<i>Waxy</i> ( <i>Wx</i> ) gene, which encodes a granule-bound starch synthase I (GBSSI), promotes rapid seed germination and embryo growth in rice by enhancing starch-to-sugar conversion

Efficient seed germination and seedling establishment are critical for achieving high rice yields. This process is regulated by numerous metabolic pathways, including the mobilization of stored starch and the utilization of sugars. The Waxy (Wx) gene, which controls amylose content (AC) in rice grains, plays a key role in this process. Premature transcription termination of Wx determines whether the seed exhibits a waxy or non-waxy phenotype. While Wx is well established as a critical regulator of eating and cooking quality (ECQ) in rice, the present study identifies a novel role for Wx in mediating brassinosteroid (BR)-regulated seed germination. Analysis of Wx-related near-isogenic and transgenic lines revealed that seeds carrying the Wxa allele showed faster germination and superior post-germinative growth than those carrying the wx allele. This enhanced post-germinative growth was associated with increased Wx expression, which significantly boosted amylase activity and led to extensive starch degradation. Additionally, the higher expression and activity of α-amylase in Wxa seeds resulted in greater efficiency of starch-to-sugar conversion. In vitro embryo culture assays demonstrated that the Wxa allele and glucose acted synergistically to promote plumule growth. Moreover, differential Wx expression influenced ABA biosynthesis and catabolism in germinated seeds. Collectively, these findings suggest that appropriately modifying Wx expression to optimize starch composition could achieve the dual goal of improving both ECQ and germination-related traits in rice breeding.