Integration of signals inducing reproductive phase transition occurs at the nodal enlarged vascular bundles in sacred lotus, Nelumbo nucifera.

Abstract

Sacred lotus (Nelumbo nucifera) is a perennial aquatic plant classified into basal eudicots, propagating through both sexual (flowering) and vegetative (clonal) reproduction. In lotus, the transition from sexual to clonal reproductive phases (swelled rhizome formation) is induced by short-day (SD) photoperiods, but the molecular mechanism remains unclear. Recent studies have shown that signals inducing reproductive phase transitions, such as flowering and storage organ formation, are integrated into the regulation of FLOWERING LOCUS T/TERMINAL FLOWER 1 (FT/TFL1) family gene expression. Elucidating the expression dynamics of lotus FT/TFL1 genes is expected to deepen our understanding of the molecular mechanisms underlying different reproductive modes. In this study, we identified candidate FT/TFL1 genes involved in growth phase transition in lotus through spatiotemporal expression analyses and transgenic assays using Arabidopsis. Interestingly, clear correlations between the phase transition and expression changes of putative inducers of flowering and rhizome swelling, NnFT2 and NnFT3, were observed not in leaves, but in underground tissues, including nodes and internodes of the rhizome. NnBFT1, a putative floral inhibitor in lotus, may mediate the transition from sexual to clonal reproduction by suppressing floral development through SD-responsive upregulation. Furthermore, histological observations revealed that lotus possesses enlarged vascular bundles within its nodes, similar to those reported only in some species of monocots. Our results suggest that this distinctive tissue is where the crucial regulation of FT/TFL1 gene expression occurs. This study sheds light on the unique mechanism of reproductive phase transition in lotus and the novel function of nodes in plant propagation.