Os07g0505200

The rice Os07g0505200 was reported as OsSPL13. OsSPL13 belongs to the SQUAMOSA PROMOTER BINDING PROTEIN (SBP) family of transcription factors.

Annotated Information

Function

• OsSPL13 gene locates on the a major quantitative trait locus, GLW7, and encodes the plant-specific transcription factor OsSPL13, positively regulates cell size in the grain hull, resulting in enhanced rice grain length and yield, but does not regulate grain width.OsSPL13 has an important role in the accumulation of dry matter in rice grains.OsSPL13 regulates grain shape through activation of cell size regulation machinery.
• A tandem-repeat sequence in the 5′ UTR of OsSPL13 alters its expression by affecting transcription and translation and that high expression of OsSPL13 is associated with large grains in tropical japonica rice.
• OsSPL13 is one of the targets of OsmiR156.OsSPL13 directly targets SRS5, and functions as a positive regulator of SRS5 expression in the regulation of hull cell size in rice.

Mutation

• To determine whether the functional differences in regulation of grain size between the two OsSPL13 haplotypes(large-grain varieties and small-grain varieties) are attributable to the polymorphisms in the promoter or coding regions, scientists generated three transgene constructs—construct I (OsSPL13SGH), construct CP (OsSPL13LGH) and construct III(pOSPL13SGH::OsSPL13LGH) (Fig. 1b)—which were used to generate transgenic plants (Online Methods). In comparison with wild-type Dongjing plants, the construct CP and III transgenic lines had significant increases in both grain length and grain thickness, resulting in approximately 10% and 9% increases in 1,000-grain weight, respectively (Fig. 1c–f). Although transgenic plants with construct I had increased grain size, their grains were smaller than those of transgenic plants with construct CP. No obvious alterations in grain width were observed in any of these transgenic plants. These transgene studies show that the polymorphisms in the promoter region of OsSPL13 are not responsible for the differences in grain size among japonica varieties. Scientists further mutated different sites in the OsSPL13SGH construct to generate additional constructs (IV–X; Fig. 1b) and carried out transgenic analysis in Dongjing plants. Both construct V and VI transgenic plants had significantly improved grain size, which was similar to that of transgenic plants with construct CP. Transgenic plants with construct IV and four 3′ UTR mutations (constructs VII–X) had grain phenotypes similar to those of transgenic plants with construct I (Fig. 1c–f). Analyses of the various transgenic plants showed higher levels of OsSPL13 transcript and protein in the transgenic lines with constructs CP, III, V and VI than in wild-type Dongjing and the transgenic lines with the other constructs (Fig. 1g,h). Taking these results together, Scientists identify a tandem repeat of the CCATTC sequence from –146 to –135 bp in the 5′ UTR of OsSPL13 as the major cause of the GLW7 effect on variation in grain size among japonica rice , with two CCATTC copies in the OsSPL13 5′ UTR causing reduced expression levels of OsSPL13 and resulting in small grains.

Figure 1. Comparative analyses of the OsSPL13 locus between the small-grain and large-grain haplotypes. .

• Scientists carried out RNA interference (RNAi) to suppress the expression of OsSPL13 in the small-grain japonica variety Dongjing and the large-grain japonica variety GP579. The corresponding transgenic plants showed significantly decreased 1,000-grain weight, grain length and grain thickness when compared with the wild-type plants. No changes were detected in grain width (Fig. 2a,b,d,e). Control plants with pTCK303 did not show any differences in grain shape for the transgenic plants.

Figure 1. Analyses of grain shape for wild-type Dongjing, transgenic plants and a T-DNA mutant .

Expression Pattern

• According to rice spikelet developmental stages29, OsSPL13 was detected when the primary- and secondary branch primordia initiated (Fig. 3a–c) and was strongly expressed in floral organ primordia (Fig. 3d). OsSPL13 was highly expressed in the middle of the lemma and palea when florets were about 1 mm in length (Fig. 3e), and expression gradually decreased during the growth of the florets and stamen (Fig. 3f). When florets reached 2.2 mm in length, only weak OsSPL13 expression was detected in the hulls of the florets (Fig. 3g). Interestingly, OsSPL13 expression was strong in the apices of the lemma and palea when the hulls were 4–5 mm in length and gradually decreased from apices to the middle of the hull (Fig. 3h–j). No signal was detected in the negative control (Fig. 3k).

Figure 1. In situ hybridization for OsSPL13 in primary branch formation .

Subcellular localization

• The nuclear localization of an OsSPL13-GFP fusion protein is consistent with the notion that OsSPL13 encodes a putative transcription factor. OsSPL13 contains a putative bipartite nuclear localization signal at the C-terminal end of the highly conserved SBP domain, which is necessary and sufficient for DNA binding and nuclear localization of SPLs

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Evolution

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Labs working on this gene

• National Center for Gene Research, Chinese Academy of Sciences Center for Excellence of Molecular Plant Sciences, Institute of Plant Physiology and Ecology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai, China.
• State Key Laboratory of Rice Biology, China National Rice Research Institute, Chinese Academy of Agricultural Sciences, Hangzhou, China.
• Crop Biotech Institute, Kyung Hee University, Yongin, Republic of Korea. Correspondence should be addressed to B.H.