Abstract
Because of their high level of diversity and complex evolutionary histories, most studies on plant receptor-like kinase subfamilies have focused on their kinase domains. With the large amount of genome sequence data available today, particularly on basal land plants and Charophyta, more attention should be paid to primary events that shaped the diversity of the RLK gene family. We thus focus on the motifs and domains found in association with kinase domains to illustrate their origin, organization, and evolutionary dynamics. We discuss when these different domain associations first occurred and how they evolved, based on a literature review complemented by some of our unpublished results.
Highlights
It was assumed for decades that most cell-to-cell communication in plants would occur via cytoplasmic bridges called plasmodesmata because of the presence of cell walls separating plant cells from one another
Comparative genome analysis between Arabidopsis and other sequenced genomes again highlighted that receptor-like kinases (RLKs) were highly similar to the Drosophila Pelle protein kinase and the mammalian INTERLEUKIN1 RECEPTOR-ASSOCIATED KINASE (IRAK). These results provided a first indication that (a) RLKs have a monophyletic origin and are close relatives to animal Pelle kinases and IRAKs, (b) RLK kinase domains evolve more rapidly than kinase domains of other protein kinases, (c) some extracellular ligand-binding domain (ECD) found in plants are unfamiliar in animals, and (d) the ECDs are similar, kinase domains of some of these RLK receptors do not cluster together in the phylogenetic trees, suggesting that several associations between these ECDs (e.g., S-domain or leucine-rich repeat (LRR)) and different kinase domains could have occurred independently
The presence of symbiotic gene homologs within the entire green lineage revealed that homologous sequences of lysin motif (LysM), malectin-like (CrRLK1L) and malectin-like LRR-domain RLKs were present in charophyte lineages, so these associations appeared before the emergence of land plants [34, 152] (Figure 3)
Summary
We pay tribute to the pioneering researchers who discovered the first members of each of the new structural organizations observed in these receptors. The first plant RLK was described in 1990, when Walker & Zhang [154] amplified and cloned, from a complementary DNA (cDNA) library of maize roots, the ZmPK1 gene. This open reading frame, representing an almost complete transcript, encoded an 817–amino acid (AA) protein. Some of these domain organizations are found only in association with specific kinase subgroups. The relationships between these domain organizations and the group types classified according to the kinase phylogeny proposed by Lehti-Shiu & Shiu [77] are provided in
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