Receptor kinases form a superfamily of transmembrane proteins that activate intracellular signaling cascades in response to extracellular stimuli. Much of our knowledge on receptor kinase signaling came from studies in animals, in which case the ligand binding induces dimerization of receptor kinases, leading to kinase activation.
Recently, molecular identity of a few plant ligand molecules and downstream signaling components have been revealed, providing an insight as to how receptor-like kinases (RLK) are activated by such ligands leading to signal transduction in plants. Worthy of mention in this context is the role of extracytoplasmic leucine-rich repeats (LRR) motif that defines a versatile ligand-binding domain.
In a review article, published in the October issue of Current Opinion in Plant Biology, Keiko U. Torii of the University of Washington, Seattle, updates researchers on the recent advances in this exciting field. She begins the review by referring to a role of the RLK subfamilies in regulating several developmental processes, phytohormone perception and defense responses, including CLAVATA1 (CLV1), which control meristem cell fate in Arabidopsis, S receptor-like kinases (SRK), which determine self incompatibility in Brassica, BRASSINOSTEROID INSENSITIVE 1 (BRI1), a putative brassinosteroid hormone receptor, and a race specific disease resistance gene product (Xa21**) of rice.
The author then summarizes the latest development in the identification of specific genes encoding ligands for SRK and CLV1 RLK. The gene product of CLV3, whose mutation confers the clavata phenotype, has been identified as a possible ligand for the CLV1 receptor.
The author recounts a recent report of two Arabidopsis loci, FLAGELLIN SENSING (FLS1) and FLS2, which trigger defense response to bacterial flagellin protein. FLS2 has been shown to encode LRR (leucine-rich-repeats) -RLK (receptor like kinase) indicating that this gene is likely to act as a receptor for bacterial flagellin protein. A 15-amino acid peptide, discovered in the conserved region of the flagellin protein, may define the minimal ligand domain because it is capable of triggering the defense mechanism in Arabidopsis plants.
The author further discusses models for receptor complex assembly and signal transduction, implicated from studies on CLV1 and Xa21. In the first model, she speculates that at the plasma membrane level, CLV1 forms a receptor core complex with CLV2 by means of disulphide linkage. The heterodimer is then activated by a presumed ligand, CLV3. This activation links it to downstream elements such as kinase-associated protein phosphate (KAPP) and Rho like GTPase (Rop). In the second model, she uses the example of transgenic plants expressing Xa21D. It is suggested that the secreted protein either forms a heterodimer with receptor like kinase or forms a homodimer. Binding of the latter to ligand (presumably Avr***Xa21), activates the cytoplasmic kinase domain triggering defense response.
Although it is a long way to go to determine mechanisms of plant receptor like kinase (RLK) activation, recent biochemical studies have proved to be of immense value in understanding their unique modes of action explaining the way signal transduction works in plants.
* Brassinosteroid is a hormone required for normal development of plants
** Genes in the Xa21 locus confer resistance to Xanthomonas oryzae pv. oryzae (bacterial blight).
*** Avr = Avirulence gene present in the pathogen. According to a gene-for-gene hypothesis, avirulence gene of pathogens are recognized by a corresponding disease resistance gene of host plants, thus triggering a defense response in invaded plants. Molecular cloning of avirulence genes and their corresponding disease resistance genes reveals that they encode ligand and receptors, respectively.