BIN2 at the crossroads between brassinosteroids, abscisic acid and auxin
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.
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