BIN2 at the crossroads between brassinosteroids, abscisic acid and auxin

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One exciting finding is that GSK3 may mediate the crosstalk between BR signalling and other hormone signalling pathways [41-42]. Abscisic acid (ABA) inhibits BR signalling, enhances BES1 phosphorylation and induces the expression of BR-suppressed genes [41]. This effect was alleviated when BIN2 activity was blocked using lithium chloride (LiCl), a GSK3 inhibitor. The effect of ABA on BR signalling was independent of the early BR perception events [41]. However, whether ABA has a direct effect on BIN2 phosphorylation or whether it affects the upstream BSK/BSU components remains to be elucidated (Figure 2). Auxin, unlike ABA, has no effect on the phosphorylation state of BES1 [43]. However, ARF2, an auxin response factor (ARF) that acts as a transcriptional repressor, has numerous putative GSK3 recognition sites and the interaction between BIN2 and ARF2 has been confirmed in yeast [42]. In vitro studies also showed that BIN2 is capable of phosphorylating ARF2 and consequently inhibiting its DNA-binding and repressor activity [42]. These findings provide a molecular explanation for the effect of BRs on the auxin response by suggesting that BIN2 facilitates binding of activator ARF proteins by removing repressor ARFs from regulatory elements in the promoters of auxin-responsive genes ([42] and Figure 2). It is possible that hormone signalling may impinge upon plant GSK3s in additional ways. For example, AtSK31 gene expression is upregulated by auxin in roots [44], although AtSK31 does not have a demonstrated role in BR or auxin signalling.