Seed priming with H2S and Ca2+ trigger signal memory that induces cross-adaptation against nickel stress in zucchini seedlings

In this study, the effect of seed priming with sodium hydro sulfide (NaHS) and CaCl2 as well as the possible relationship between them in inducing post-germinative cross-adaptation in zucchini seedlings (cv Courgette d'Italie) were investigated. Results showed that Ni toxicity reduced plant growth and photosynthetic pigments, decreased the content of ascorbate (AsA) and total thiols, increased hydrogen peroxide (H2O2) content and electrolyte leakage (EL), up-regulated the transcription levels of -dependent protein kinase (CDPK) and phytochelatin (PCs) genes and elevated H2S content in leaves of zucchini seedlings. Individual or combined seed priming with Ca2+ and NaHS improved the content of photosynthetic pigments and seedling biomass, reduced H2O2 content and EL, increased the content of AsA and total thiols, decreased ascorbate peroxidase activity and enhanced glutathione reductase activity in leaves. These findings suggest the last time effect of seed priming with Ca2+ and NaHS on inducing cross-adaptation in seedlings under Ni stress. H2S accumulation and other responses induced with Ca2+ in leaves were weakened with hypotaurine (HT as H2S scavenger), denoting seed priming with Ca2+ established cross-adaptation in a H2S-dependent manner. Seed priming with NaHS amplified CDPK transcripts in leaves of seedlings and seed priming with ethylene glycol tetraacetic acid (as Ca2+ chelator), lanthanum chloride and verapamil (as plasma membrane channel blockers) reduced transcript levels of CDPK and PCs genes and reversed impacts of seed priming with NaHS. These results indicated that the cross-adaptation induced with NaHS is mediated through Ca2+ signaling. Overall our findings suggest that two-side cross-talk between Ca2+ and H2S is involved in the acquisition of a signal memory in seed embryo cells which can be employed upon a later Ni-exposure and more strongly enhance AsA-GSH cycle, redox homeostasis and phytochelatin transcripts in leaves of zucchini seedlings grown from primed seeds.