Sage, R.F.: How Plants Sense, Signal, and Respond to Carbon Dioxide

Most effects of atmospheric CO2 increase arise indirectly as a result of photosynthetic stimulation, inhibition of respiration, or improved water use efficiency. Only the guard cells that control stomatal function are known to directly sense variation in CO2 and transduce it into a regulated response. High CO2 reduces stomatal aperture, while low CO2 increases it. The mechanism of this signal-transduction response remains unclear. At the level of the internal leaf biochemistry, the expression of only a few enzymes may be directly regulated by CO2 level. For example, carbonic anhydrase expression is reduced by high CO2, but the mechanism for this control is also unclear. Increasing CO2 promotes photosynthesis through its effect as a substrate in carbon fixation and as an inhibitor of photorespiration. Long-term (weeks to years) exposure to high CO2 increases leaf carbohydrate status, which in turn reduces photosynthetic gene expression, causing a decline in photosynthetic capacity. This reduction in photosynthetic capacity is the main acclimation response to high CO2, and is similar to acclimation responses to other environmental agents that promote carbohydrate accumulation. Factors that affect carbohydrate accumulation following transfer to high CO2 modulate the strength of the acclimation response. For example, low sink capacity and low nutrient status promote carbohydrate accumulation and a strong CO2 acclimation response. Carbohydrates status alone does not drive CO2 acclimation. Instead, it appears leaves must be primed to respond to high levels of carbohydrate, probably by long-distance signals such as ABAand cytokinins that are sent from the roots and other sinks. Identifying the mechanisms of interaction between carbohydrate status, plant hormones and other developmental signals is a major requirement for understanding how plants will respond to atmospheric CO2 enrichment.