Maize plants prime anti-herbivore responses by the memorizing and recalling of airborne information in their genome

Abstract

Intact maize plants prime for defensive action against herbivory in response to herbivore-induced plant volatiles (HIPVs) emitted from caterpillar-infested conspecific plants. The recent research showed that the primed defense in receiver plants that had been exposed to HIPVs was maintained for at least 5 d after exposure. Herbivory triggered the receiver plants to enhance the expression of a defense gene for trypsin inhibitor (TI). At the upstream sequence of a TI gene, non-methylated cytosine residues were observed in the genome of HIPV-exposed plants more frequently than in that of healthy plant volatile-exposed plants. These findings provide an innovative mechanism for the memory of HIPV-mediated habituation for plant defense. This mechanism and further innovations for priming of defenses via plant communications will contribute to the development of plant volatile-based pest management methods in agriculture and horticulture.

Plants can adapt to critical environmental changes to promote their own and their population’s survival. For instance, plants have an ability to defend against herbivores by emitting and receiving herbivore-induce plant volatiles (HIPVs).¹ Such airborne signals allow plants experiencing them, but themselves not under attack, to tailor their defenses to their current and expected risk of herbivory. In some cases, these receiver plants do not show immediate changes in their level of defenses, but respond more strongly and more rapidly than non-receiver plants upon damage by herbivores.¹ Since HIPVs are emitted by damaged plants for only a period of days after the onset of damage,² neighboring plants can only receive volatile signals when emitter plants are damaged, or slightly later. However, plants cannot be aware of how much later the herbivores will arrive, indicating that plants do not know how long they should record volatile signals. Therefore, questions about plants’ ability to employ an appropriate time scale and about the mode of sustaining priming arise. This is important in order to develop semiochemical-based pest management methods based on plant communications.

In response to Mythimna separata attack, infested maize plants emit high levels of HIPVs comprising terpenoids (myrecene, [E]-β-ocimene, [E]-4,8-dimethyl-1,3,7-nonatriene, linalool, [E]-β-caryophyllene, [E]-α-bergamotene, [E]-β-farnesene and [{E,E}-4,8,12-trimethyltrideca-1,3,7,11-tetraene], green leaf volatiles [{Z}-3-hexen-1-yl acetate] and indole³ [Fig. 1]). In the leaves of maize plants exposed to the blend of those HIPVs, the expression of a defensive gene for Bowman-Birk type trypsin inhibitor (TI), was not promoted directly by HIPVs but rather was recalled by the plant when it was later fed on.⁴ Moreover, this priming was recalled strongly after 5 d of post-exposure maintenance. Jasmonic acid signaling is not involved in the recalled expression but is critically involved in the activation of this defense gene when plants are attacked by chewing herbivores (Fig. 2). What is the mechanism for the memorizing of priming anti-herbivore responses? A major hint was hidden in a recent hot topic: epigenetic modifications by which DNA methylation, chromatin, and small RNA-based mechanisms can contribute separately or together to phenotypes by regulating gene expression in response to stress effects.⁴ We therefore investigated the DNA methylation status in receiver maize plants, and observed a suite of demethylation sites in the promoter region of TI.³ These observations are consistent with previous reports linking stress-induced methylation changes to transcriptional control of inducible genes in other plant systems.^5,6 There are linear pathways from reception of signals and propagation of effectors to a type of memory that may be described by terms such as learning, habituation or priming.⁷ So, there should be a storage and recall memory based on a complex network including epigenetic modifications of DNA and histones in any cases of plant communications mediated by HIPVs.

Figure 1. Volatile emissions from maize plants infested with Mythimna separata after 1 day.

Figure 2. Possible mechanisms underlying the synergy of the HIPV-primed demethylation of the TI promoter region (priming) and herbivore-induced JA signaling activation for the enhanced TI expression in receiver maize plants. The primed status in the receiver plants is maintained for up to 5 d after exposure to HIPVs, leading to intensified defense actions toward herbivore pests. HIPVs, herbivore-induced plant volatiles; JA, jasmonic acid; TI, trypsin inhibitor.

Although priming anti-herbivore responses are abolished at least 10 d after exposure to HIPVs in receiver maize plants, this non-lasting effect is not known to contribute to any cases, for instance, of defenses in which herbivore or pathogen attack of plants can generate particular defense phenotypes across generations. Epigenetic modifications frequently contribute to transgenerational defense in which priming of the offspring generation cause more rapid induction following subsequent pest attack.⁸ It is therefore speculated that maize receiver plants can maintain their memory for a long time and, in some cases, it may be maintained across generations if the receiver plants are sufficiently mature. In our study, we used immature seedlings (about 10–20 cm tall) that became about 3 times bigger after 10 d of post-exposure maintenance, indicating that freshly propagated, vegetative organs/cells that had not experienced exposure to HIPVs occupy the grown plant body. However, if receiver plants are already mature, the volatile information can be experienced and sustained long-term in most cells, and even transmitted to their offspring generation. The nature and mechanisms of the epigenetic basis of the lasting effect of HIPVs should be investigated in more detail in order to develop realistic “volatile agriculture based on plant communications.”

Disclosure of Potential Conflicts of Interest

No potential conflicts of interest were disclosed.