In plants, extensive efforts have been devoted to understanding the crosstalk between salicylic acid (SA) and jasmonic acid (JA) signaling in pathogen defenses, but this crosstalk has scarcely been addressed during senescence. In this study, the effect of SA application on methyl jasmonate (MeJA)-induced leaf senescence was assessed. We found that low concentrations of SA (1-50 μM) played a delayed role against the senescence promoted by MeJA. Furthermore, low concentrations of SA enhanced plant antioxidant defenses and restricted reactive oxygen species (ROS) accumulation in MeJA-treated leaves. When applied simultaneously with MeJA, low concentrations of SA triggered a nitric oxide (NO) burst, and the elevated NO levels were linked to the nitric oxide associated 1 (NOA1)-dependent pathway via nitric oxide synthase (NOS) activity. The ability of SA to up-regulate plant antioxidant defenses, reduce ROS accumulation, and suppress leaf senescence was lost in NO-deficient Atnoa1 plants. In a converse manner, exogenous addition of NO donors increased the plant antioxidant capacity and lowered the ROS levels in MeJA-treated leaves. Taken together, the results indicate that SA at low concentrations counteracts MeJA-induced leaf senescence through NOA1-dependent NO signaling and strengthening of the antioxidant defense.

(J.L., Y.J., J.Z., D. X.).PI3K and its product PI3P are both involved in plant development and stress responses. In this study, the down-regulation of PI3K activity accelerated leaf senescence induced by methyl jasmonate (MeJA) and suppressed the activation of vacuolar H + -ATPase (V-ATPase). Yeast two-hybrid analyses indicated that PI3K bound to the V-ATPase B subunit (VHA-B). Analysis of bimolecular fluorescence complementation in tobacco guard cells showed that PI3K interacted with VHA-B2 in the tonoplasts. Through the use of pharmacological and genetic tools, we found that PI3K and V-ATPase promoted vacuolar acidification and stomatal closure during leaf senescence. Vacuolar acidification was suppressed by the PIKfyve inhibitor in 35S:AtVPS34-YFP Arabidopsis during MeJA-induced leaf senescence, but the decrease was lower than that in YFP-labeled Arabidopsis. These results suggest that PI3K promotes V-ATPase activation and consequently induces vacuolar acidification and stomatal closure, thereby delaying MeJA-induced leaf senescence.

Crosstalk between salicylic acid (SA) and jasmonic acid (JA) signaling plays an important role in regulation of plant senescence. our previous work found that SA could delay methyl jasmonate (MeJA)-induced leaf senescence in a concentration-dependent manner. Here, the effect of low concentration of SA (LCSA) application on MeJA-induced leaf senescence was further assessed. High-throughput sequencing (RNA-Seq) results showed that LCSA did not have dominant effects on the genetic regulatory pathways of basal metabolism like nitrogen metabolism, photosynthesis and glycolysis. the clusterone was applied to identify discrete gene modules based on protein-protein interaction (PPI) network. Interestingly, an autophagy-related (ATG) module was identified in the differentially expressed genes (DEGs) that exclusively induced by MeJA together with LCSA. RT-qPCR confirmed that the expression of most of the determined ATG genes were upregulated by LCSA. Remarkably, in contrast to wild type (Col-0), LCSA cannot alleviate the leaf yellowing phenotype in autophagy defective mutants (atg5-1 and atg7-2) upon MeJA treatment. Confocal results showed that LCSA increased the number of autophagic bodies accumulated in the vacuole during MeJAinduced leaf senescence. Collectively, our work revealed up-regulation of autophagy by LCSA as a key regulator to alleviate MeJA-induced leaf senescence. Senescence in green plants is a complex and orderly regulated process that is crucial for transiting from nutrient assimilation to nutrient remobilization 1-5. During senescence, the most visible characteristic is leaf yellowing, which is the consequence of a succession of changes in cellular physiology including chlorophyll degradation and photosynthetic activity reduction 3,4. Chloroplast as an early senescence signaling response organelle, its dismantling plays an important role in the major nitrogen source recycling and remobilization 6. The progression of leaf senescence can be prematurely induced by multiple environmental and endogenous factors, such as temperature, light, humidity and phytohormones 7. Hormone signaling pathways play roles at all the stages of leaf senescence, including the initiation, progression, and the terminal phases of senescence 7. Recent progresses show that senescence can be coordinately regulated by several phytohormones like cytokinins, ethylene, abscisic acid, salicylic acid (SA), and jasmonic acid (JA) 8-12. However, the detailed molecular mechanisms for these phytohormone signals in plant senescence remain poorly understood. JA has been known as a key plant hormone for promoting senescence, based on the findings that exogenously applied methyl jasmonate (MeJA, methyl ester of JA) leads to a rapid loss of chlorophyll content and accompany with reduction of photochemical efficiency 13,14. Studies with JA-insensitive mutant coronatine insensitive 1 (coi1) that exhibited defective senescence response to MeJA treatment 11 , supporting the notion that JA signaling pathway is crucial for leaf senescence. Some other evi...

15Crosstalk between salicylic acid (SA) and jasmonic acid (JA) signaling plays an important 16 role in molecular regulation of plant senescence. Our previous works found that SA could 17 delay methyl jasmonate (MeJA)-induced leaf senescence in a concentration-dependent 18 manner. Here, the effect of low concentration of SA (LCSA) application on MeJA-induced 19 leaf senescence was further assessed. High-throughput sequencing (RNA-Seq) results 20 showed that LCSA did not have dominant effects on the genetic regulatory pathways of 21 basal metabolism like nitrogen metabolism, photosynthesis and glycolysis. The 22ClusterONE was applied to identify discrete gene modules based on protein-protein 23 interaction (PPI) network. Interestingly, an autophagy-related (ATG) module was identified 24 in the differentially expressed genes (DEGs) that exclusively induced by MeJA together 25 with LCSA. RT-qPCR confirmed that the expression of most of the determined ATG genes 26 were upregulated by LCSA. Remarkably, in contrast to wild type (Col-0), LCSA cannot 27 alleviate the leaf yellowing phenotype in autophagy defective mutants (atg5-1 and atg7-2) 28 upon MeJA treatment. Confocal and western blot results showed that LCSA increased the 29 number of autophagic bodies and autophagic flux during MeJA-induced leaf senescence. 30 Collectively, our work revealed up-regulation of autophagy by LCSA as a key regulator to 31 alleviate MeJA-induced leaf senescence. 32 salicylic acid 34 35 RNA-Seq analysis 132 Detached 3rd and 4th rosette leaves from 4-week old plants were immersed in 3 mM MES 133 buffer (pH 5.8) containing 10 μM SA, 50 μM MeJA, and MeJA together with SA for 24 h. 134Total RNA for RNA-Seq was extracted from leaves using a Hipure plant RNA kit (Magen, 135