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

Dark-induced senescence (DIS) is a coordinated physiological process associated with chlorophyll degradation, macromolecular turnover, and nutrient remobilization under prolonged darkness. <i>EXO70B1</i>, a subunit of the exocyst complex, has been implicated in intracellular membrane trafficking, autophagy-associated vacuolar transport, and salicylic acid-dependent immunity. However, whether <i>EXO70B1</i> contributes to DIS remains unknown. Here, we show that <i>EXO70B1</i> expression increases with leaf age and is transiently induced during the early phase of dark treatment. Accordingly, as a consequence of loss of <i>EXO70B1</i>, 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 <i>EXO70B1</i> mutant background depends on <i>NYE1</i> 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 <i>exo70b1-1</i>. Before visible necrosis, mature <i>exo70b1-1</i> 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 <i>EXO70B1</i> as an age-dependent modulator of DIS and indicate that the enhanced dark sensitivity of mature <i>exo70b1-1</i> leaves is associated with defense-related transcriptional reprogramming and <i>NYE1</i>-dependent chlorophyll degradation.