Plant growth and development, which determinate plant form, require the integration of a variety of environmental signals with the intrinsic genetic program. Fundamental to this process are several growth regulators called the plant hormones or phytohormones. In accordance with definition, the plant hormones are signal molecules produced within the plant and occur in extremely low concentrations. They are often synthesized in one part of the plant and are transported to another location through the plant vasculature system or, at least in the case of auxin, through a complex cell-to-cell transport system. They interact with specific target tissues to cause physiological responses and unlikely to animals responses are often the result of two or more hormones acting together. Central to comprehending hormonal control of plant growth and development is the understanding of how the hormones are perceived and the signal is transduced. Despite decades of study, only recently receptors for several of these substances have been identified, providing plant physiologists with a much clearer picture of hormonal control. Moreover, studies have revealed a quite novel model of signal transduction in which ubiquitin ligases function as hormone receptors. In eukaryotes, ubiquitin ligases operate in the ubiquitin-proteasome system (UPS) participating in the control of signal transduction events by selectively eliminating regulatory proteins. E3 ubiquitin ligases specifically bind degradation substrates and mediate their poly-ubiquitylation and later degradation by the 26S proteasome. In Arabidopsis thaliana more than 1400 genes encode components of the UPS. About 90% of these genes encode subunits of the E3 ubiquitin ligases comprise large family of protein or protein complexes containing a RING-finger, U-box domain or a HECT domain. Analysis of nucleotide sequences have revealed also that the UPS is strongly conserved in plants kingdom. The most thoroughly studied in plants is the SCF class of E3 ubiquitin ligases. The name of this class is derived from three of its four subunits: SKP1, cullin and the F-box protein (FBP). FBP represent the largest superfamily in Arabidopsis, comprising 2,7% of this plant genome. The recent study revealed that plant proteins: AtFIR1 (A. thaliana Transport Inhibitor Response 1), OsGID1 (O. sativa Gibberellin Insensitive Dwarf 1) and AtCOI1 (A. thaliana CORonatine Insensitive 1), which play important role respectively in auxin, jasmonate and gibberellin signal transduction, are F-box proteins (AtTIR1, AtCOI1) or closely related (OsGID1), whereas the role of the SCF is to degrade repressors of hormone response. In the case of auxin, auxin-responsive genes arc repressed by AUX/IAA proteins heterodimerizing with ARF transcription factors. Upon an auxin stimulus, the TIR1 binds auxin, enabling the recruitment of AUX/IAA proteins to the SCF complex for ubiquitination. Subsequently, AUX/IAA degradation by the 26S proteasome derepresses the ARF transcription factors. In the case of jasmonate, JAZ proteins negatively regulate jasmonate response by repressing MYC2 transcriptional activity. After binding of jasmonate, the SCFCOI1 ubiquitin-ligase targets JAZ proteins for ubiquitin-mediated proteolysis, derepressing MYC2. In addition to auxin and jasmonate, gibberellin is yet another plant hormone whose perception involves an SCF ubiquitin ligase complex. DELLA proteins repress GA response by negatively regulating GAMyb, PIF3, PIF4, and presumably other transcription factors that control the expression of GA-inducible gnes. DELLA proteins also promote the expression of several GA-repressible genes, some of which encode GA biosynthetic enzymes and components of the response pathway including the GID1 receptors. In the consequence of GA binding, the GID1 receptor interacts with DELLA and the new created complex (GA-GID1-DELLA) is recognized by the SCFGID2/SLY1 ubiquitin-ligase, which targets DELLA for ubiquitin-mediated degradation. It is seen, that GAs is not directly sensed by an F-box protein but instead by an extension molecule of the SCF substrate receptor subunit. Given the importance of SCFs to signal transduction, it is not surprising that SCF assembly and function are highly regulated. So far, tree proteins or protein complexes have been implicated in SCF regulation: RUB I (Related to UBiquitin 1), CAND1 (Cullin Associated Neddylation Dissociated 1) and the COP9 signalosome (CSN). The mechanism of hormonal signal transduction presented in this paper has been described only in plants, however, due to extensive occurrence of ubiquitin ligases in eukaryotic cells, it can be supposed, that this novel hormone-signaling mechanism may also exist in other organisms.
