Functional analysis of related Cr RLK 1L receptor‐like kinases in pollen tube reception

Abstract

The Catharanthus roseus Receptor-Like Kinase 1-like (CrRLK1L) family of 17 receptor-like kinases (RLKs) has been implicated in a variety of signaling pathways in Arabidopsis, ranging from pollen tube (PT) reception and tip growth to hormonal responses. The extracellular domains of these RLKs have malectin-like domains predicted to bind carbohydrate moieties. Domain swap analysis showed that the extracellular domains of the three members analyzed (FER, ANX1, HERK1) are not interchangeable, suggesting distinct upstream components, such as ligands and/or co-factors. In contrast, their intercellular domains are functionally equivalent for PT reception, indicating that they have common downstream targets in their signaling pathways. The kinase domain is necessary for FER function, but kinase activity itself is not, indicating that other kinases may be involved in signal transduction during PT reception.

Receptor-like cytoplasmic kinase MARIS functions downstream of Cr RLK1L-dependent signaling during tip growth

“…FER was recently found to mediate the inhibition of primary root growth by forming a receptor-ligand pair with the rapid alkalinization factor 1 (RALF1) peptide (20). Moreover, it was shown that the intracellular kinase domains of three CrRLK1Ls members are interchangeable and that the various CrRLK1Ls share downstream signaling components (21). This finding has been confirmed for at least one signaling component so far, namely the reactive oxygen species (ROS)-producing NADPH oxidases, also called "respiratory burst oxidase homologs" (Rboh).…”

Signals fly when kinases meet Rho-of-plants (ROP) small G-proteins

“…Moreover, kinase activity is 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 not always necessary for receptor-like kinase-mediated downstream signalling towards ROPs, such as in the case of FERONIA-mediated pollen tube rupture in Arabidopsis [59] or PANGLOSS1-regulated stomatal patterning in maize leaves [31]. These kinases may serve as scaffolds for the assembly of large complexes, potentially including other kinases.…”

“…One possible way in which FERONIA could regulate downstream signalling is through the direct phosphorylation of RopGEFs. However, even though the kinase domain of FERONIA is necessary for pollen tube reception, kinase activity is not [59]. Consequently, FERONIA is hypothesized to serve as a co-receptor, recruiting other factors: potentially other kinases mediating the extracellular signals towards cellular targets [59].…”

“…These kinases may also serve as upstream regulators of ROP GTPase signalling. While the kinase domains of three members of the family (FERONIA, ANXUR1 and HERKULES1) were interchangeable in an in planta functional assay, their extracellular domains were not [59]. Therefore, these receptor kinases Potential receptor kinase-RopGEF pairs could be hypothesized based on the similarities between mutant phenotypes, coupled with co-expression analysis and traditional proteinprotein interaction assays.…”

RLKs orchestrate the signaling in plant male-female interaction

“…Sequence analysis suggests that the extracellular domain of FER was under positive selection during species evolution, thus the signal perception of FER could function as an interspecies cross barrier (Escobar-Restrepo et al, 2007). In contrast, the cytoplasmic kinase domain of FER can be replaced with that of ANX for full function of FER (Kessler et al, 2015). Although the ligand of FER from the pollen tube is not yet identified, studies in plant growth have unveiled that a secreted peptide-RALF1 (rapid ALkalization factor 1), most likely acts as the ligand of FER during seedling growth (Haruta et al, 2014).…”