A plant surface receptor for sensing insect herbivory

Herbivory, the feeding on living plant parts by animals, is a fundamental ecosystem process affecting both global autotroph biomass production in natural habitats and crop production in agricultural settings (1). Invasions by herbivorous insects are an ancient threat to food security as evidenced, for example, by their inclusion as one of the 10 Biblical plagues. Insect pests remain a major threat to the world’s food security both in terms of regular annual crop loss as well as periodic catastrophic losses such as those caused by locust swarms that have repeatedly swept over large parts of East Africa (2). Modern integrated pest management strategies comprise mechanical methods (barriers, traps, tillage), the use of synthetic insecticides, the application of biological control agents (pest-parasitizing insects, insecticidal nematodes), and molecular marker-based breeding strategies (3). Biotechnological transfer of insect resistance traits holds great potential for the production of crops with enhanced pest resilience. This strategy requires a detailed understanding of mechanisms underlying insect pest recognition in host plants, which until recently, was lacking. In PNAS, Steinbrenner et al. (4) reveal the molecular identity of a plant immune receptor sensing herbivory-inflicted host tissue damage.

Plants employ a germline-encoded innate immune system to combat microbial infections and insect pest invasions (5, 6). This surveillance system is activated upon sensing patterns of danger derived either from the infectious agent or from the host plant itself. Immunogenic microbial surface signatures are referred to as pathogen-associated molecular patterns (PAMPs). In analogy to PAMPs, plant defense-stimulating compounds found in saliva, oral secretions (OSs), or frass of plant-chewing (caterpillars, beetles) or sucking (mites, planthoppers) herbivorous insects have been termed herbivore-associated molecular patterns (HAMPs) (7, 8). Plant-derived cellular breakdown products released upon herbivory, mechanical damage, or microbial infection are termed damage-associated molecular patterns (DAMPs) or endogenous danger signals (5, 6). Collectively, pattern recognition by plant cell surface-resident pattern recognition receptors (PRRs) triggers nonspecific, generic immune responses. Plant defenses mounted in response to microbial and insect cues overlap but are not identical (9).

Numerous plant immune receptors sensing microbe- or plant-derived patterns have been identified in recent years (10, 11). In several cases, PRR transfer was shown to confer enhanced immunity to microbial infection to plant species even across family boundaries (10, 11). However, plant receptors for sensing insect-derived patterns have been elusive so far. In earlier studies, Schmelz et al. (12–14) identified an 11-amino-acid proteolytic fragment, termed inceptin, which is released from armyworm digestion of the plant chloroplastic adenosine triphosphate (ATP) synthase. Inceptin elicits hallmark immune responses in cowpea plants (12). Now, with the identification of an inceptin receptor (INR) in species of the legume subtribe Phaseolinae, Steinbrenner et al. (4) have opened routes to engineer insect resistance in a variety of crop plants. The authors’ experimental approach to identify INR was guided 1) by previous work suggesting receptor-mediated HAMP perception at the cell surface (4) and 2) by reports on substantial genetic variation in PAMP sensitivity among accessions (subspecies) of a given plant species (11). To map INR, a series of cowpea (Vigna unguiculata) accessions was treated with an inceptin variant that was biologically less active than the wild-type peptide. This screen revealed remarkable defense response variation within cowpea germplasm that, somewhat surprisingly, could not be observed when using wild-type inceptin. A cross between inceptin-sensitive and -insensitive accessions was used to build a recombinant inbred line collection for quantitative trait loci (QTL) mapping. Subsequently, a very elegant combination of genome-wide association and QTL mapping approaches culminated in the identification of a single genetic locus associated with inceptin sensitivity. Within this locus, the authors identified a transmembrane leucine-rich repeat (LRR)–receptor-like protein (RLP)–encoding gene (INR), which conferred inceptin sensitivity to Nicotiana benthamiana. Functional INR homologs were found in closely related Vigna radiata and common bean (Phaseolus vulgaris), but not in soybean (Glycine max), suggesting that the ability of plants to perceive inceptins has evolved rather recently. INR specifically binds inceptin; as inferred from receptor–ligand docking simulations, binding is likely mediated by two positively charged amino acid residues within INR that correspond to a negative charge in inceptins. Importantly, site-directed mutagenesis revealed that these residues were required for both receptor–ligand binding and inceptin-inducible defenses. As commonly observed for LRR-RLPs implicated in PAMP recognition, INR builds a ternary complex with two coreceptors, Suppressor of BIR1 (SOBIR1) and members of the Somatic Embryogenesis Receptor Kinase (SERK) family (6, 10, 11). Such heteromeric complex formation is triggered upon inceptin binding to INR, which recruits SERKs into preformed INR SOBIR1 complexes. Stable transformation into N. benthamiana or tobacco plants conferred enhanced resistance to infection with beet armyworm (Spodoptera exigua), a generalist Lepidopteran herbivore. Thus, plants unrelated to cowpea link ectopically expressed HAMP receptors to endogenous insect defense pathways, suggesting that INR may be used to engineer insect resistance in species within and across plant family boundaries. Importantly, INR allele strength variation observed in cowpea was mirrored upon expression of INR variants in N. benthamiana plants, hence offering ways to fine tune insect resistance levels in transgenic crops.

