Minisymposium 15: In Response to Water

Abs # 27002: Circadian regulation of Ca²⁺ signalling and stomatal guard cells

Presenter:	Dodd, Antony N, antony.dodd@plantsci.cam.ac.uk
Authors	Dodd, Antony N (A)   Love, John  (A)   Webb, Alex AR (A)
Affiliations:	(A): Department of Plant Sciences, University of Cambridge
Web Site:	http://www.plantsci.cam.ac.uk/Webb/home.html

Plants generate an internal estimate of the time of day without reference to external cues, because they are equipped with an endogenous circadian clock. Circadian clocks are believed to optimise and synchronise aspects of physiology with the external 24 h cycle. Circadian [Ca²⁺]_cyt oscillations¹ could function as a clock input, a clock output, or a component of the core oscillator. We are examining the role of circadian [Ca²⁺]_cyt oscillations within the guard cell, due to the highly-characterised nature of Ca²⁺-based signalling in the guard cell, and because stomata exhibit circadian patterns of opening and closure. The position of circadian [Ca²⁺]_cyt oscillations within the circadian signalling network is being established using a combination of mutant analysis, targeted protein misexpression and pharmacological interference with Ca²⁺ signalling. We have developed a novel infra-red gas analyser that can accurately measure circadian rhythms of stomatal conductance and CO₂ assimilation in Arabidopsis. The instrument incorporates multiple whole-plant gas exchange cuvettes, and the environmental conditions of each channel are independently controlled to within 1 % error. By examining lines with genetically perturbed clocks, we have discovered that the circadian clock plays a critical role in the optimisation of water loss and net CO₂ assimilation. Overexpression of the clock-associated gene CCA1² abolishes the circadian rhythm of stomatal opening and CO₂ assimilation. Diurnal gas exchange in CCA1-ox is compromised by the failure of stomatal opening to predict dawn, excessive stomatal opening during the light period, and reduced net CO₂ assimilation, compared to the wild-type. This clock-stopped line exhibited poor efficiency of water use compared to the wild type. The rates of biomass accumulation and growth were compromised in several lines with perturbed circadian clocks. Our data indicate that it is advantageous for a plant to possess i) a functional circadian clock, and ii) a clock with a free-running period near to 24 h. This work was supported by the BBSRCUK, the Royal Society of London and the Isaac Newton Trust, whom we gratefully acknowledge. ¹ Johnson CH, Knight MR, Kondo T, Masson P, Sedbrook J, Haley A, Trewavas A. Science 269 (1995) pp1863-1865. ² Wang ZY, Tobin EM. Cell 93 (1998) pp1207-1217.