At the core of global bioenergy: polyamines and transglutaminases in chloroplasts.

Chloroplasts are the energy factories of photosynthetic life. The energy and the plant biomass of the planet depend on the activity of chloroplasts. Their efficiency results from a delicate balance between membrane organization, metabolic regulation and the maintenance of protein homeostasis. Among the molecular factors that support chloroplast structure and function, polyamines (PAs) and transglutaminases (TGases) have emerged as key regulators of photosynthetic performance and stress tolerance. PAs stabilize thylakoid membranes, influence proton-motive force partitioning, modulate chlorophyll biosynthesis and protect the photosynthetic apparatus from oxidative damage, thereby contributing to increased photoprotection and delayed senescence. TGases, particularly the plastid-localized isoforms, catalyze the covalent conjugation of PAs to stromal and thylakoid proteins, including RuBisCo and light-harvesting complexes (LHC). These reactions affect protein stability, supramolecular assembly, and the dynamic remodeling of LHC. Light-, pH- and redox-dependent activation of TGases links PA dependent protein modification to the regulation of photosynthetic efficiency and stress responses. This review integrates biochemical, structural and physiological evidence to highlight how PAs and TGases operate at the interface of light-to-chemical energy conversion. Both are involved in membrane organisation and maintaining protein quality. Understanding this molecular network provides new perspectives for improving plant performance, enhancing tolerance to abiotic stress and sustaining biomass production, notably that of agriculture interest, under changing environmental conditions.