Distinct leaf developmental and gene expression responses to light quantity depend on blue-photoreceptor or plastid-derived signals, and can occur in the absence of phototropins.

Leaf palisade cell development and the composition of chloroplasts respond to the fluence rate of light to maximise photosynthetic light capture while minimising photodamage. The underlying light sensory mechanisms are probably multiple and remain only partially understood. Phototropins (PHOT1 and PHOT2) are blue light receptors regulating responses which are light quantity-dependent and which include the control of leaf expansion. Here we show that genes for proteins in the reaction centres show long-term responses in wild type plants, and single blue photoreceptor mutants, to light fluence rate consistent with regulation by photosynthetic redox signals. Using contrasting intensities of white or broad-band red or blue light, we observe that increased fluence rate results in thicker leaves and greater number of palisade cells, but the anticlinal elongation of those cells is specifically responsive to the fluence rate of blue light. This palisade cell elongation response is still quantitatively normal in fully light-exposed regions of phot1 phot2 double mutants under increased fluence rate of white light. Plants grown at high light display elevated expression of RBCS (for the Rubisco small subunit) which, together with expected down-regulation of LHCB1 (for the photosynthetic antenna primarily of photosystem II), is also observed in phot double mutants. We conclude that an unknown blue light photoreceptor, or combination thereof, controls the development of a typical palisade cell morphology, but phototropins are not essential for either this response or acclimation-related gene expression changes. Together with previous evidence, our data further demonstrate that photosynthetic (chloroplast-derived) signals play a central role in the majority of acclimation responses.