Proteome Analysis in Arabidopsis Reveals Shoot- and Root-Specific Targets of Cytokinin Action and Differential Regulation of Hormonal Homeostasis    

Abstract

Republic, CZ-165 02 Prague, Czech Republic (B.P., J.M.)The plant hormones cytokinins (CKs) regulate multiple developmental and physiological processes in Arabidopsis (Arabidopsis thaliana). Responses to CKs vary in different organs and tissues (e.g. the response to CKs has been shown to be opposite in shoot and root samples). However, the tissue-specific targets of CKs and the mechanisms underlying such specificity remain largely unclear. Here, we show that the Arabidopsis proteome responds with strong tissue and time specificity to the aromatic CK 6-benzylaminopurine (BAP) and that fast posttranscriptional and/or posttranslational regulation of protein abundance is involved in the contrasting shoot and root proteome responses to BAP. We demonstrate that BAP predominantly regulates proteins involved in carbohydrate and energy metabolism in the shoot as well as protein synthesis and destination in the root. Furthermore, we found that BAP treatment affects endogenous hormonal homeostasis, again with strong tissue specificity. In the shoot, BAP up-regulates the abundance of proteins involved in abscisic acid (ABA) biosynthesis and the ABA response, whereas in the root, BAP rapidly and strongly up-regulates the majority of proteins in the ethylene biosynthetic pathway. This was further corroborated by direct measurements of hormone metabolites, showing that BAP increases ABA levels in the shoot and 1-aminocyclopropane-1-carboxylic acid, the rate-limiting precursor of ethylene biosynthesis, in the root. In support of the physiological importance of these findings, we identified the role of proteins mediating BAP-induced ethylene production, METHIONINE SYNTHASE1 and ACC OXIDASE2, in the early root growth response to BAP.

Abscisic Acid Is a General Negative Regulator of Arabidopsis Axillary Bud Growth

“…In Arabidopsis, endogenous cytokinins promoted ABA accumulation but suppressed ABA response (Nishiyama et al, 2011), and the cytokinin receptors HIS KINASE2, HIS KINASE3, and CYTOKININ RESPONSE1 were shown to negatively regulate ABA response in Arabidopsis germination assays (Tran et al, 2007). Another study demonstrated that application of the cytokinin 6-benzylaminopurine transiently increased Arabidopsis shoot ABA levels ( Zd'árská et al, 2013), while overexpression of the cytokinin biosynthesis enzyme isopentenyl transferase resulted in reduced ABA accumulation in petunia flowers (Chang et al, 2003). Overall, the existing evidence may suggest that it is more probable that some of the effects of cytokinins are transduced by ABA rather than vice versa.…”

Ethylene and Hormonal Cross Talk in Vegetative Growth and Development

“…Originally, it was shown that CKs inhibit root growth and that this inhibition was mediated via the ethylene signaling pathway (Cary et al, 1995). Furthermore, CKs can induce ethylene biosynthesis by stabilizing ACS5 protein levels (Chae et al, 2003), resulting in a higher endogenous ACC content ( Zd'árská et al, 2013). CKs also up-regulate the levels of S-adenosyl methionine synthetase and ACO in the root ( Zd'árská et al, 2013), most likely leading to a local increase in ethylene production and, consequently, to the inhibition of root elongation.…”

“…Furthermore, CKs can induce ethylene biosynthesis by stabilizing ACS5 protein levels (Chae et al, 2003), resulting in a higher endogenous ACC content ( Zd'árská et al, 2013). CKs also up-regulate the levels of S-adenosyl methionine synthetase and ACO in the root ( Zd'árská et al, 2013), most likely leading to a local increase in ethylene production and, consequently, to the inhibition of root elongation. Contradictorily, Kushwah et al (2011) showed that CKs can induce root elongation instead of inhibition and that this response is also mediated by the ethylene signaling pathway.…”

Ethylene Inhibits Cell Proliferation of the Arabidopsis Root Meristem

“…Cytokinin also induces ethylene biosynthesis (Vogel et al, 1998a(Vogel et al, , 1998bChae et al, 2003;Hansen et al, 2009;Zd'árská et al, 2013), raising the possibility that the inhibitory effect of cytokinin on cell proliferation might exhibit ethylene dependence. We, therefore, examined the RAM response to the cytokinin benzyladenine (BA) in the wild type, the ethylene-insensitive mutants etr1-1 and ein2-1, and the constitutive ethylene-response mutant ctr1-2 (Fig.…”