Abstract
Plant memory is an adaptive mechanism that plants can use to increase their fitness and cope with adverse environmental stresses. In this study, mRNA-sequencing (mRNA-seq), whole-genome bisulfite sequencing (WGBS) and real-time quantitative PCR (RT-qPCR) methods were applied for evaluating formation and maintenance of somatic transcriptional memory after treatment with ultrasound and drought stimuli in tomatoes. In addition, the effects of repeated stimuli, as well as the association-forming ability of plants were studied when they were trained previously with combined stimuli. Two days after exposure to the two stimuli applied alone or in combination, significantly altered gene transcription and DNA methylation were revealed. Using four selected target genes, we demonstrated that plants memorized stimuli for 5-10 d, in a gene- and stimulus-dependent way. The repeated application of the stimuli caused various alterations in gene transcription behavior, such as habituation, sustained induction or modified reinduction. Plants were able to use one conditioned stimulus as a predictor of the other, unconditioned one, after conditioning in the case of 3 out of 4 target genes, and used their transcriptional memory associatively. The exploitation of plant memory and associative learning may contribute to the development of new strategies to increase plant stress resilience.