qAC2, a novel QTL that interacts with Wx and controls the low amylose content in rice (Oryza sativa L.).

KEY MESSAGE: This manuscript reports the fine mapping of a novel QTL, qAC2 controlling the low amylose in rice. The action mechanism of the qAC2 is also investigated by the analysis of genetic interactions to Wx (a), Wx (b), du1, du2 and du3. Amylose content of the rice (Oryza sativa L.) endosperm greatly affects starch properties and eating quality of cooked rice. Seeds of japonica rice cultivar Kuiku162 have low amylose content (AC) and good eating quality. Our analysis revealed a novel QTL, designated as qAC2 that contributed to the low AC of Kuiku162. qAC2 was fine mapped within a 74.9-kb region between two insertion and deletion markers, KID3001 and KID5101, on the long arm of chromosome 2. Seven genes are predicted in this region, but none of them is known to be related to the regulation of AC. The AC of a near-isogenic line (NIL110) carrying qAC2 (Kuiku), the Kuiku162 allele of qAC2, in the genetic background of japonica cultivar Itadaki was lower by 1.1% points than that of Itadaki. The chain length distributions of amylopectin were similar in NIL110 and Itadaki; therefore, the low AC of NIL110 was caused by a decrease in the actual AC, but not by a difference in the amylopectin structure. The interaction analyses revealed that qAC2 (Kuiku) has epistatic interaction with Wx (a). The qAC2 (Kuiku) has epistatic interactions with two loci, du1 and du2, on Wx (b), whereas the genetic effect of qAC2 (Kuiku) has additive to that of du3 on Wx (b). Thus, similar to du1 and du2, qAC2 may have a function related to Wx (b) mRNA splicing.

qEMF3, a novel QTL for the early-morning flowering trait from wild rice, Oryza officinalis, to mitigate heat stress damage at flowering in rice, O. sativa.

A decline in rice (Oryza sativa L.) production caused by heat stress is one of the biggest concerns resulting from future climate change. Rice spikelets are most susceptible to heat stress at flowering. The early-morning flowering (EMF) trait mitigates heat-induced spikelet sterility at the flowering stage by escaping heat stress during the daytime. We attempted to develop near-isogenic lines (NILs) for EMF in the indica-type genetic background by exploiting the EMF locus from wild rice, O. officinalis (CC genome). A stable quantitative trait locus (QTL) for flower opening time (FOT) was detected on chromosome 3. A QTL was designated as qEMF3 and it shifted FOT by 1.5-2.0 h earlier for cv. Nanjing 11 in temperate Japan and cv. IR64 in the Philippine tropics. NILs for EMF mitigated heat-induced spikelet sterility under elevated temperature conditions completing flower opening before reaching 35°C, a general threshold value leading to spikelet sterility. Quantification of FOT of cultivars popular in the tropics and subtropics did not reveal the EMF trait in any of the cultivars tested, suggesting that qEMF3 has the potential to advance FOT of currently popular cultivars to escape heat stress at flowering under future hotter climates. This is the first report to examine rice with the EMF trait through marker-assisted breeding using wild rice as a genetic resource.

Protein disulfide isomerase like 1-1 participates in the maturation of proglutelin within the endoplasmic reticulum in rice endosperm.

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.

Soluble starch synthase I effects differences in amylopectin structure between indica and japonica rice varieties.

The effect of soluble starch synthase I (SSI) on differences of amylopectin structure between the indica and japonica rice varieties was investigated. Native-PAGE/active staining analysis showed that the SSI activity of an indica rice variety, "Kasalath", was significantly lower than that of a japonica rice variety, "Nipponbare", and that the low activity in "Kasalath" was maintained during seed development. The result of northern blot analyses suggests that the low expression of SSI in "Kasalath" is controlled at the transcription levels of SSI mRNA. Chain length distribution of amylopectin in F3 endosperms derived from a cross between two varieties showed that not only SSIIa but also SSI regulated the population of short chains. These results indicate that the low activity of SSI gives rise to the decrease of short chains in amylopectin of indica rice varieties, suggesting that SSI effects the differences in physicochemical properties between two varieties.