Identification of Transcriptomic and Phytohormonal Biomarkers to Monitor Drought and Evaluate the Potential of Acibenzolar-S-Methyl Root Application to Prime for Drought Resistance in Two Apple Rootstock Genotypes

Droughts are predicted to intensify with climate change, posing a serious threat to global crop production. Increasing drought tolerance in plants requires understanding of underlying mechanisms. This study measured physiological, phytohormonal and transcriptomic responses to drought in two apple rootstocks to identify drought ‘biomarkers’ and investigated whether root application of acibenzolar-S-methyl (ASM) could enhance drought tolerance. Two potted-plant trials were conducted on dwarfing (M9) and semi-dwarfing (CG202) apple rootstocks. In both trials, response patterns in roots and leaves were compared in irrigated and non-irrigated plants over a 14-day period. In trial 2, ASM was applied 14-days before and immediately before with-drawing irrigation. Drought induced significant decreases in transpiration, photosynthesis and stomatal conductance in both trials. This was accompanied by accumulation of abscisic acid (ABA) metabolites and upregulation of ABA pathway transcripts (CYP707A1/A2 & NCED3), a decrease in 12-oxophytodienoic acid (cis-OPDA) and downregulation of ABA-receptor genes (PYL4). Responses to drought were greater in roots than leaves, broadly similar across both rootstocks, but differed in strength and timing between the rootstocks.  Root application of ASM did not significantly affect responsiveness to drought in either rootstock. The identified phytohormonal and transcriptomic biomarkers require further validation across a broader range of genotypes.