Minisymposium 14: Hormones II

Abs # 26003: The Diageotropica Gene of Tomato Encodes a Novel Player, a Cyclophilin, in Auxin Signaling.

Presenter:	Oh, KwangChul , ohk@science.oregonstate.edu
Authors	Oh, KwangChul  (A)   Ivanchenko, Maria G (A)   White, Tj  (A)   Lomax, Terri L (A)
Affiliations:	(A): Oregon State University, Botany and Plant Pathology, Cordely Hall 2082, Corvallis, OR 97331-2902

The single gene, auxin-resistant diageotropica (dgt) mutant of tomato displays a pleiotropic phenotype that includes a slow gravitropic response, lack of lateral roots, reduced apical dominance, altered vascular development, and reduced fruit growth. Interestingly, the dgt mutation appears to affect the expression of only a subset of auxin-regulated genes. We hypothesize that the Dgt gene encodes a component in the auxin signal transduction pathway since dgt plants do not exhibit altered levels, metabolism, or transport of auxin. By combining map-based cloning with a comparative microsynteny approach, we determined that the Dgt gene encodes a cyclophilin (LeCYP1), a previously unidentified component of auxin signaling. Cyclophilins (CYPs) are cellular receptors for the immunosuppressive drug, cyclosporine A (CsA), with intrinsic peptidyl-prolyl cis-trans isomerase activity. CYPs are known to be involved in numerous aspects of signal transduction pathways and protein folding, interacting with many other signal transduction components in various eukaryotes. However, there is little information as to a role for CYPs in the regulation of plant growth and development. Each known dgt allele contains a unique mutation in the coding sequence of LeCyp1. Single nucleotide point mutations generate an amino acid change (G₁₃₇R) and stop codon (W₁₂₈stop) in dgt^1-1 and dgt^1-2, respectively, and dgt^dp has an amino acid change (W₁₂₈C∆129-133) preceding a 15 bp deletion. All three mutations reside in the CsA-binding domain. Complementation of dgt mutant plants with LeCyp1 restored lateral root initiation from the primary root and the PPIase activity of glutathione S-transferase–LeCYP1 fusion proteins was reduced for nullified by the dgt mutations in vitro. RT-PCR and immunoblot analyses indicate that the expression of LeCyp1 mRNA is not affected by the dgt mutation. However, the accumulation of LeCYP1 protein is greatly reduced in all three alleles, suggesting that each is a null mutation. The CYP inhibitor CsA retards auxin-induced adventitious root initiation in hypocotyl sections and reduces auxin-induced expression of the early auxin response genes, LeIAA10 and 11. Our results indicate that the tomato CYP encoded by the Dgt gene plays a role in the expression of early auxin-response genes and is a novel player in auxin signal transduction pathways.