Auxin metabolism and the modulation of plant growth

In the face of global climate change, several unprecedented challenges are currently faced by agriculture. To achieve food security, understanding the developmental program from seed formation and germination, through early seedling establishment until plant growth and crop yield, is required to increase agricultural production and ensure sustainability. Natural auxin, a heterogeneous group of aromatic carboxylic acids, is one of the most important plant hormones, mediating several endogenous developmental signals and exogenous environmental cues that profoundly affect virtually all plant growth and development processes. There must be a balance in endogenous auxin dynamics between synthesis, influx, efflux, degradation, receptor binding, and downstream signaling to modulate plants responses. While the genes and biochemical reactions for endogenous auxin metabolism are well understood, the involvement of auxin in plant central metabolism (e.g. photosynthesis and respiration) remains poorly known. Nevertheless, it is already known that endogenous auxin acts as the main epigenetic regulator responsible for mesophyll cell expansion and thus, indirectly, for photosynthesis. Furthermore, endogenous auxin response factors have been identified that mediate sugar and starch metabolism, as well as abiotic stress tolerance, indicating that auxin should be further explored as a key molecule to improve plant performance under normal and stressful conditions in crops. Here, we summarize recent advances in dissecting auxin metabolism, their importance on central metabolism, and discuss the functions of endogenous auxin in the overall control of plant growth. We further provide an overview of the pivotal role of endogenous auxin and how mutations in different auxin signaling modulate photosynthetic and respiratory processes, which is likely crucial for coordinating cellular responses.