GBSS‐BINDING PROTEIN, encoding a CBM48 domain‐containing protein, affects rice quality and yield

Abstract

The percentage of amylose in the endosperm of rice (Oryza sativa) largely determines grain cooking and eating qualities. Granule‐bound starch synthase I (GBSSI) and GBSSII are responsible for amylose biosynthesis in the endosperm and leaf, respectively. Here, we identified OsGBP, a rice GBSS‐binding protein that interacted with GBSSI and GBSSII in vitro and in vivo. The total starch and amylose contents in osgbp mutants were significantly lower than those of wild type in leaves and grains, resulting in reduced grain weight and quality. The carbohydrate‐binding module 48 (CBM48) domain present in the C‐terminus of OsGBP is crucial for OsGBP binding to starch. In the osgbp mutant, the extent of GBSSI and GBSSII binding to starch in the leaf and endosperm was significantly lower than wild type. Our data suggest that OsGBP plays an important role in leaf and endosperm starch biosynthesis by mediating the binding of GBSS proteins to developing starch granules. This elucidation of the function of OsGBP enhances our understanding of the molecular basis of starch biosynthesis in rice and contributes information that can be potentially used for the genetic improvement of yield and grain quality.

A Review of Starch Biosynthesis in Relation to the Building Block-Backbone Model

“…Loss of PTST1 in Arabidopsis results in loss of GBSSI from the starch granules and a waxy phenotype [ 183 ], and its conservation throughout the plant Kingdoms argues for a critical role in amylose biosynthesis. However, studies in barley and rice endosperm suggest the impact of loss of PTST1 is greatest in chloroplasts, since effects on amylose synthesis in endosperm amyloplasts were highly variable [ 184 , 185 ].…”

The A to B of starch granule formation in wheat endosperm

“…PTST1-like proteins have been studied in Arabidopsis leaves (Seung et al 2015), cassava roots (Bull et al 2018) as well as, barley and rice endosperm (Wang et al 2019; Zhong et al 2019). In Arabidopsis, rice and barley, these proteins have been shown to interact with the amylose synthesizing GRANULE BOUND STARCH SYNTHASE (GBSS).…”

“…In Arabidopsis, rice and barley, these proteins have been shown to interact with the amylose synthesizing GRANULE BOUND STARCH SYNTHASE (GBSS). Mutations of this protein in Arabidopsis (Seung et al 2015), cassava (Bull et al 2018) and rice (Wang et al 2019) reduce amylose amounts in starch, while barley ptst1 mutants do not accumulate endosperm starch (Zhong et al 2019).…”

Amylose in starch: towards an understanding of biosynthesis, structure and function

“…It seems reasonable that GBSS is more dependent on PTST1 to locate starch granules in chloroplasts, where granules are dispersed through the stroma, than in endosperm amyloplasts, where granules occupy the majority of the amyloplast volume. Also, although both rice GBSS1 and GBSS2 isoforms can interact with PTST1 (Wang et al ., 2019), it is possible that GBSS1 has other features that allow it to associate with starch in the absence of PTST1. In stark contrast to the results in rice, CRISPR/Cas9‐generated knockout mutants of PTST1 in barley produced nonviable seeds with no starch in the endosperm (Zhong et al ., 2019).…”

“…Currently, the role of PTST1 in cereal endosperms is not clear. A recent study in rice used CRISPR/Cas9 to create knockout mutants of PTST1 (also referred to as Os GBP by the authors) (Wang et al ., 2019). Like the Arabidopsis mutant, the rice mutants produced amylose‐free starch in leaves and had undetectable levels of GBSS2.…”