The Role of IBR5 in Regulating Plant Auxin Response through the SCF(TIR1) Complex
Plant growth and development is a highly regulated process that involves synthesis, cellular transport, and perception of the growth hormone auxin, or indole-3-acetic acid (IAA). Cellular responses to auxin involve the degradation of the Aux/IAA family of repressors through SCF TIR1/AFBs complex, which is composed of ASK1, CUL1, RBX1, and the F-box protein TIR1/AFBs, subsequently modulating the expression of auxin-related genes to control growth and development. Previous studies identified IBR5 as a gene involved in the auxin response pathway, as primary root growth of ibr5 mutants exhibited insensitivity to indole-3-butyric acid (IBA), a precursor to IAA in plants, as well as IAA and other auxin analogues. Additionally, ibr5 is defective in several other hormone and stress response pathways. Interestingly, Aux/IAAs are rapidly degraded in ibr5 mutants, which is contrary to other mutant genes identified in the auxin signaling pathway. This research sought to characterize the role of IBR5 in regulating the auxin response pathway through the SCF TIR1/AFBs complex. Previous results indicated that SCF TIR1/AFBs subunit, ASK1, interacts in vitro with IBR5. Results of this research indicate that steady-state levels of ASK1 and TIR1 proteins are elevated in ibr5 mutant and 35S:IBR5-Myc overexpression lines. Since SGT1b is also known to regulate TIR1 abundance, the genetic interaction between IBR5 and SGT1b was also analyzed. ibr5, sgt1b double mutants show increased auxin resistance compared to single mutants, suggesting these proteins act partially independently in auxin response. Additionally, post-translational modifications of TIR1, including those mediated by HSP90-SGT1b chaperone modules, were shown to be unaffected in ibr5 mutants. Together, the results in this study suggest that IBR5 is not involved with the HSP90-mediated stabilization of TIR1, however future testing may reveal IBR5 to be required for proper HSP90-mediated nuclear localization of TIR1.
Auxin, Plant development, Hormones, Protein degradation
Cioffi, T. J. (2019). <i>The role of IBR5 in regulating plant auxin response through the SCF(TIR1) complex</i> (Unpublished thesis). Texas State University, San Marcos, Texas.