Exogenous application of poly-γ-glutamic acid enhances stress defense in Brassica napus L. seedlings by inducing cross-talks between Ca²⁺, H₂O₂, brassinolide, and jasmonic acid in leaves

Poly-γ-glutamic acid (γ-PGA) is a microbe-secreted isopeptide shown to promote growth and enhance crop stress tolerance. However, its downstream signaling pathways are unknown. Here, we studied γ-PGA-induced tolerance to salt and cold stresses. Pretreatment with γ-PGA contributed to enhance stress tolerance of canola seedlings by promoting proline accumulation and total antioxidant capacity (T-AOC) improvement. Further, Ca²⁺, H₂O₂, brassinolide, and jasmonic acid were found to be involved in the γ-PGA-induced process. First, using signal blockers, we concluded that γ-PGA activated Ca²⁺ fluctuations in canola seedling leaves. Second, the activated Ca²⁺ further elicited H₂O₂ production by Ca²⁺-binding proteins CBL9, CPK4, and CPK5. Third, the H₂O₂ signal promoted brassinolide and jasmonic acid biosynthesis by upregulating key genes (DWF4 and LOX2, respectively) for synthesizing these compounds. Lastly, brassinolide and jasmonic acid increased H₂O₂ which promoted proline accumulation and T-AOC improvement and further enhanced Ca²⁺-binding proteins including CaM, CBL10, and CPK9.