Twenty Years of Progress in Physiological and Biochemical Investigation of RALF Peptides

Abstract

RALF isoforms play many biological roles and their specific functions are defined by combinatorial interactions with dynamic receptor complexes that vary more than initially thought. Author contributions: All authors participated in writing the Update. KEYWORDS: Rapid Alkalinization Factor (RALF), Carantheus roseus Receptor Like Kinase 1 Like (CrRLK1L), FERONIA (FER), THESEUS1 (THE1), ANXUR1/2 (ANX1/2), BUDDHA'S PAPER SEAL (BUPS), HERCULES1 (HERK1) A family of secreted peptides known as the RALFs (Rapid ALkalinization Factors) induce rapid alkalinization of the extracellular compartment of plant cells. RALFs are perceived by a family of Carantheus roseus Receptor Like Kinase 1 Like proteins (CrRLK1Ls), a structurally related family of plasma membrane-enriched receptors. RALF perception coincides with the downregulation of plasma membrane H +-ATPase function, which reduces energy for solute uptake and results in growth inhibition. In this review, we discuss the initial discovery of this ubiquitous family of peptide growth regulators and their cognate receptors. We describe recent detailed structural studies of RALF-CrRLK1Ls interactions and the identification of a subset of downstream signaling components. We then summarize the current state of RALFs-CrRLK1L research with emphasis on the Arabidopsis (Arabidopsis thaliana) RALFs (AtRALFs) and their roles in cellular regulation of plant growth and development, and in biotic and abiotic stress responses. RALF DISCOVERY, STRUCTURE AND DIVERSITY The origins of the RALF field lie in the work of Clarence "Bud" Ryan's group at Washington State University, with the discovery of the first RALF peptide using an activity assay originally designed to detect systemins, which are small, ~18 amino acid peptide hormones involved in the wound-response, in tobacco (Nicotiana tabacum) plants. This assay targeted the alkalinization caused by systemins and was used to test HPLC-fractionated tobacco leaf extracts. Surprisingly, an HPLC fraction eluting later than systemins elicited an unexpectedly fast and prominent increase in the pH of the culture medium. Edman degradation later confirmed that this activity

The Role of Mechanoperception in Plant Cell Wall Integrity Maintenance

“…Since FER activity is modified by interaction with RALF1, the available data suggest that FER could form a node in a regulatory network coordinating CWI with ABA-based signaling. This notion is further supported by the observation that RALF1-FER interaction leads to increased FER phosphorylation, which in turn inhibits the activity of ARABIDOPSIS H + ATPase2 (AHA2) [86,97] ( Figure 1B). The FER-ABA regulatory feedback loop is not the only one described, which explains to some extent the multitude of functions ascertained to FER.…”

“…This would allow detection of CWD and/or displacement of wall versus membrane ( Figure 2B,C). With the binding specificity of FER and THE1 for RALF peptides apparently changing in a pH-dependent manner, RALFs interacting with more than one CrRLK, and FER regulating AHA2 activity, additional regulatory levels exist, which suggests that fine-tuning of the signaling activity and coordination between different CrRLKs may occur [79,86,97]. Signals from THE1 and FER are possibly relayed via NO-and/or ROS-based signaling cascades [73,80,94].…”

Evidence for multiple receptors mediating RALF‐triggered Ca2+ signaling and proton pump inhibition

“…RALF peptides participate in a plethora of responses (Liao et al ., 2017; Blackburn et al ., 2020). Most RALFs control apoplastic pH during growth, as in developing pollen tubes (Ge et al ., 2017; Mecchia et al ., 2017) and root hairs (Ge et al ., 2017).…”

Family-wide evaluation of RALF peptides inArabidopsis thaliana

“…For instance, different RALF peptides might be secreted upon diverse stimuli in different tissues and this, in turn, would trigger the formation of receptor complexes with a combination of the above-mentioned assembly modules. There are numerous studies analysing individual aspects of this complex signalling network (Blackburn et al, 2020). However, the functional role of the majority of RALF peptides and their bioactivity is largely unknown.…”