Unraveling the Nyctinastic movement of Oxalis triangularis 'Purpurea': the role of circadian clock, hormones, and Ca signaling.

Oxalis triangularis 'Purpurea' is an ornamental plant that exhibits nyctinastic movement. However, the underlying mechanisms remain unclear. The nyctinastic movement of the leaflets is regulated by a motor organ termed the pulvinus, in which the flexor cells and extensor cells adjust their osmosis potential antagonistically to drive the opening or closure of the leaflets. This study investigates the factors essential for the nyctinastic movement in O. triangularis 'Purpurea'. The critical structural and subcellular changes of the pulvini that facilitate the opening or closing movement of leaflets were revealed by sectioning and scanning electron microscope (SEM). The dynamic and significant changes of phytohormones [Auxin (IAA), Abscisic acid (ABA), Cytokinin (cis-Zeatin, cZ and trans-Zeatin, tZ), Gibberellin (GA), Salicylic acid (SA) and 1-aminocyclopropane-1-carboxylate (ACC, the ethylene precursor)] were detected during the leaflets opening or closing, and their contributions to nyctinastic movement were determined by exogenous application. Furthermore, dynamic but opposite Ca flux was observed in the flexor and extensor cells during leaflets opening or closure, and the Ca channel blocker disrupted normal leaflets movement. Transcriptomic analysis of the pulvini revealed key differentially expressed genes (DEGs) during leaflets opening and closing, and these DEGs are enriched in the function categories of circadian clock, hormone signaling, and ion channels. Notably, the core circadian clock gene, OtLHY, was demonstrated to be required for the nyctinastic movement of O. triangularis 'Purpurea' by virus-induced gene silencing (VIGS). Based on these findings, we propose a regulatory network involving the circadian clock, phytohormones, and ion channels that coordinate the nyctinastic movement of O. triangularis 'Purpurea'.