Steinbrenner et al. (4) report an intriguing piece of research that illustrates, in unprecedented molecular detail, the complexity of chemical warfare underpinning plant–insect interactions. As summarized in Fig. 1, caterpillar feeding on cowpea plants causes massive plant tissue destruction and release of intracellular materials, including chloroplasts. Subsequently, fodder intake by the insect initiates proteolytic processing, release, and accumulation of plant inceptins in OS. Feeding-mediated delivery of OS into intact plant tissue may subsequently activate INR, trigger antiinsect defenses, and eventually halt herbivore attack.

Fig. 1.

Pattern generation and perception in herbivory-induced plant immune activation. Plant leaf materials taken up by caterpillars during feeding (1) serve as a source for herbivory-associated patterns. Caterpillar gut proteases release peptides from plant chloroplast ATP synthase (blue), called inceptins (2). Via caterpillar OSs, these peptides are transferred back to the plant (3) where they are recognized by a PRR complex made of the INR, the adaptor kinase SOBIR1, and SERK-type coreceptors (4). Inceptin recognition induces plant defenses that ultimately stop herbivore growth (5).

Inceptins are commonly called HAMPs (4, 12–14). However, due to their plant origin, these peptides may as well be considered host endogenous DAMPs. Parasitic plant-associated molecular patterns (ParAMPs), such as newly identified Cuscuta spp. glycine-rich cell wall proteins, may not be found in parasitic plants alone (15). Likewise, PAMPs are often found in both pathogens and commensals and are thus also referred to as microbe-associated molecular patterns (5, 6). This confounding mix of terms exemplifies difficulties in a strict definition of these molecules and may argue for using the sole term “pattern” to describe various immunogenic molecules.

A feature common to all of these patterns is their recognition by plant cell surface PRRs (10, 11). Plant PRRs are distinguished by the architecture of their ligand-binding ectodomains and the presence or absence of a cytoplasmic protein kinase domain. Plant immune receptors with LRR ectodomains, such as INR, predominantly sense proteinaceous patterns, most likely because the LRR domain is predestined to form protein–protein interactions (16). Ligand-dependent recruitment of SERK protein family members into PRR complexes, such as the INR SOBIR1 complex, is another uniform feature of LRR-type immune receptors. The current report on INR (4) now reinforces the view on plant immunity as a generic surveillance system for patterns of danger regardless of their phylogenetic origin. Rather, the molecular nature of microbe-, insect-, or plant-derived patterns may define the type of surface receptor implicated in pattern recognition and activation of plant immunity.