Transcriptional reference map of hormone responses in wheat spikes

Phytohormones are key regulators of plant growth, development, and signalling networks involved in responses to diverse biotic and abiotic stresses. Transcriptional reference maps of hormone responses have been reported for several model plant species such as Arabidopsis thaliana, Oryza sativa, and Brachypodium distachyon. However, because of species differences and the complexity of the wheat genome, these transcriptome data are not appropriate reference material for wheat studies. We comprehensively analysed the transcriptomic responses in wheat spikes to seven phytohormones, including indole acetic acid (IAA), gibberellic acid (GA), abscisic acid (ABA), ethylene (ET), cytokinin (CK), salicylic acid (SA), and methyl jasmonic acid (MeJA). A total of 3386 genes were differentially expressed at 24 h after the hormone treatments. Furthermore, 22.7% of these genes exhibited overlapping transcriptional responses for at least two hormones, implying there is crosstalk among phytohormones. We subsequently identified genes with expression levels that were significantly and differentially induced by a specific phytohormone (i.e., hormone-specific responses). The data for these hormone-responsive genes were then compared with the transcriptome data for wheat spikes exposed to biotic (Fusarium head blight) and abiotic (water deficit) stresses. Our data were used to develop a transcriptional reference map of hormone responses in wheat spikes. Only flowering heads were used for hormone treatments. Two florets of each fully developed spikelet from a whole spike at the mid-anthesis stage were treated with 10 μl 10% methanol:water solution with or without (control) 2 mM IAA, 1 mM SA, 1 mM MeJA, 0.38 mM ABA, 0.5 mM GA3, or 0.5 mM trans-zeatin (a type of cytokinin). Regarding the ET treatment, each head was sealed in a plastic bag with 2 ml freshly prepared 25 mM ethephon (pH = 11). All of the hormone solutions were used to treat florets within 1 h of being prepared. The heads were collected at 24 h after the hormone treatments, and then ground to a fine powder in liquid nitrogen. Each treatment was completed with three biological replicates, each of which comprised at least five heads. No unusual morphological changes were observed in the treated heads. The hormone concentrations of the treatments were based on previous studies