EXO70B1 Modulates Dark-Induced Leaf Senescence in an Age-Dependent Manner Associated with NYE1-Dependent Chlorophyll Catabolism

Dark-induced senescence (DIS) is a coordinated physiological process associated with chlorophyll degradation, macromolecular turnover, and nutrient remobilization under prolonged darkness. EXO70B1, a subunit of the exocyst complex, has been implicated in intracellular membrane trafficking, autophagy-associated vacuolar transport, and salicylic acid-dependent immunity. However, whether EXO70B1 contributes to DIS remains unknown. Here, we show that EXO70B1 expression increases with leaf age and is transiently induced during the early phase of dark treatment. Accordingly, as a consequence of loss of EXO70B1, an acceleration of dark-induced leaf yellowing, chlorophyll degradation, and decline in photosynthetic performance was observed. Notably, this hypersensitivity was strongly age-dependent, being evident in mature (4-week-old) plants but not in younger plants. Genetic analyses indicated that the accelerated chlorophyll degradation in the EXO70B1 mutant background depends on NYE1 function. To investigate the molecular basis underlying this age-specific transition, we performed stage-resolved transcriptomic profiling, which identified the 4-week stage as a major point of divergence between Col-0 and exo70b1-1. Before visible necrosis, mature exo70b1-1 leaves displayed substantial transcriptional reprogramming, including enrichment of salicylic acid (SA) signaling, systemic acquired resistance (SAR), and other defense-related pathways. Collectively, our findings support a role for EXO70B1 as an age-dependent modulator of DIS and indicate that the enhanced dark sensitivity of mature exo70b1-1 leaves is associated with defense-related transcriptional reprogramming and NYE1-dependent chlorophyll degradation.