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.