MdATG5a-mediated autophagy improves apple performance under low light.

Apples are sun-loving plants, and excessive canopy closure in orchards leads to poor ventilation and reduced light penetration, ultimately limiting yield and fruit quality. Autophagy is a conserved mechanism for recycling cellular materials and energy in eukaryotes and is known to participate in multiple stress responses. However, whether autophagy contributes to low-light adaptation remains largely unclear. Here, we show that MdATG5a enhances the tolerance of apple plants to low-light conditions. Under low light, MdATG5a-overexpressing (OE) plants exhibited higher photosynthetic capacity, greater biomass, and increased carbohydrate accumulation compared with wild-type (WT) plants. During acclimation to low light, OE plants enhanced light harvesting and promoted the turnover of photosynthetic proteins by increasing leaf area and the leaf nitrogen content. Relative to WT, OE plants displayed a lower light compensation point and light saturation point, reduced dark respiration rate, higher net photosynthetic rate and stomatal conductance, and less structural and functional damage to chloroplasts and photosystem II. Metabolite profiling further revealed that OE plants had more stable levels of flavonoids and carbohydrates and their derivatives. OE plants also showed higher autophagic activity than WT plants, and pharmacological inhibition of autophagy abolished the growth advantage of OE plants under low light. Moreover, we found that MdATG5a is positively regulated by MdPIF7, a key transcription factor in shade-related light signaling. Together, these findings indicate that MdATG5a-mediated autophagy, regulated by MdPIF7, is crucial for the adaptation of apple to low light by enhancing autophagic flux, improving photosynthetic performance, and stabilizing metabolic homeostasis.