-shaped grain1 ( bsg1 ) and abnormal flower and dwarf1 ( afd1 ) were also reported 35 – 37 .…”

Control of inflorescence architecture in tomato by BTB/POZ transcriptional regulators

“…Notably, mutations in the closest homolog of TMF in rice simplifies panicle architecture (Yoshida et al 2013), but loss of the closest homologs in Arabidopsis (LIGHT-SENSITIVE HYPOCOTYL3 and LIGHT-SENSITIVE HYPOCOTYL4) has no effect on inflorescences, although overexpressing these genes suppresses lateral organ differentiation (Takeda et al 2011). A handful of other ALOG genes in Arabidopsis and rice have reported roles in light signaling and floral organ development (Zhao et al 2004;Yoshida et al 2009;Cho and Zambryski 2011;Takeda et al 2011;Sato et al 2014). Thus, ALOG proteins represent a new, poorly understood family of growth regulators with prominent, species-specific roles in reproductive development.…”

Polar patterning of the spikelet is disrupted in the <i>two opposite lemma</i> mutant in rice

“…For example, the DEPRESSED PALEA1 (DP1) gene encoding an AT-hook DNA binding protein regulates proper development of the central region of the palea (Jin et al, 2011), whereas the G1 gene encoding a plant-specific nuclear factor specifies sterile lemma identity by repressing lemma identity . The TRIANGULAR HULL1 (TH1) gene, belonging to the same gene family as G1, is involved in structural fine-tuning of the lemma and palea (Sato et al, 2014). Although Arabidopsis has homologs of these genes, there is no report that they are involved in flower development in Arabidopsis.…”