Looking BAK again: Is an old acquaintance of innate immunity involved in the detection of herbivores?

The membrane-based receptor-like kinase BAK1 has been reported to interact with a number of other membrane-based receptors to contribute to a variety of signaling responses to exogenous and endogenous cues. These include brassinosteroid hormones as well as conserved microbe-derived and endogenous patterns. More recently, several lines of evidence have been reported to expand this concept also to the detection and deterrence of insect herbivores. We hereby present results that further support this hypothesis as they show that in Arabidopsis thaliana, herbivore oral secretions trigger two hallmark responses of plant innate immunity and that these responses are significantly reduced in plants that lack functional BAK1 receptors.

The membrane-bound receptor-like kinase (RLK) BAK1 (BRI1-associated kinase) has been shown to interact with a variety of other RLKs once the latter bind their respective ligands. This has been observed for BRI1 (Brassinosteroid insensitive 1), the receptor for brassinosteroid phytohormones, as well as a number of pattern recognition receptors that are known to elicit PTI-like (Pattern-Triggered Immunity) responses in plants. In Arabidopsis thaliana, their ligands include the microbial-elicitors flg22 and elf18 as well as the endogenous plant elicitor peptides (PEPs). For all these interactions, BAK1 is believed to be a positive contributor to downstream signaling events as plants lacking a functional BAK1 system commonly display decreased physiological responses to the respective cues. Again in Arabidopsis and additional to a role in the detection and deterrence of microbial invaders, BAK1 has recently been proposed to be involved in the recognition of yet unknown viral elicitors as bak1 mutants both displayed decreased PTI responses upon treatment with viral extracts as well as an increased susceptibility to viral infections. Finally, Yang et al. (2011a) observed a decreased accumulation of several marker genes for defense responses against herbivores in BAK1-silenced Nicotiana benthamiana plants, suggesting a role for BAK1 in the recognition of herbivore-derived elicitors, too. These findings were later supported by reports of BAK1 being required for the full amplitude of a selection of PTI responses upon treatment with aphid-derived elicitors.

Investigating the relevance of PEP signaling in the deterrence of herbivores in Arabidopsis thaliana, we collected additional data to support a potential role of BAK1 in herbivore detection. In a similar experimental setup as described by Körner (2013), we assessed the amplitude of 2 hallmark PTI-responses, namely ethylene production and seedling growth inhibition (SGI) upon exposure to Spodoptera littoralis Boisduval (Lepidoptera: Noctuidae) oral secretions (OS), and we discovered that these responses were reduced in mutants with an impaired BAK1 signaling system (Fig. 1). This reduction was similar to the previously reported impaired responses of bak1-mutants to the microbial elicitor flg22. Intriguingly, no such reduction of ethylene production and SGI upon elicitation with Spodoptera OS was observed in mutants impaired in jasmonic acid (JA) signaling, a known master hormone for the integration of plant defense responses against herbivores: Both a mutant in JA biosynthesis (aos, allene oxide synthase) and signaling (coi1-1, coronatine-insensitive 1) displayed ethylene and SGI responses comparable to wild type plants (Fig. 2).

Figure 1.

OS induced defense responses are dependent on BAK1. (A) Arabidopsis seedlings of the lines Col-0 (wild-type), bak1-4 and bak1-5 were grown under sterile conditions for 5 d and then transferred into MS medium either containing 1µM of flg22, 0.5% OS (v/v) or no elicitor (control). Seedling growth was assessed 10 d after transfer by weighing the fresh weight of seedlings. (B) Leaf disks of Arabidopsis Col-0 (wild type), bak1-4 and bak1-5 plants were treated with 1µM flg22, 0.5% (v/v) OS 1µM or no elicitor (control). Ethylene production was assessed 5 hours after after the application of the respective elicitor. Error bars show +/− 1 SE of 6 independent replicates. Asterisks represent statistically significant differences to the respective treatment in wild-type plants (t-test, *= p < 0.05; ns = not significant).

Figure 2.

OS induced defense responses are indedependent of JA signaling. (A) Arabidopsis seedlings of the lines Col-0 (wild-type), aos and coi1-1 were grown under sterile conditions for 5 d and then transferred into MS medium either containing 1µM of flg22, 0.5% OS (v/v) or no elicitor (control). Seedling growth was assessed 10 d after transfer by weighing the fresh weight of seedlings. (B) Leaf disks of Arabidopsis Col-0 (wild type), aos and coi1-1 plants were treated with 1µM flg22, 0.5% (v/v) OS 1µM or no elicitor (control). Ethylene production was assessed 5 hours after after the application of the respective elicitor. Error bars show +/− 1 SE of 6 independent replicates. Asterisks represent statistically significant differences to the respective treatment in wild-type plants (t-test, *= p < 0.05; ns = not significant).

Taken together, these results provide additional lines of evidence for an involvement of BAK1 as a facilitator in the recognition of specific elicitors in herbivore OS. Moreover, our results suggest that this recognition and signaling may be independent of the JA pathway as JA mutants do not display an impairment of the assessed PTI-responses upon OS treatment. However, neither the elicitors nor the corresponding receptors for this interaction have yet been identified and there also remains a possibility of the PTI-causing elicitors being derived from a microbial flora in the herbivore regurgitant, as for instance reported and proposed by Chung et al. (2013). Still, since membrane-bound receptors for several herbivore elicitors have been proposed but not yet verified, it remains tempting to speculate that these could be RLKs and that they might interact with BAK1 in a similar fashion to the ones responsible for the detection of other danger signals. Following these observations and lines of evidence, BAK1 might prove a powerful tool to help identify the receptors and ligands involved in this signaling pathway.