Abstract
The second messenger cyclic guanosine 3',5'-monophosphate (cGMP) plays an important role in plant development and responses to stress. Recent studies indicated that cGMP is a secondary signal generated in response to auxin stimulation. cGMP also mediates auxin-induced adventitious root formation in mung bean and gravitropic bending in soybean. Nonetheless, the mechanism of the participation of cGMP in auxin signalling to affect these growth and developmental processes is largely unknown. In this report we provide evidence that indole-3-acetic acid (IAA) induces cGMP accumulation in Arabidopsis roots through modulation of the guanylate cyclase activity. Application of 8-bromo-cGMP (a cell-permeable cGMP derivative) increases auxin-dependent lateral root formation, root hair development, primary root growth, and gene expression. In contrast, inhibitors of endogenous cGMP synthesis block these processes induced by auxin. Data also showed that 8-bromo-cGMP enhances auxin-induced degradation of Aux/IAA protein modulated by the SCF(TIR1) ubiquitin-proteasome pathway. Furthermore, it was found that 8-bromo-cGMP is unable to directly influence the auxin-dependent TIR1-Aux/IAA interaction as evidenced by pull-down and yeast two-hybrid assays. In addition, we provide evidence for cGMP-mediated modulation of auxin signalling through cGMP-dependent protein kinase (PKG). Our results suggest that cGMP acts as a mediator to participate in auxin signalling and may govern this process by PKG activity via its influence on auxin-regulated gene expression and auxin/IAA degradation.
Highlights
The plant hormone auxin plays a central role in plant responses to physiological and environmental changes
It has been demonstrated that the auxin signal transduction system operates via the SCFTIR1/AFB proteasome machinery (TIR1/AFB is TRANSPORT INHIBITOR RESPONSE1/AUXIN RECEPTOR-BOX), which plays a key role in the regulatory process of transcription and leads to auxin-related developmental responses
In this study we have provided evidence that cGMP, as a positive modulator, is involved in the auxinregulated signalling response in Arabidopsis roots, which depends on cGMP-dependent protein kinase (PKG) activity
Summary
The plant hormone auxin plays a central role in plant responses to physiological and environmental changes. Recent studies demonstrated that auxin acts as a ‘molecular glue’ in binding to TIR1 and stabilizing the interaction between TIR1 and Aux/IAA proteins (Santner and Estelle, 2009; Greenham et al, 2011) This interaction results in Aux/IAA ubiquitination and subsequent degradation by the 26S proteasome and releases the AUXIN RESPONSE FACTOR (ARF) proteins to regulate the expression of target genes (Tan et al, 2007). The SCF regulatory proteins AXR1, ECR1, and RCE1 are involved in the RUB/NEDD8 conjugation of CUL1 Mutations in these components confer auxin-resistant phenotypes and result in defects in auxin-related developmental processes (del Pozo et al, 2002; Dharmasiri et al, 2003). Auxin signalling components have been conserved throughout the evolution of land plants and have proliferated and specialized to control specific developmental processes (Chapman and Estelle, 2009)
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