NnSBE1 encodes a starch branching enzyme involved in starch biosynthesis in lotus seeds

Lotus seed starch holds vast potential for utilization across various industries, with its content and structure directly influencing the commercial value of lotus seeds. However, there has been limited information available on the molecular mechanisms underlying lotus seed starch biosynthesis. In this study, three starch branching enzyme homologs were identified in the lotus genome, designated as NnSBE1 to NnSBE3, which possess conserved CBM_48 and α_Aamy domains. Among them, NnSBE1 exhibited predominant expression, with abundant transcript levels observed in lotus seeds and flower-related organs. Expression of NnSBE1 remained consistently up-regulated in lotus cotyledons from 6 to 21 days after pollination. Additionally, a C2H2-type finger protein encoding gene, NnLOL1, co-expressed with NnSBE1 in lotus cotyledons. As a seed-predominantly expressed transcription factor, NnLOL1 was confirmed to activate NnSBE1 expression. Transient overexpression of NnSBE1 in lotus cotyledons resulted in a significant increase in both amylopectin and total starch content compared to the control. Furthermore, multiple variation sites within the NnSBE1 gene gave rise to diverse haplotypes between seed-lotus and other lotus varieties. These findings contribute to our understanding of the regulation mechanisms involved in lotus seed starch biosynthesis, offering valuable theoretical insights for the genetic improvement of lotus seed starch by molecular breeding strategies.