FERONIA and Her Pals: Functions and Mechanisms^{<a href="#fn1" rid="fn1" class=" fn">1</a>,}^{<a href="#fn2" rid="fn2" class=" fn">[OPEN]</a>}

Abstract

FERONIA and 16 closely related proteins form a distinct clade within the Arabidopsis (Arabidopsis thaliana) superfamily of receptor-like kinases (RLKs), transmembrane proteins with an extracellular domain for signal perception and a cytoplasmic domain that phosphorylates target molecules and induces cellular responses to incoming signals. Several members of this family, such as THESEUS1 and ANXUR1,2, are known to play distinct roles in growth and reproduction; FERONIA is unique in being critically involved in both plant growth and reproduction. The FERONIA family of proteins from Arabidopsis is distinguished from other RLKs by having extracellular protein motifs that share homology with malectin, an animal protein with the capacity to bind dimeric and oligomeric Glc. The possibility that these malectin-like motifs might interact with carbohydrates has generated widespread speculations that these receptor kinases could act as cell-wall sensors, communicating perturbations at the frontline of cell-cell and plant-environment interaction to the cytoplasm to induce responses. Here, we discuss emerging understanding of the functional roles and signaling mechanisms of FERONIA and its related proteins. We also highlight pressing questions, as well as the functional potential of the broader malectin-like domain-containing RLK family that exists across the plant kingdom. We believe FERONIA and her pals provide a rich ground for research with many emerging opportunities for uncovering novel insights into how plants strive for growth and survival.

FERONIA/SIRÈNE was first identified genetically more than ten years ago as a key regulator of female fertility in Arabidopsis (Rotman et al., 2003; Huck et al., 2003). It was later determined to be a receptor kinase (Escobar-Restrepo et al., 2007) and one of 17 closely related receptor-like kinases (RLKs) in Arabidopsis (Fig. 1; Hèmaty and Höfte, 2008; Boisson-Dernier et al., 2011; Cheung and Wu, 2011). The name FERONIA (after an Etruscan goddess of fertility) will be used from hereon. Arabidopsis has more than 600 RLKs (Shiu and Bleecker, 2003). Several discoveries made at about the same time led to an extraordinary level of interest in FERONIA and related RLKs. These include: (1) a member of this group, THESEUS1 (named after the Greek mythological figure that slew Procustes the brigand) is a critical regulator of cell growth and appears to function as a surveyor of cell-wall status (Hèmaty et al., 2007); (2) FERONIA functions broadly throughout development and is fundamental to cell and plant growth (Guo et al., 2009; Deslauriers and Larsen, 2010; Duan et al., 2010); and (3) a closely related pair of these RLKs, ANXUR1 and ANXUR2 (named after the consort of FERONIA), is essential for male fertility (Boisson-Dernier et al., 2009; Miyazaki et al., 2009). Last but not least was the report of malectin, a novel protein from animals with the capacity to bind dimeric and oligomeric Glc-binding protein (Schallus et al., 2008) and the realization that FERONIA and related RLKs contain malectin-like motifs (PFAM CL0468) in their extracellular domains (Fig. 1). This led to widespread speculations that FERONIA and related RLKs might interact with carbohydrate moieties and function as sensors of perturbations in the cell wall, communicating conditions at the cell surface to induce appropriate cellular responses (Hèmaty and Höfte, 2008; Boisson-Dernier et al., 2011; Cheung and Wu, 2011; Lindner et al., 2012). FERONIA and related malectin-like domain-containing RLKs are often referred to as the CrRLK1-like RLKs (see e.g. Nibau and Cheung, 2011), after its founding member identified in Catharanthus roseus, CrRLK1 (Schulze-Muth et al., 1996), but for which no functional work has been reported. THESEUS1 was the first member of the group for which a clear functional role was demonstrated, and FERONIA is the most prevalently studied among these RLKs. To provide a functional context for our discussion here, we will refer to the FERONIA-related RLKs in Arabidopsis as the THESEUS1/FERONIA-related RLK family. We update current knowledge about these RLKs from Arabidopsis and highlight pressing questions and emerging opportunities from these and related malectin-like domain-containing RLKs, which are present throughout the plant kingdom (Hèmaty and Höfte, 2008; Antolin-Llovera et al., 2014; Nguyen et al., 2015).

Figure 1.

FERONIA protein domain structure and phylogenetic tree of the Arabidopsis THESEUS1/FERONIA receptor kinase family. A, Deduced FERONIA structural domains. SS, ECD, TM are, respectively, signal peptide, extracellular domain, transmembrane domain. MALA and MALB are tandem malectin-like domains. exJM, extracellular juxtamembrane region. Numbers indicate amino acid residues. B, The THESEUS1/FERONIA protein family.

Extracellular homology with malectin (Schallus et al., 2008; Fig. 1A) distinguishes the THESEUS1/FERONIA-related RLKs from other members of the Arabidopsis RLK family. Malectin, named after its in vitro ability to bind maltose (Glc α1-4 Glc), is a conserved animal protein located in the lumen of the endoplasmic reticulum where it is involved in protein quality control in the early steps of secretion (Schallus et al., 2008; Qin et al., 2012). A majority of the Arabidopsis THESEUS1/FERONIA family (Fig. 1B) has tandem malectin-like motifs (Fig. 1A; Boisson-Dernier et al., 2011) in their extracellular domains. We focus our discussion on FERONIA, THESEUS1, and three other members of the family, ANXUR1, ANXUR2, and ERULUS/[Ca^]_cyt-associated Protein Kinase1, for which clear biological roles have been demonstrated (Miyazaki et al., 2009; Boisson-Dernier et al., 2009; Bai et al., 2014). A contribution to cell growth and morphogenesis has also been reported for HERCULES1 (Guo et al., 2009) and CURVY (Gachomo et al., 2014), but details regarding their functions remain limited.

Acknowledgments

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