5-Aminolevulinic acid and hydrogen sulphide alleviate chilling stress in pepper (Capsicum annuum L.) seedlings by enhancing chlorophyll synthesis pathway

5-Aminolevulinic acid (ALA) as a precursor in chlorophyll (Chl) synthesis and hydrogen sulphide (H2S) as a gas signalling molecules can alleviate various abiotic stresses by enhancing photosynthesis. However, little is known about their mechanisms ameliorating photosynthesis under chilling stress, or interactions of ALA and H2S in Chl synthesis. In this study, we explored the effects of exogenous ALA and H2S on chilling stress-induced photosynthesis damage in pepper (Capsicum annuum L.) seedlings. Chilling inhibited the photosynthetic capacity of pepper seedlings, ALA or H2S treatment alone could alleviate this inhibition, and ALA + H2S treatment was even more effective for improving photosynthetic capacity. Additionally, levels of Chl synthesis pathway substances including endogenous ALA, protoporphyrin IX (Proto IX), Mg-protoporphyrin (Mg-Proto IX), protochlorophyllide (Pchl) and Chl (Chl a and Chl b) were significantly decreased, and chilling down-regulated upstream genes HEMA1, HEMB, FAR1, FHY3, CHLH, HEME1, HEMF and PORA. ALA + H2S treatment significantly increased levels of Chl and upstream substances, and up-regulated expression of HEMA1, HEMB and FAR1. In conclusion, exogenous ALA and H2S enhanced chlorophyll synthesis pathway, and thus improved the photosynthesis of pepper seedlings under chilling stress.