Abstract
Mechanisms that govern the size of plant organs are not well understood but believed to involve both sensing and signaling at the cellular level. We have isolated loss-of-function mutations in the four genes comprising the transmembrane kinase TMK subfamily of receptor-like kinases (RLKs) in Arabidopsis. These TMKs have an extracellular leucine-rich-repeat motif, a single transmembrane region, and a cytoplasmic kinase domain. While single mutants do not display discernable phenotypes, unique double and triple mutant combinations result in a severe reduction in organ size and a substantial retardation in growth. The quadruple mutant displays even greater severity of all phenotypes and is infertile. The kinematic studies of root, hypocotyl, and stamen filament growth reveal that the TMKs specifically control cell expansion. In leaves, TMKs control both cell expansion and cell proliferation. In addition, in the tmk double mutants, roots and hypocotyls show reduced sensitivity to applied auxin, lateral root induction and activation of the auxin response reporter DR5: GUS. Thus, taken together with the structural and biochemical evidence, TMKs appear to orchestrate plant growth by regulation of both cell expansion and cell proliferation, and as a component of auxin signaling.
Highlights
During development, it is essential for all multicellular organisms to perceive and process information from extracellular signals via cell surface receptors to control organ size and growth
The evolutionarily conserved receptor tyrosine kinase (RTK) signaling pathway plays a central role in regulating cell proliferation, cell growth, cell differentiation and survival [1], [2]
We explore the biological function of all four members in the TMK subfamily of receptor-like kinases (RLKs) in Arabidopsis and present evidence that the four members of the TMK subfamily play a critical role in the control of cell expansion and cell proliferation
Summary
It is essential for all multicellular organisms to perceive and process information from extracellular signals via cell surface receptors to control organ size and growth. The evolutionarily conserved receptor tyrosine kinase (RTK) signaling pathway plays a central role in regulating cell proliferation, cell growth, cell differentiation and survival [1], [2]. RTKs comprise a large family of integral membrane proteins with highly divergent extracellular domains coupled to a conserved intracellular tyrosine kinase motif. Plant receptor-like kinases (RLK) exhibit a basic structural similarity with animal RTKs, and display several unique characteristics. Most plant RLKs are serine/threonine kinases and have extracellular domains distinct from ligand-binding motifs of RTKs in animals [3], [4]. Structural features of plant RLKs support the idea that they may function as entry points for signaling pathways
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