An endogenous basis for synchronization manners of the circadian rhythm in proliferatingLemna minorplants

Endogenous circadian rhythms in plants play a role in adaptation to day-night cycles. The circadian clock is a cell-autonomous system that functions through the coordination of time information in the plant body. Synchronization of cellular clocks is based on coordination mechanisms; the synchronization manners in proliferating plants remain unclear. We performed spatiotemporal analysis of the circadian rhythm of fronds (leaf-like plant units) of proliferating Lemna minor plants carrying a circadian bioluminescence reporter, AtCCA1:LUC . Noninvasive observations of the bioluminescence of fast-growing two-dimensional plants allowed us to analyze the circadian rhythms at a cell-level resolution and obtain information regarding frond lineage. We focused on spontaneous circadian organization under constant light conditions for plants with light/dark treatment (LD-grown) or without it (LL-grown). Even fronds developing from an LL-grown parental frond showed coherent circadian rhythms among them. This allowed the maintenance of circadian rhythmicity in proliferating plants. Inside a frond, a centrifugal phase/period pattern was observed in LD-grown plants, whereas various phase patterns with traveling waves were formed in LL-grown plants. These patterns were model-simulated by local coupling of cellular circadian oscillators with different initial synchronous states in fronds. Taken together with similar patterning previously reported for detached leaves of Arabidopsis , it is strongly suggested that local coupling is the primary force for the development of these phase patterns in plants lacking long-distance communication. We propose a basic framework of spontaneous phase patterning with three stages of circadian organization: initial phasing, evolution of patterning, and desynchronization/randomizing of phase, in association with altering cell-cell coupling.