Endosperm is the major tissue for seed reserve accumulation in rice grains and serves as a nutrient source for seed germination and seedling establishment. Starch and seed storage proteins (SSPs) are main component of endosperm storage material. Rice endosperm is occupied by homogeneous polygonal starch granules from inside to outside and these starch granules are composed of amylose (linear a-1,4-polyglucans) and amylopectin (a-1,6-branched polyglucans), which are synthesized in amyloplast (specialized plastids contained in endosperm) (She et al., 2010; Wang et al., 2017). Enzymes involved in starch biosynthesis are regulated by RSR1, FLO2, MADS6, BP-5, EBP-89, RIPBF and RISBZ1 transcription factors. Glutelins are the major SSPs accumulated in rice endosperm account to approximately 60?80% of the total proteins in the endosperm. Rice glutelins are synthesized at the endoplasmic reticulum (ER) as precursors (pro-glutelins), and are transported to protein storage vacuoles, where they are processed into mature proteins(Ren et al., 2014).
