KNOX Protein OSH15 Induces Grain Shattering by Repressing Lignin Biosynthesis Genes

Abstract

Seed shattering is an agronomically important trait. Two major domestication factors are responsible for this: qSH1 and SH5. Whereas qSH1 functions in cell differentiation in the abscission zone (AZ), a major role of SH5 is the repression of lignin deposition. We have determined that a KNOX protein, OSH15, also controls seed shattering. Knockdown mutations of OSH15 showed reduced seed-shattering phenotypes. Coimmunoprecipitation experiments revealed that OSH15 interacts with SH5 and qSH1, two proteins in the BELL homeobox family. In transgenic plants carrying the OSH15 promoter-GUS reporter construct, the reporter gene was preferentially expressed in the AZ during young spikelet development. The RNA in situ hybridization experiment also showed that OSH15 messenger RNAs were abundant in the AZ during spikelet development. Analyses of osh15 SH5-D double mutants showed that SH5 could not increase the degree of seed shattering when OSH15 was absent, indicating that SH5 functions together with OSH15. In addition to the seedshattering phenotype, osh15 mutants displayed dwarfism and accumulated a higher amount of lignin in internodes due to increased expression of the genes involved in lignin biosynthesis. Knockout mutations of CAD2, which encodes an enzyme for the last step in the monolignol biosynthesis pathway, caused an easy seed-shattering phenotype by reducing lignin deposition in the AZ. This indicated that the lignin level is an important determinant of seed shattering in rice (Oryza sativa). Chromatin immunoprecipitation assays demonstrated that both OSH15 and SH5 interact directly with CAD2 chromatin. We conclude that OSH15 and SH5 form a dimer that enhances seed shattering by directly inhibiting lignin biosynthesis genes.

The anatomy of abscission zones is diverse among grass species

“…A pattern of nonlignified cells next to lignified cells is important for a functional AZ in fruits of Arabidopsis thaliana and rice (Liljegren et al., , ; Rajani and Sundaresan, ; Mitsuda and Ohme‐Takagi, ; Zhou et al., ; Yoon et al., , ). However, our study found approximately equal numbers of species with and without differential lignification (8 vs. 9, respectively) (Fig.…”

“…Thin and nonlignified walls are more accessible for degradation, and differential lignification creates physical tension between cell types when tissue dries and loses turgor pressure, which eventually leads to breakage at the weakly attached AZ (Sexton and Roberts, ). Rice mutants with ectopic lignification in the AZ exhibit increased tensile strength of tissues holding the grain onto the plant and reduced or loss of grain abscission (Zhou et al., ; Yoon et al., , ; Lv et al., ). In fruit abscission (also called dehiscence) of Arabidopsis , mutants with ectopic lignification in the AZ (Rajani and Sundaresan, ) or nonlignification in the cells adjacent to the AZ (Liljegren et al., , ; Mitsuda and Ohme‐Takagi, ) both inhibit dehiscence, demonstrating the importance of differential lignification in the abscission process.…”

BELL1‐like homeobox genes regulate inflorescence architecture and meristem maintenance in rice

“…In rice, two BEL1‐type homeobox genes, qSH1 and SH5 , which are closely related to Arabidopsis PNY and PNF , are required for formation of the abscission zone that promotes seed shattering (Konishi et al ., ; Yoon et al ., ). OSH15, a BP‐like KNOX‐type homeodomain protein, is also involved in this process through its formation of a heterodimer with qSH1 and SH5 (Yoon et al ., ). Recently, it has been reported in maize that mutations in the two BELL1‐LIKE HOMEOBOX (BLH) genes BLH12 and BLH14 , which are close homologs of PNY and PNF , cause abnormalities in internode patterning and vascular bundles anastomosis, in addition to indeterminate branch formation in the tassel (Tsuda et al ., ).…”

Chromatin interacting factor OsVIL2 increases biomass and rice grain yield

“…The Co-IP assays were performed as reported earlier (Cho et al, 2016;Yoon et al, 2017). Briefly, fusion molecules were coexpressed in rice Oc cell protoplasts.…”

“…Transgenic plants expressing OsVIL2-Myc were used for ChIP analysis, as described previously (Yang et al, 2013;Yoon et al, 2017), with the anti-Myc monoclonal antibody (Cell Signaling; #2276) and the anti-H3K27me3 monoclonal antibody (Millipore; 07-449). The assays were performed as reported earlier (Haring et al, 2007).…”