Functional interplay between protein kinase CK2 and salicylic acid sustains PIN transcriptional expression and root development.

We have previously reported that CK2-defective Arabidopsis thaliana plants (CK2mut plants) were impaired severely in root development and auxin polar transport, and exhibited transcriptional misregulation of auxin-efflux transporters (Plant J., 67, 2011a, 169). In this work we show that CK2mut roots accumulate high levels of salicylic acid (SA) and that the gene that encodes isochorismate synthase (SID2) is overexpressed, strongly suggesting that CK2 activity is required for SA biosynthesis via the shikimate pathway. Moreover, SA activates transcription of CK2-encoding genes and, thus, SA and CK2 appear to be part of an autoregulatory feed-back loop to fine-tune each other's activities. We also show that exogenous SA and constitutive high SA levels in cpr mutants reproduce the CK2mut root phenotypes (decrease of root length and of number of lateral roots), whereas inhibition of CK2 activity in SA-defective and SA-signalling mutants lead to less severe phenotypes, suggesting that the CK2mut root phenotypes are SA-mediated effects. Moreover, exogenous SA mediates transcriptional repression of most of PIN-FORMED (PIN) genes, which is the opposite effect observed in CK2mut roots. These results prompted us to propose a model in which CK2 acts as a link between SA homeostasis and transcriptional regulation of auxin-efflux transporters. We also show that CK2 overexpression in Arabidopsis has neither impact on SA biosynthesis nor on auxin transport, but it improves the Arabidopsis root system. Thus, unlike the outcome in mammals, an excess of CK2 in plant cells does not produce neoplasia, but it might be advantageous for plant fitness.