Rice endosperm occupies most of the space in rice seed. Many genetic regulators related to seed size control also determine endosperm size directly or indirectly (Zhou et al., 2013). Seed size is composed of three elements: seed length, width and thickness. miRNAs play important role in controlling rice seed size and panicle branching by targeting several transcription factors, e.g; GRFs and SPL genes. Three miRNAs, miR396, miR397 and miR156 are involved in this process (Tang et al., 2017). miRNA 156 targets SPL genes (SPL13, SPL14, SPL16), which inturn regulates various other downstream genes. miR397 regulated brassinosteroid mediated regulation of grain length and width via LAC gene. miR396 regulate growth regulating factors (GRF6 and GRF4), which controls panicle auxin mediated branching by controlling the expression of ARFs, MADS-box and TAW1 genes (Tang et al., 2017). in addition to miRNAs regulatory module, seed size is controlled by polygenes, and numerous QTLs associated with seed length, weight, thickness and seed filling. GS3, GW2, and GW5 are major negative regulators of seed size. GS3 is major loci for seed length and weight buit minor loci for seed width and thickness (Zhou et al., 2013). GW2, a RING-type protein with E3 ubiquitin ligase activity controls rice grain width and weight (Song et al., 2007). GW5 interacts with polyubiquitin and deletion of GW5 results in a significant increase in sink size owing to an increase of cell number (Weng et al., 2008). GIF1, FLO2, GS5 and GW8 are positive regulators of grain size. GIF1 encodes a cell-wall invertase, which is required for carbon partitioning during early grain filling, and enhanced expression of which results in increased grain size and yield (Wang et al., 2008). FLO2 is a tetratricopeptide repeat motif-con- taining protein and regulates grain size and starch quality by affecting the accumulation of storage substances in endosperm (She et al., 2010). GS5, a serine carboxypeptidase, is a positive regulator of grain size (Li et al., 2011). OsSPL16 is a positive regulator of cell proliferation and enhanced expression of which promotes cell division and grain filling, resulting in increased grain width and yield (Wang et al., 2012). OsWRKY78 also regulates rice endosperm size by affecting cell elongation (Zhang et al., 2011).
