DEG_SCF_TIR1_1

Phytohormones are a diverse set of endogenous chemicals that control many different aspects of plant development and growth. Some well-studied plant hormones include auxins, ethylene, gibberellins, jasmonates and abscisic acid. Their activity depends on hormone synthesis, transport, conjugation to other substances, and degradation, as well as extensive cross talk between the different hormones (Garay-Arroyo,2012).
The auxin family of plant hormones, with indole-3-acetic acid (IAA) as the most important member, plays a key role in plant development and growth by acting as a signal for cell division, elongation and differentiation. This hormone regulates a wide variety of processes, including embryogenesis, root formation, apical dominance and tropic responses to light and gravity (Mockaitis,2008, Hayashi,2012). Local auxin abundance is determined by the coordinated control of regulatory pathways involved in the metabolism and transport of auxin. Responses to auxin are mediated by a variety of signalling mechanisms that control the expression of specific sets of genes or trigger transcription-independent responses. Genes transcriptionally activated by auxin include GH3 genes involved in synthesis of inactive IAA-amino acid conjugates, the Small Auxin-Up RNA (SAUR) genes of unknown function, and the Aux/IAA genes. Important mediators of the transcriptional response to auxin are the auxin response factor (ARF) transcription factors that directly bind to the promoters of auxin-responsive genes to regulate their expression. The auxin-responsiveness of the ARFs depends on the Aux/IAA proteins, transcriptional repressors that heterodimerize with ARF proteins, thereby inactivating ARF activity and blocking transcription of auxin-responsive genes. In the presence of auxin, the Aux/IAA repressors are targeted for ubiquitin-dependent proteasomal degradation by the SCF (Skp1-Cul1-Rbx1-Fbox protein) E3 ubiquitin ligase. The auxin-dependent degradation of the Aux/IAAs relieves inhibition of ARF activity and results in expression of auxin-responsive genes. As these genes also include the Aux/IAA-encoding genes, this mechanism provides a negative feedback loop for control of auxin signalling (Mockaitis,2008, Hayashi,2012).
Degradation of the Aux/IAA proteins by the SCF complex depends on the Transport Inhibitor Response 1 (TIR1) and Auxin signalling F-Box (AFB) proteins that function as auxin receptors and substrate recognition subunits of the SCF. The Aux/IAAs are recruited to the SCF by a TIR1/AFB F-box protein, subsequently ubiquitylated by the SCF, and thereby marked for degradation by the proteasome. Binding to the F-box subunit is mediated by the auxin-dependent SCF-TIR1-binding degron motif. Auxin promotes an increase of Aux/IAA degradation by enhancing the interaction between the motif and TIR1/AFB (Parry,2006, Calderon-Villalobos,2010). The components of this transcriptional auxin response are conserved in plants, although some Aux/IAA proteins appear to lack a functional TIR1-binding degron motif (Dreher,2006, Paponov,2009).
The oxylipin jasmonic acid (JA) and its metabolites constitute a family of plant hormones collectively referred to as jasmonates. Production of the major bioactive isoform of the hormone, N-[(3R,7S)-(+)-7-iso-Jasmonoyl]-(S)-isoleucine (JA-Ile), requires conjugation of the JA prohormone to isoleucine (Fonseca,2009). Jasmonates play an important role in normal plant development and growth processes, including root growth, including inhibition of root growth and seed germination and stimulation of senescence. In addition, they mediate defensive responses to biotic and abiotic stress signals such as pathogenic infection, wounding and UV irradiation (Wasternack,2007). Jasmonate-induced responses depend on signalling mechanisms that control the expression of specific sets of genes. An important mediator of the transcriptional response to jasmonates is the transcription factor MYC2/RAP1, which directly binds to the promoters of JA-responsive genes. Genes regulated by MYC2 include wound-responsive genes and genes involved in redox signalling (Dombrecht,2007). The JA-responsiveness of MYC2 and the related transcription factors MYC3 and MYC4 depends on the JAZ/TIFY proteins that function as transcriptional repressors and inhibit positive regulation of gene expression by MYC2. In the absence of bioactive jasmonates, the JAZ proteins bind to MYC2 through their C-terminal region. The inhibitory function of JAZ proteins depends on their ZIM domain that binds to the adaptor protein NINJA, which in turn recruits the general corepressor TOPLESS (Pauwels,2011, Kombrink,2012). In the presence of active JA, the JAZ proteins are targeted for ubiquitin-dependent proteasomal degradation by the SCF (Skp1-Cul1-Rbx1-Fbox protein) E3 ubiquitin ligase. The JA-dependent degradation of the JAZ proteins relieves inhibition of MYC2 activity and results in expression of JA-responsive genes. As these genes also include the JAZ-encoding genes, this mechanism provides a negative feedback loop for control of jasmonate signalling (Fonseca,2009).
Degradation of the JAZ proteins by the SCF complex depends on the Coronatine-insensitive protein 1 (COI1), which functions as a JA receptor and substrate recognition subunit of the SCF. The JAZ proteins are recruited to the SCF by the COI1 F-box protein, subsequently ubiquitylated by the SCF, and thereby marked for degradation by the proteasome. Binding to the F-box subunit is mediated by the JA-dependent SCF-COI1-binding degron motif located in the Jas domain, which mediates binding to MYC2. JA-Ile promotes an increase of JAZ degradation by enhancing the interaction between the motif and COI1 (Gfeller,2010, Kombrink,2012). The components of this transcriptional JA response are conserved in higher plants, although some JAZ proteins appear to lack a functional COI1-binding degron motif (Pauwels,2011, Shyu,2012).