Rice (Oryza sativa) grain shape, which is controlled by quantitative trait loci (QTL), has a strong effect on yield production and quality. However, the molecular basis for grain development remains largely unknown. In this study, we identified a novel QTL, Slender grain on chromosome 7 (SLG7), that is responsible for grain shape, using backcross introgression lines derived from 9311 and Azucena. The SLG7 allele from Azucena produces longer and thinner grains, although it has no influence on grain weight and yield production. SLG7 encodes a protein homologous to LONGIFOLIA 1 and LONGIFOLIA 2, both of which increase organ length in Arabidopsis. SLG7 is constitutively expressed in various tissues in rice, and the SLG7 protein is located in plasma membrane. Morphological and cellular analyses suggested that SLG7 produces slender grains by longitudinally increasing cell length, while transversely decreasing cell width, which is independent from cell division. Our findings show that the functions of SLG7 family members are conserved across monocots and dicots and that the SLG7 allele could be applied in breeding to modify rice grain appearance.KEYWORDS rice; quantitative trait loci; grain shape; cell elongation R ICE (Oryza sativa L.) is a staple food for half of the world's population (Khush 2001). Three major components, panicle number per plant, grain number per panicle, and grain weight, determine rice yield production. Grain weight is associated with grain size and shape, which are defined as grain length, grain width, and grain thickness (Duan et al. 2014). There is a striking diversity of grain size among the rice species worldwide. The grains of domesticated rice range from 3 to 11 mm in length and from 1.2 to 3.8 mm in width (Fitzgerald et al. 2009). Despite the influence of several environmental factors on plant growth and development, such as water supply and fertilizer level, the final grain size of rice is reasonably constant within a given species.Rice grain traits are quantitatively inherited. In the past decade, several quantitative trait loci (QTL) controlling grain size and shape have been cloned. GS3, encoding a transmembrane protein containing four putative domains, was the first characterized QTL that regulates grain length (Fan et al. 2006). qGL3 encodes a putative protein phosphatase with a Kelch-like repeat domain, and an aspartate-to-glutamate transition in the second Kelch domain leads to a long-grain phenotype (Zhang et al. 2012). GW6 encodes a GNAT-like protein that harbors intrinsic histone acetyltransferase activity, and an elevated expression enhances grain length and weight by enlarging spikelet hulls and accelerating grain filling (Song et al. 2015). GW2, GW5/qSW5, GS5, and GW8 were identified as regulators of rice grain width. GW2 encodes a previously unknown RING-type protein with E3 ubiquitin ligase, which negatively regulates cell division by degrading its substrate(s) through the ubiquitin-proteasome pathway (Song et al. 2007). GW5/qSW5 encodes a nuclearlocated protein t...
Background The heterotrimeric G protein β subunit RGB1 plays an important role in plant growth and development. However, the molecular mechanisms underlying the regulation of rice growth by RGB1 remain elusive. Results Here, the rgb1 mutants rgb1–1 (+ 1 bp), rgb1–2 (− 1 bp), and rgb1–3 (− 11 bp) were isolated using the CRISPR/Cas9 system, and they were arrested at 1 day after germination and ultimately exhibited seedling lethality. The dynamic anatomical characteristics of the embryos of the rgb1 seedlings and WT during early postgermination and according to TUNEL assays showed that the suppressed growth of the rgb1 mutants was caused by cell death. In addition to the limited shoot and root development, the development of the embryo shoot-root axis was suppressed in the rgb1 mutants. RGB1 was expressed mainly in the root epidermal and vascular tissues of the embryo. Moreover, transcript profiling analysis revealed that the expression of a large number of auxin-, cytokinin-, and brassinosteroid-inducible genes was upregulated or downregulated in the rgb1 mutant compared to the wild type during seedling development. Conclusions Overall, the rgb1 mutants provide an ideal material for exploring the molecular mechanism underlying rice seedling formation during early postgermination development by G proteins. Significance statement The heterotrimeric G protein β subunit RGB1 acts as a crucial factor in promoting early postgermination seedling development in rice. Electronic supplementary material The online version of this article (10.1186/s12284-019-0313-y) contains supplementary material, which is available to authorized users.
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