Attention, neighbors: Methyl salicylate mediates plant airborne defense

Plants have evolved to generate and release a wide array of volatile organic compounds (VOCs) when challenged by environmental stimuli such as biotic and abiotic stresses, which facilitate their reproduction, defense responses, and plant-plant communication (Karban, 2021). Once emitted, some VOCs can elicit defense signaling in neighboring plants by interacting with specific receptor(s), a phenomenon referred to as airborne defense (AD) (Loreto and D’Auria, 2022).

Aphids, as one of the major agricultural pests, cause tremendous yield loss globally by either direct damage to plant organs or indirect virus-transmitting ability within plants (Jaouannet et al., 2014). Many studies have shown that aphid infestation can induce VOC emission from host plants, and methyl salicylate (MeSA) is considered one of the major components of VOCs (Staudt et al., 2010). MeSA can enhance plant defense by repelling or inhibiting the survival rate of insect pests as well as by attracting their natural enemies (Dong and Hwang, 2017; Ninkovic et al., 2021). In addition, MeSA can act as a mobile signal, not only conferring within-plant and long-distance systemic acquired resistance (SAR) but also establishing interplant communication under microbial pathogen infection (Shulaev et al., 1997; Park et al., 2007).

MeSA plays important roles in plant AD. However, the mode of action of MeSA bridging interplant communication and inducing plant AD remains unclear. The recent study of Gong et al. (2023) unveiled an integral MeSA-mediated AD signal circuit composed of MeSA, salicylic acid-binding protein-2 (SABP2), the transcription factor NAC2, and salicylic acid-carboxylmethyltransferase-1 (SAMT1). This investigation deciphers the details of the molecular genetic mechanism by which MeSA is generated and perceived by neighboring plants as a plant AD agent (Gong et al., 2023).

To investigate virulence strategies of the CMV1a protein (CMV1a) from cucumber mosaic virus (CMV), Gong et al. (2023) identified the transcription factor NAC2 from Nicotiana benthamiana as an interactor of CMV1a and demonstrated that NAC2 is required for plant antiviral defense through virus infection assays on NAC2 knockout plants. They also found that nac2 plants attracted more aphids than wild-type (WT) plants (Gong et al., 2023). The chemical analysis together with the aphid behavior assay showed that the lack of MeSA emission is responsible for nac2-mediated attractiveness to aphids. The aphid preference for nac2 plants still exists after gaseous MeSA treatment, confirming the role of NAC2 in volatile MeSA-induced plant antixenosis resistance to aphids (Gong et al., 2023).

To examine the role of NAC2 in plant AD, the authors mimicked the natural environment by using aphid-attacked plants as emitters (with no-aphid-attacked emitters as control) and tested how aphids react to WT/nac2 receivers, respectively. Compared with WT receivers, nac2 receivers lose the ability to respond to the mobile signal from the aphid-attacked emitters, indicating that MeSA-mediated plant AD is dependent on transcription factor NAC2.

To further investigate how NAC2 regulates MeSA biosynthesis at a molecular level, Gong et al. (2023) identified numerous differentially expressed genes between WT and nac2 plants, of which SAMT1 was selected for further analysis due to its essential role in the conversion of SA into MeSA (Dudareva et al., 1998). Further genetic studies showed that NAC2 transcriptionally regulates SAMT1 expression, and the expression level of SAMT1 is dependent on NAC2. Biochemical assays showed that NAC2 binds to the SAMT1 promoter at the putative NAC transcription-factor-binding site, thus activating reporter gene transcription. Additional genetic and physiological assays showed that NAC2 enhances MeSA production in plants, and volatile MeSA production under aphid attack is dependent on SAMT1. In addition, exogenous application of MeSA or aphid attack significantly increases the mRNA levels of SAMT1 and NAC2. Taken together, these results indicate that the NAC2–SAMT1 module governs the regulation of MeSA biosynthesis in plants. Previous studies have shown that intracellular MeSA is converted into SA and thus activates SAR (Park et al., 2007). Gong et al. (2023) demonstrated that MeSA-mediated SAMT1 and NAC2 expression is SA-dependent, suggesting that SA acts as the cue to initiate NAC2–SAMT1-mediated MeSA volatilization.

As shown in previous reports, SABP2, an esterase of the α/β-fold hydrolase superfamily, is essential for the conversion of MeSA into SA in plant intracellular spaces (Forouhar et al., 2005; Park et al., 2007). Gong et al. (2023) demonstrated that SABP2 binds to MeSA at a physiological concentration and MeSA-mediated plant AD to aphids is SABP2 dependent, suggesting that SABP2 is an odorant-binding protein-like receptor sensing and perceiving the mobile signal MeSA. Besides, virus infection also elicits MeSA production and MeSA-mediated plant antiviral defense in tobacco. Collectively, Gong et al. (2023) proposed a signaling cascade in which SA triggers plant MeSA emission by activating the NAC2-modulated transcription of SAMT1. Mobile MeSA is perceived and converted to SA through SABP2 in neighboring receiver plants. SA, as the cue, elicits self-NAC2–SAMT1-mediated MeSA production in receiver plants, thereby conferring plant resistance against aphids and viruses (Figure 1A).

