UC San Diego

The plant hormone auxin plays an important role in plant growth and development. Studies in flowering plants have uncovered many aspects of auxin signaling underlying molecular mechanisms. The SGT1 protein is known to be required for auxin receptor TIR1-mediated auxin response, and known to function as a HSP90 co-chaperone. However, the precise molecular function of SGT1 in auxin responses remains unknown. In this thesis, I seek to understand the function of SGT1 in auxin responses in the moss Physcomitrella patens by taking advantage of its efficient recombination system. My experiments indicate that PpSGT1 has an important role in moss development. PpSGT1 inhibited auxin-responsive Aux/IAA gene expressions when it was induced by high temperature or ectopically expressed under a strong promoter. In addition, auxin- responsive reporter expression was suppressed by PpSGT1 overexpression. Strong growth inhibition resulting from overexpression of auxin receptor AFB2 was compromised by PpSGT1 overexpression. A reduction in auxin receptor accumulation in lines overexpressing PpSGT1 implies that the reduced auxin-responsive gene expression is caused by decreased auxin receptor level. Contrarily, HSP90 appears to have a positive effect on auxin receptor accumulation and auxin reporter expression, which may suggest independent function of SGT1 and HSP90 on the auxin receptors in moss. Overall, overexpression of PpSGT1 destabilizes auxin receptor AFB, and negatively regulates auxin signaling