Phytohormones and related genes function as physiological and molecular switches regulating water stress response in the sunflower

Water deficit stress reduces crop yield in field crops, including sunflowers, at any growth stage. In response, most plants activate hormonal and gene expression patterns to mitigate damage. In this study, we evaluated changes in the physiological and gene transcription levels of two sunflower (Helianthus annuus L.) inbred lines -one sensitive (B59 line) and one water stress-tolerant (B71)-in response to water stress, by using mannitol to simulate water deficit conditions, which provides moderate stress in both sunflower lines. The analyses of the accumulation of various phytohormones under this stress revealed that Jasmonic acid (JA) significantly increased in the shoots of both lines. Similarly, Salicylic acid (SA) increased in the shoots of both lines, although it also accumulated in B71 roots. In addition, Abscisic acid (ABA) and Indole-3-acetic acid (IAA) showed a considerable increase in the B59 shoots. Regarding the JA and SA pathways, the WRKY70 transcription levels were higher in the shoots of both lines and the roots of B71. The B59 line showed overtranscription of a gene related to the ABA pathway (XERICO) and genes associated with IAA (ARF9 and ARF16 genes). The B71 line, on the other hand, simultaneously triggered the JA, SA and ABA hormonal pathways in response to this stress condition. The ABA and JA hormonal pathways activated different TFs, such as RD20, RD22, RD26, ANAC19 and ANAC29, through MYC2. Both the JA and SA hormonal pathways activated the WRKY70 transcription factor. Altogether, each line triggered the hormonal and transcriptional pathways in response to water stress, although at varying intensities. The results suggest that the hormonal pathways of JA, SA, IAA and ABA, along with their primary associated genes, are activated in response to water deficit at the early growth stage in sunflower seedlings, which mitigates damage.