Poster: Vegetative development

Abs # 332: Auxin-associated growth is mediated by the TMK subfamily of receptor kinases from Arabidopsis

Presenter:	Dai, Ning , ndai@wisc.edu
Authors	Dai, Ning  (A)   Wang, Wuyi  (A)   Shiu, Shinhan  (B)   Bleecker, Anthony B (A)
Affiliations:	(A): Department of Botany and Laboratory of Genetics, University of Wisconsin-Madison (B): Department of Ecology and Evolution, University of Chicago

The four genes representing the TMK subfamily of receptor-like kinases (RLK subfamily LRR IX) from Arabidopsis code for proteins containing a leucine-rich-repeat extracellular domain, a single transmembrane domain, and a cytoplasmic serine/threonine kinase domain. Three of the four members; TMK1, TMK3, and TMK4; are expressed globally throughout development at abundant levels while the fourth, TMK2, is expressed at low levels only in flowers and fruits. Mutational analysis of T-DNA insertions generating lost function mutants for all four genes revealed no phenotype for any single-gene mutant nor for most double-mutant combinations. However, the double mutant showed a three-fold reduction in root and etiolated-hypocotyl growth at four days after germination, a ten-fold reduction in final sixth leaf size, a two-week delay in bolting time, and a 60% reduction in fertility relative to wild type. The triple mutant displayed an increased severity in all of the above phenotypes including a four-fold reduction in root and etiolated-hypocotyl growth, a twenty-fold reduction in final sixth leaf size, a three-week delay in bolting time, and a 90% reduction in fertility compared to wild type, while the quadruple mutant was , in addition, completely infertile. Kinematic studies of cell division and cell expansion in the mutants revealed that the TMK mutations specifically affected cell expansion in roots, hypocotyls, meristem, the early proliferative phase of leaf development, and during megagametophyte development. Interestingly, callus induced from mutant tissue did not show any growth reduction. Hormonal effects in the TMK mutants were investigated using a root growth inhibition assay. The roots of double and triple mutant showed relatively normal responses to applied cytokinin, abscisic acid, and ethylene. By contrast, the double mutant showed a reduced sensitivity to applied auxin, while the triple mutant was completely insensitive to applied auxin as high as 30 μM. Mutants also showed reduced sensitivity to auxin in lateral root induction and DR5::GUS activation in root and leaf primodia. We conclude that the TMK subfamily of receptor kinases act in a functionally redundant manner to mediate cell expansion during specific stages in development in pathways that are downstream of auxin.