Chloroplast development in green plant tissues: the interplay between light, hormone, and transcriptional regulation.

Chloroplasts are best known for their role in photosynthesis, but they also allow nitrogen and sulphur assimilation, amino acid, fatty acid, nucleotide and hormone synthesis. How chloroplasts develop is therefore relevant to these diverse and fundamental biological processes, but also to attempts at their rational redesign. Light is strictly required for chloroplast formation in all angiosperms and directly regulates the expression of hundreds of chloroplast-related genes. Light also modulates the levels of several hormones including brassinosteriods, cytokinins, auxins and giberellins, which themselves control chloroplast development particularly during early stages of plant development. Transcription factors such as GOLDENLIKE1&2 (GLK1&2), GATA NITRATE-INDUCIBLE CARBON METABOLISM-INVOLVED (GNC) and CYTOKININ-RESPONSIVE GATA FACTOR 1 (CGA1) act downstream of both light and phytohormone signalling to regulate chloroplast development. Thus, in green tissues transcription factors, light signalling and hormone signalling form a complex network regulating the transcription of chloroplast- and photosynthesis-related genes to control the development and number of chloroplasts per cell. We use this conceptual framework to identify points of regulation that could be harnessed to modulate chloroplast abundance and increase photosynthetic efficiency of crops, and to highlight future avenues to overcome gaps in current knowledge.