The Role of Auxin-Cytokinin Antagonism in Plant-Pathogen Interactions

It has been several decades since Skoog and Miller described the contrasting behavior of auxin and cytokinin in promoting the growth of root and shoot, respectively [1]. In recent years, a lot of progress has been made in understanding the regulation of stem cell niche and cell fate in both shoot and root apical meristems. Developmental processes such as the maintenance of root meristems [2], lateral root formation [3], leaf position determination [4], and de novo auxin-induced organogenesis [5] are finetuned by the mutual interactions between auxin and cytokinin. Auxin exerts its inhibition on cytokinin on several levels; mechanisms range from its biosynthesis to the suppression of its signaling [6]. Reciprocally, cytokinin antagonistically impacts the flux, distribution, and signaling of auxin [7]. Antagonism between auxin and cytokinin is not the only type of interaction that governs developmental outputs in plants. Rather, synergistic interaction between auxin and cytokinins also exists in processes such as nodule organogenesis [8], light-mediated leaf initiation, and organ positioning [9]. With the majority of previous studies focusing on these hormones in development, auxin-cytokinin interplay has not been extensively analyzed in the context of plant immunity. Mutual interactions between stress-specific hormones such as salicylic acid and jasmonic acid/ethylene (SA-JA/ET) are regarded as the central backbone of the immunity [10]. However, growth-promoting hormones (auxin, cytokinins, gibberellic acid, and abscisic acid) either inhibit or potentiate this balance in mediating the protection or susceptibility of the plant against the invading pathogen [10,11]. For a comprehensive understanding of hormonal crosstalk in disease, a systems-biological perspective is critical, as plant hormones act in concert [11]. We focus on recent progress regarding the individual effects of auxin and cytokinins and their combined effect on immune dynamics in plant-pathogen systems.