Crosstalk Between Auxin Signalling and <scp>Growth Regulating Factor</scp> 5: Implications for Chloroplast Division and Leaf Development

Chloroplasts are the sites of photosynthesis but also host essential metabolic and biosynthetic pathways; therefore, the regulation of the chloroplast population is of crucial importance for the viability of the plant. Chloroplast division is closely linked to leaf development, but the coordination of cell expansion, division, and chloroplast multiplication at the molecular level is still poorly understood. Auxin signalling influences leaf growth and may also mediate chloroplast biogenesis and proliferation. Most studies focused on auxin and the development of chloroplasts in the fruit, emphasising the need for further research on leaf tissue. Overexpression of Growth Regulating Factor 5 (35S:GRF5) increases cell and chloroplast division in Arabidopsis thaliana, resulting in larger leaves with more chloroplasts per cell. In this study, we utilised 35S:GRF5 plants as a model to identify auxin control points that regulate chloroplast division. By examining the impact of changes in auxin homeostasis on chloroplast division and mesophyll cell size and by analysing crosses with selected auxin homeostasis genes, we found that reactive oxygen species-auxin crosstalk influences chloroplast multiplication during the cell expansion phase. Evidence indicates that GRF5 modulates auxin responsiveness by regulating the expression of UDP-glucosyltransferase UGT74E2 and PIN-LIKES3/5, which are key players in intracellular auxin homeostasis. These findings provide potential targets for modulating chloroplast abundance to improve photosynthetic efficiency in crops and highlight key areas for further research.