Exogenous Application of Poly-γ-glutamic Acid Enhances Stress Defense in Brassica Napus L. Seedlings by Inducing Cross-talks Between Ca, HO, 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, Ca, HO, 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 Ca fluctuations in canola seedling leaves. Second, the activated Ca further elicited HO production by Ca-binding proteins CBL9, CPK4, and CPK5. Third, the HO 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 HO which promoted proline accumulation and T-AOC improvement and further enhanced Ca-binding proteins including CaM, CBL10, and CPK9.