Cryptic signals: substomatal architecture influences stomatal responses to red light and CO2

The mesophyll provides a critical signal for stomatal responses to red light (RL) and CO2 in angiosperms. By contrast, the stomatal response to blue light (BL) is largely guard cell-specific. It is not known whether substomatal or mesophyll anatomy influences the effectiveness of the mesophyll signal driving stomatal responses to RL and CO2. Here we utilize the diverse substomatal anatomy in Restionaceae to investigate whether mesophyll anatomy has an influence on stomatal responses to light and CO2. Restionaceae from the subfamily Restionoideae have distinctive nonphotosynthetic, cuticle-covered protective cells that line a large substomatal cavity, while most species from the subfamily Leptocarpoideae have a small substomatal cavity surrounded by mesophyll cells. We found that representative Restionoideae species do not have a stomatal response to RL or CO2, with only BL driving stomatal responses to light. By contrast, representative Leptocarpoideae species have a stomatal response to RL, BL and CO2. The absence of stomatal responses to RL and CO2 in species with protective cells lining the substomatal cavity demonstrates the importance of mesophyll anatomy in stomatal control and suggests that the mesophyll signal that drives stomatal responses to RL and CO2 requires close proximity of photosynthetic cells to the guard cells.