Multiple hormone treatment revealed novel cooperative relationships between abscisic acid and biotic stress hormones in cultured cells

Phytohormones have vigorous crosstalk relationships. For example, abscisic acid (ABA), a hormone involved in abiotic stress responses, has antagonistic interactions with plant hormones that play pivotal roles in defense responses, including salicylic acid (SA) and methyl-jasmonic acid (MeJA). Evidence indicates that the relationships among these plant hormones extend beyond simple antagonism. To explore the interplay between hormones in detail, we analyzed the effects of double hormone treatment on gene expression. By contrast to the antagonistic effects reported previously, our data indicates that ABA interacts with SA and MeJA cooperatively as well. Particularly many genes responded only to double hormone treatment, and, interestingly, the loci that responded to ABA+SA also responded to ABA+MeJA. The expression of early-response genes following double hormone treatment did not fit the linear superposition of individual hormone treatments, in contrast to mammalian and prokaryotic cell responses to multiple chemical stimuli. Thus, synergies in these plant hormone signalings are not simply the sum of individual responses. ABA and SA collaboratively down-regulated the expression of genes involved in cell cycle progression at G2/M phase. Presumably, plants interpret combined hormone signals differently from individual signals in order to respond appropriately to their environmental conditions.