Circadian Regulation of Plant Physiology: A New Role for Humidity, the Forgotten Zeitgeber

Abstract

Air humidity is an environmental cue that influences many physiological responses in terrestrial organisms. In early chronobiology studies, it was thought that humidity could influence the circadian clock. However, no further studies have been performed in pursuit of this idea, nor humidity been investigated as a potential Zeitgeber, despite the fact that daily humidity fluctuation has been widely observed. Here, we report that humidity is a Zeitgeber on par with light and temperature and that it can fundamentally influence plant physiology. Using 10 years of weather records obtained from Harvard Forest and Weather Underground databases, we quantitatively characterized humidity oscillation as a natural phenomenon and found that the circadian oscillation of humidity (i.e., lower during the day and higher at night) is robust and independent of climates or seasons. Moreover, there is a 2-hour overlap of high humidity with light in the morning, suggesting a possible interplay between these two environmental cues and the circadian clock. We then tested and found that humidity can indeed entrain the circadian clock as a bona fide Zeitgeber through regulation of the plant core morning clock gene CCA1 (CIRCADIAN CLOCK ASSOCIATED 1) and the core evening clock gene TOC1 (TIMING OF CAB EXPRESSION 1). However, the core morning clock gene LHY (LATE ELONGATED HYPOCOTYL), partner of CCA1, resists the humidity-entrainment, indicating that the plant circadian clock may have two intertwined loops: one is responsive to both light and humidity via CCA1, whereas the other is responsive only to light mediated by LHY. In the absence of rhythmic light signal, humidity can entrain the clock, whereas in the presence of light in the morning, humidity synergistically reinforces the clock. At the physiological level, the humidity-entrained clock provides additional advantages to plants. It can improve energy storage, reduce the flowering time and improve seed production in a circadian-dependent manner. Additionally, we found that humidity can fundamentally shape plant-microbe interactions because it is a key signal for the initial induction of the bacterial type III secretion system and effector genes required for virulence. Thus, it is necessary for plants to mount an appropriate response to pathogen infections at night when humidity naturally rises. These findings demonstrate how terrestrial organisms could adapt to humidity oscillation, a universal environmental cue, to strategically maximize their own fitness. We propose that humidity-sensing might be an ancient determinant shaping the evolution of terrestrial organisms after their transition away from aquatic environment.