Exogenous application of 5-aminolevulinic acid on wheat seedlings under drought stress enhances the transcription of psbA and psbD genes and improves photosynthesis

The compound 5-aminolevulinic acid (ALA) is an essential precursor for the biosynthesis of porphyrins, including chlorophyll, heme, and cytochromes. The protective effects of ALA on photosynthesis and the expression of photosynthetic genes in wheat under drought stress are not well understood. Two wheat cultivars, drought-tolerant Aikang-58 and drought-sensitive Chinese Spring, were exposed to drought stress induced by 20% polyethylene glycol (PEG-6000) after foliar pretreatment with ALA for 3 days. The results showed that exogenous application of ALA protected the drought-stressed wheat seedlings by significantly inhibiting the decrease in relative water and chlorophyll contents. The ALA-mediated alleviation of stress was similar between the drought-tolerant and drought-sensitive wheat cultivars. Meanwhile, compared to seedlings under drought treatment alone, the ALA-pre-treated wheat seedlings under drought stress maintained higher photosystem II (PSII) functional indexes. The ALA pretreatment reduced the drought-driven accumulation of both H2O2 and ABA and increased the stomatal conductance. Real-time PCR analysis showed that the psbA and psbD gene transcripts were both upregulated under drought stress following ALA pretreatment. The present study suggests that the exogenous foliar application of ALA alleviates the drought stress on wheat seedlings, which is associated with the enhancement of PSII function by inducing chlorophyll synthesis and psbA and psbD transcription. Moreover, the protective effect of ALA pretreatment was not related to the decline in stomatal conductance caused by ABA or H2O2 accumulation.