An NnSnRK1-centered regulatory network of shade-induced early termination of flowering in lotus

The flowering process of lotus is highly sensitive to environmental conditions such as canopy shade and cloudy days, which often lead to abortion of the flower buds. However, the underlying mechanisms remain largely unknown. Based on transcriptome analysis and genetics screens, we found that silencing of SNF1-related protein kinase 1 (NnSnRK1) gene in lotus could promote flowering and reduce flower bud abortion under carbon starvation induced by 70% shade. Transcriptome analysis of NnSnRK1-overexpressed or -silenced lotus revealed that DEGs were mainly associated with ROS homeostasis, cell enlargement, and autophagy processes. Excessive autophagy levels and cell death were detected in the aborting flower buds. We further used NnSnRK1 as the bait to screen the yeast two-hybrid library constructed with RNA isolated from mixed lotus flower buds, and identified 14 putative interacting proteins of NnSnRK1. Two novel negative regulators of NnSnRK1, mediator of ABA regulated dormancy1 (NnMARD1) and 4-coumarate-CoA ligase-like 5 (Nn4CLL5), were identified. Overexpression of NnMARD1 and Nn4CLL5 could promote flowering of lotus under 70% shade, while silencing of them further inhibited flowering. Our results revealed a NnSnRK1-centring regulatory network that produced aborted flower buds during carbon starvation, which could improve reproductive success, and act like an energy-efficient measure of plants.