Light and the control of chloroplast development in Arabidopsis

Plants require light as their energy source for growth, and therefore need to optimise light capture. They do so by sensing their light environment and modifying their development accordingly. In flowering plants neither leaves, the ‘solar panels’, nor chloroplasts, the photosynthetic organelles or ‘solar cells’, develop in the dark in the first place. Specialised light sensors, belonging to the phytochromes, cryptochromes and phototropins groups, are present in plant cells. Our work aims at understanding how the development of chloroplasts and leaves is controlled by light, in the hope of obtaining clues to boost the processes and consequently plant performance.

One stream of our work focuses on how photoreceptors control the expression of genes for photosynthetic proteins, the building blocks of chloroplasts. We are using a combined genetic and molecular approach in the model plant species Arabidopsis, whose genome is fully sequenced. Chloroplasts belong to a broader class of organelle called plastids. Plastids themselves need to be functional for photosynthetic nuclear genes to be expressed. This implies the existence of a "plastid-functionality signal" which carries information from plastids to the nucleus of plant cells, where gene expression takes place. We, among others, obtained evidence that implies a class of chloroplast metabolites, the chlorophyll precursors, as plastid signals. We are attempting to ascertain whether other classes of plastid signals exist, and whether they can lead us to central ‘plastid development’ regulators. Novel genes involved in chloroplast biogenesis are being sought.

In another stream, our laboratory is exploring the control mechanisms of leaf development in the light. To form a leaf, cells need to proliferate, grow and eventually achieve their final form and function. Plants are always simultaneously old and young– they mature in adult regions while continuing to grow in the shoot apices or ‘meristems’; sites which have been described as ‘stem cells that make stems’. In meristems, some cells divide and differentiate, while others nearby remain embryonic to replenish the former. Their light control has allowed us to observe some of the earliest processes of leaf inception, including the entry into cell proliferation and growth. We are using ‘transcriptomics’ approaches, in which the transcript level or expression of the majority of individual genes (around 28,000 in Arabidopsis) is measured in parallel. We have observed the unfolding of cell expansion, differentiation and chloroplast development, as well as a wealth of regulatory processes, many of them involving the action of plant hormones, including auxin and cytokinin. The interrelatedness of those processes requires our work, as experimentalists, side-by-side with that of computer scientists to attempt to unravel the underlying leaf development regulatory networks.

Presence or absence of light and "plastid-functionality signals" are not the only regulators of chloroplast and leaf development. The quantity of light that the leaf experiences also affects both. Under intense light chloroplasts boost their light utilisation capacity and minimise the risk of excess, damaging light absorption, while leaves develop extra cell layers that create internal shading and allow extra CO2 fixation per unit area. This light quantity sensor may be related to chloroplast function (be a photosynthetic signal) or be a separate photoreceptor. Our evidence indicates both types of light quantity sensor actually occur. We are trying to understand how each of these signals act.