Abstract:

Aims As anisohydric plants lacking vascular tissue, mosses are thought to be influenced on their water movement and photosynthesis mainly by the water status and light of the ambient environment. Do the mosses develop an intrinsic circadian regulation mechanism similar to that of vascular plants as these environmental conditions change with day and night alteration? Understanding its rhythmic response characteristics is of great significance for the conservation and utilization of the mosses.

Methods The Brachythecium thraustum, Hypnum plumaeforme and Mnium lycopodioides were trained to 12 h/12 h light-dark (LD) cycles before transfer to 24 h/0 h constant light. We measured the three mosses’ net photosynthetic rate (P_n) and transpiration rate (T_r) on the constant light and last day of the light-dark cycles. Subsequently, non-structural carbon (NSC) content was measured for Brachythecium thraustum. Finally, we sequenced the Brachythecium thraustum transcriptomes in a time series in constant light conditions.

Important findings We found the significant rhythmic changes in the P_n and T_r of the three mosses when excluding environmental influences such as light and vapor pressure difference (VPD), and rhythmic regulation could explain 23.4% and 30.2% of the diurnal changes in P_n and T_r. Moreover, the rhythmic effect of T_r, which is rarely observed in vascular plants, appeared in all the mosses tested, and the rhythmic responses of P_n and T_r were different among species. The species with the strongest rhythm in this study was Brachythecium thraustum, and it was further analyzed for the rhythmic responses of time-series transcriptome and its NSC content under full light. The results showed that 35.1% of starch and carbohydrate biosynthesis-related genes showed rhythmic expression, which was closely related to NSC content change. In addition, the rhythmic regulation of T_r was associated with the transcript changes of abscisic acid signaling and stomatal regulation. Furthermore, the photosynthesis rhythms were associated with transcript changes of photosynthesis-related protein, like photosynthetic electron transport and carbon sequestration proteins. Although the CCA1/LHY genes, the core feedback loop of the circadian clock, have been lost in the mosses, the core circadian clock function concentrates in the PRRs family. The rhythmic response to carbon sequestration and water consumption is highly conserved.

Key words: circadian rhythm, transcriptome, photosynthesis, transpiration, stomata, Brachythecium thraustum, Hypnum plumaeforme, Mnium lycopodioides