Figure 1.

Model representing the plant-aphid-virus interplay during airborne defense

(A) MeSA mediates plant AD against aphids and viruses. Aphid attack induces biosynthesis of SA in emitter plants and thus activates the NAC2–SAMT1 module to produce volatile MeSA. Receiver plants perceive and convert volatile MeSA into SA by the receptor SABP2, eliciting NAC2–SAMT1-mediated defense against aphids and viruses.

(B) Virus and aphid counterdefense against plant AD. Viruliferous aphids carry viruses that can secrete helicase-contained viral proteins (such as CMV1a), which alters the subcellular localization and leads to the degradation of NAC2, thus inhibiting the NAC2–SAMT1 module and promoting aphid infestation and virus transmission.

Gong et al.’s (2023) findings revealed that AD reduces aphid’s ability to transmit CMV. Conversely, viruliferous aphids inhibit AD, promoting aphid infestation or virus infection. To examine the molecular mechanisms by which aphids benefit from virus transmission when fighting against plant AD, Gong et al. (2023) found CMV1a relocates NAC2 from the nucleus to the cytoplasm and promotes the degradation of NAC2 through the 26S proteasome, thus suppressing aphid-induced MeSA production and MeSA-induced AD. By using AlphaFold-Multimer, the authors also modeled the structure of the CMV1a–NAC2 complex and revealed that the glycine residue at position 983 within the C-terminal ATP-dependent helicase domain of CMV1a is essential for the CMV1a–NAC2 interplay and interplant AD. Moreover, it was also revealed that some other viruses contain a conserved glycine residue at position 983 of CMV1a, suggesting that some aphid-transmitted viruses employ a conserved strategy to interfere with plant AD.

Concluding remarks and future prospectives

MeSA has been extensively studied as a volatile compound repelling aphids, as well as a within-plant and long-distance mobile signal-inducing SAR to pathogens (Shulaev et al., 1997; Park et al., 2007; Dong and Hwang, 2017). Gong et al. (2023) discovered a signal transduction circuit comprising the MeSA–SABP2–NAC2–SAMT1 complex in plant AD against aphid infestation and virus infection, for the first time deciphering the genetic mechanism of MeSA-mediated plant AD and the mode of action of airborne signal MeSA in plant-plant communication.

In the interplay of aphid-plant-virus, aphid attack induces a high level of SA in plants, which activates NAC2-modulated SAMT1 transcription, thus upregulating biosynthesis and volatilization of MeSA, and conferring plant SAR against viruses (Figure 1A). As an airborne signal, volatile MeSA disperses and is then perceived by neighboring plants through the odorant-binding protein-like receptor SABP2, which converts MeSA into SA, leading to NAC2–SAMT1 activation to produce more MeSA against aphid infestation (Figure 1A). To counteract plant AD, CMV deploys a helicase domain-containing protein, possibly a conserved tactic among multiple virus species, to relocate and degrade NAC2 and thus promote aphid survival and virus infection by undermining the MeSA–SABP2–NAC2–SAMT1 signaling cascade (Figure 1B).

Gong et al. (2023) have achieved a breakthrough in describing signal transduction in VOC-mediated plant AD against aphid colonization, and for the first time, found the plant receptor sensing and perceiving airborne signal MeSA. In addition, the authors discovered that plant viruses can promote aphid infestation by manipulating plant AD, offering new insights into mutualism between aphids and aphid-vectored viruses as well as the long-lasting co-evolution of plant-aphid-virus tri-trophic relationships. More importantly, this study provides novel strategies for future crop protection from the perspective of plant AD enhancement, such as gene editing of plant receptors for more effective perception of VOCs. As plants can release many other volatiles, those possibly act as interplant mobile signals (Loreto and D’Auria, 2022), and additional questions need to be answered, including the following: (1) Are there receptors sensing and receiving these VOCs in plants? (2) Are the receptors VOC-specific, or can some receptors recognize more than one VOC? (3) If some VOCs are not part of the receptor–ligand system, then what are the molecular targets of these VOCs in plants? (4) How do different VOCs work together in the field conditions to reprogram plant defense and immunity against viruses and aphids? Lastly, although the impact of VOCs on plant immunity is highly context-dependent, are there any general patterns that underlie these diverse effects? Answers to these questions could provide a mechanistic understanding of plant VOC signaling cascades and their possible use in future agriculture practices.

Funding

We are sincerely thankful for the funding by National Natural Science Foundation of China (32250410283, 31800386, and 32000201), Jiangsu Outstanding Postdoctoral Program (2022ZB678), and Natural Science Foundation of Jiangsu Province (BK20211319).

Published: November 10, 2023