Exogenous methyl jasmonate mediates tolerance of heat stress in Korean fir (Abies koreana)

Heat stress is a major environmental stress that affects the growth and development of plants. Korean fir (Abies koreana), a rare species endemic to South Korea, is sensitive to global climate change. The effect of exogenous methyl jasmonate (MeJA) on heat stress tolerance was, therefore, investigated in this species. During heat stress, the expression levels of eight genes (AkNAC19, AkMPK6, AkERF4, AkEFP, AkNAC2, AkbHLH, AkHSP17.6, and AkMYB123) were assessed in needles of A. koreana following treatment with 0, 0.1, 1.0, or 2.0 mM MeJA. Optimal upregulation of expression of most genes was observed 24 h post-treatment with 2.0 mM MeJA. Similar results were obtained when gene expression was analyzed 1, 2, 4, and 8 days post-treatment with 2.0 mM MeJA. Under heat stress conditions, plants treated with 2.0 mM MeJA initially showed a rapid decline in electrolyte leakage and higher chlorophyll content after 28 days of heat stress; however, opposite trends were observed in untreated plants, indicating that MeJA mediated tolerance to heat stress. Higher levels of expression of AkERF4, AkNAC2, and AkHSP17.6 were observed in MeJA-treated needles than in untreated needles, indicating these genes were strongly associated with MeJA-mediated heat tolerance. Therefore, these results suggest that the ability of Korean fir to tolerate abiotic stress is associated with endogenous MeJA synthesis or signaling, and identifies AkERF4, AkNAC2, and AkHSP17.6 as potential candidates for genes involved in the stress-tolerance mechanism.