Highlight qEMF3, a novel QTL for the early-morning flowering trait to mitigate heat-induced spikelet sterility at flowering in rice, was identified using a wild rice, Oryza officinalis, as a genetic resource.

The rice esp2 mutation was previously characterized by the abnormal accumulation of elevated levels of proglutelin and the absence of an endosperm-specific protein disulfide isomerase like (PDIL1-1). Here we show that Esp2 is the structural gene for PDIL1-1 and that this lumenal chaperone is asymmetrically distributed within the cortical endoplasmic reticulum (ER) and largely restricted to the cisternal ER. Temporal studies indicate that PDIL1-1 is essential for the maturation of proglutelin only when its rate of synthesis significantly exceeds its export from the ER, a condition resulting in its build up in the ER lumen and the induction of ER quality control processes which lower glutelin levels as well as those of the other storage proteins. As proglutelin is initially synthesized on the cisternal ER, its deposition within prolamine protein bodies in esp2 suggests that PDIL1-1 helps retain proglutelin in the cisternal ER lumen until it attains competence for ER export and, thereby, indirectly preventing heterotypic interactions with prolamine polypeptides.

To identify the chromosomal regions controlling the eating quality of Koshihikari rice, we performed a quantitative trait locus (QTL) analysis using two backcross inbred lines (BILs): N-BILs (79 lines derived from a cross of Nipponbare/Koshihikari//Nipponbare) and K-BILs (89 lines derived from a cross of Nipponbare/ Koshihikari//Koshihikari). We evaluated several components of the eating quality of cooked rice, namely glossiness, taste, stickiness, hardness, and overall evaluation, based on sensory tests by a trained panel, and amylose and protein contents. Ten QTLs for these components were detected in N-BILs (two regions of chromosome [chr.] 3 and one of chr. 11), and six in K-BILs (chr. 3 and chr. 6). Each QTL explained 11.6% to 32.0% of the total phenotypic variance. QTLs at the distal end of the short arm of chr. 3 were commonly identified in both BILs. The Koshihikari alleles at these QTLs increased eating quality. The genetic effect of the Koshihikari alleles was confirmed by analysis of a chromosome segment substitution line containing a Koshihikari segment of the short arm of chr. 3 in the Nipponbare background.

We had previously identified eight mutants, esp2 and g(G)lups1 to 7, which accumulated abnormally high amounts of proglutelin, the major storage protein in rice seeds. Analysis of their seed proteins by SDS-PAGE, their levels of the luminal chaperone BiP and gene-gene interactions indicated that these mutants fell into four classes. The most epistatic class consisted of esp2, which encodes a defective protein disulfide isomerase (PDI). A second class consisting of Glup1, glup2 and glup7 was hypostatic to esp2, and showed abnormally high levels of BiP, suggesting that maturation and export of proglutelins from the ER are inhibited in this class of mutants. The third class containing glup4, Glup5 and glup6 mutations was hypostatic to esp2, Glup1, glup2 and glup7. Since the glup4 allele encodes the small GTPase Rab5a, which participates in the trafficking of proglutelin from Golgi apparatus to the protein storage vacuole (PSV), this third class of mutants is likely affected in this process. Lastly, glup3, which encodes a vacuolar processing enzyme, which proteolytically processes proglutelin into acidic and basic subunits within the PSV, was hypostatic to the other mutants. Overall, these gene relationships are consistent with the sequential intracellular transport and processing of proglutelin and provide novel insights on the trafficking of proglutelin to the PSV.