Abstract
Plants respond to herbivory by perceiving herbivore danger signal(s) (HDS(s)), including “elicitors”, that are present in herbivores’ oral secretions (OS) and act to induce defense responses. However, little is known about HDS-specific molecules and intracellular signaling. Here we explored soybean receptor-like kinases (RLKs) as candidates that might mediate HDS-associated RLKs’ (HAKs’) actions in leaves in response to OS extracted from larvae of a generalist herbivore, Spodoptera litura. Fractionation of OS yielded Frα, which consisted of polysaccharides. The GmHAKs composed of their respective homomultimers scarcely interacted with Frα. Moreover, Arabidopsis HAK1 homomultimers interacted with cytoplasmic signaling molecule PBL27, resulting in herbivory resistance, in an ethylene-dependent manner. Altogether, our findings suggest that HAKs are herbivore-specific RLKs mediating HDS-transmitting, intracellular signaling through interaction with PBL27 and the subsequent ethylene signaling for plant defense responses in host plants.
Highlights
Plants respond to herbivory by perceiving herbivore danger signal(s) (HDS(s)), including “elicitors”, that are present in herbivores’ oral secretions (OS) and act to induce defense responses
Given that high molecular weight substances consisting of unknown oligosaccarides in OS of Spodoptera spp. function as putative elicitors in Arabidopsis host plants[12], we focused on the receptor-like kinases (RLKs) superfamily based on our hypothesis that some soybean herbivore-derived danger signals (HDSs)-associated RLKs’ (HAKs) (GmHAKs) are structurally similar to AtCERK1 (LysM-RLK), which may recognize various unknown elicitors
We mined 19 soybean RLKs showing amino acid sequence similarity to AtCERK1 (~44% identities). Of these 19 putative soybean RLKs far mined as GmHAK candidates, the corresponding cDNAs of 15 genes were successfully amplified (Supplementary Fig. 1a), and we generated the respective 15 transgenic Arabidopsis lines independently expressing RLK genes under the control of the constitutive cauliflower mosaic virus (CaMV) 35S promoter (35SP) (R1-R15) (Supplementary Table 1)
Summary
Plants respond to herbivory by perceiving herbivore danger signal(s) (HDS(s)), including “elicitors”, that are present in herbivores’ oral secretions (OS) and act to induce defense responses. Our findings suggest that HAKs are herbivore-specific RLKs mediating HDS-transmitting, intracellular signaling through interaction with PBL27 and the subsequent ethylene signaling for plant defense responses in host plants. It has been shown that the Arabidopsis LRR-RLK, brassinosteroid insensitive1-associated kinase 1 (BAK1), is required for the defense response to aphid attack[16]. It remains to be clarified, for all of these membrane proteins, whether these proteins are specific to elicitors from the respective insects, and if so, what type of elicitor molecule(s) act as the HDS.
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