Light fluence rate and chloroplasts are sources of signals controlling mesophyll cell morphogenesis and division

As part of the acclimation of the photosynthetic apparatus to high fluence rates of light, mesophyll (photosynthetic) leaf cells change in morphology (they elongate anticlinally or perpendicular to the leaf surface) and undergo extra cell divisions. This results in increased leaf thickness and internal, protective shading among chloroplasts. Here we have examined whether the chloroplasts themselves are sources of intracellular signals that trigger these changes, by monitoring the Arabidopsis thaliana chm1 variegated mutant, in which albino (chloroplast-defective) and green (with functional chloroplasts) sectors coexist in one leaf. Our results have uncovered two separable responses. The increase in mesophyll cell elongation was substantially reduced but still observable in albino sectors, indicating that chloroplasts contribute to the cell morphogenesis response, but a chloroplast-independent light sensory mechanism must exist. In contrast the change in number of mesophyll cell layers was completely abolished when plastids were dysfunctional, indicating that plastids are sole sources of signals for the cell division response. These data highlight the importance of plastid-derived signals in the cellular responses associated with photosynthetic acclimation.