Plant Responses to Drought Stress in Future Climate CO2: A Multi-level Analysis

Abstract

Future climate scenarios indicate increasing CO2 levels in combination with more frequent drought spells. Recent studies suggest that elevated CO2 reduces the effect of drought stress, but the mechanisms underlying the reduced stress effects remain unclear. Therefore we investigated the growth, physiological, biochemical and genome-wide transcriptional responses of Arabidopsis thaliana to mild (MD) and severe drought (SD) under ambient (aCO2) and elevated CO2 (eCO2), 360 and 620ppm respectively. Kinematic analysis showed that drought reduced leaf growth by inhibiting cell division and expansion. High CO2 and drought showed opposite and interactive effects on growth. Where, eCO2 reduced the impact of MD on growth by limiting the effect on both cell number and size, and the impact of SD by reducing the effect on cell size only. Genome-wide transcriptional analysis showed that growth, photosynthesis and defense processes are more strongly affected by SD as compared to MD. These results were mirrored at the growth, physiological and biochemical levels, where SD inhibited photosynthesis and stomatal conductance and induced defense related parameters. Accumulation of antioxidants and osmolytes were not sufficient to completely recover the SD damage impact. In contrast, in SD conditions in particular, eCO2 significantly mitigated the detrimental effects of drought stress. The transcriptome and metabolic analyses did not implicate the antioxidant defense metabolism in protective effect of eCO2, with down regulation or no significant impact on most of measured antioxidant parameters. On the other hand, we found that eCO2 alleviated photorespiration; a stress-induced H2O2 generation process thus, the relaxation in antioxidant system could be a consequence of less oxidative pressure. Further induction of osmolytes was also observed and that could be associated with improved leaf relative water content under the drought and eCO2. We therefore concluded that elevated CO2 mitigates the effects of drought by maintaining plant turgor and reduced hydrogen peroxide production.