Illuminating stomatal responses to red light: establishing the role of Ci-dependent vs -independent mechanisms

The stomatal response to red light appears to link stomatal conductance (gs) with photosynthetic rates. Initially, it was suggested that changes in intercellular CO 2 (Ci) provide the main cue via a Ci- dependent response. However, evidence for Ci-independent mechanisms suggests an additional, more direct relationship with photosynthesis. While both Ci-dependent and -independent mechanisms clearly function in stomatal red-light responses, little is known regarding their relative contribution. The present study aimed to quantify the relative magnitude of Ci-dependent and - independent mechanisms on the stomatal red-light response, to characterise their interplay and to assess the putative link between plastoquinone (PQ) redox state and Ci-independent stomatal responses. Red light response curves measured at a range of Ci values for wild-type Arabidopsis thaliana (Col-0) and the CO2 hyposensitive mutant, ca1ca4, allowed deconvolution of Ci-dependent and -independent pathways. Surprisingly, we observed that both mechanisms contribute equally to stomatal red-light responses, but Ci-independent stomatal opening is suppressed at high Ci. The present data are also consistent with the involvement of PQ redox in coordinating Ci-independent stomatal movements. Overall, it seems that while Ci-independent mechanisms are distinct from responses to Ci, interplay between these two pathways is important to facilitate effective coordination between gs and photosynthesis. HighlightBy controlling intercellular CO2 (Ci) at a range of values across red-light response curves, we demonstrate independent and interactive roles of Ci-dependent and -independent pathways in coordinating stomatal red-light responses.