Aboveground plant-to-plant communication reduces root nodule symbiosis and soil nutrient concentrations.

Aboveground communication between plants is well known to change defense traits in leaves, but its effects on belowground plant traits and soil characteristics have not been elucidated. We hypothesized that aboveground plant-to-plant communication reduces root nodule symbiosis via induction of bactericidal chemical defense substances and changes the soil nutrient environment. Soybean plants were exposed to the volatile organic compounds (VOCs) from damaged shoots of Solidago canadensis var. scabra, and leaf defense traits (total phenolics, saponins), root saponins, and root nodule symbiosis traits (number and biomass of root nodules) were measured. Soil C/N ratios and mineral concentrations were also measured to estimate the effects of resource uptake by the plants. We found that total phenolics were not affected. However, plants that received VOCs had higher saponin concentrations in both leaves and roots, and fewer root nodules than untreated plants. Although the concentrations of soil minerals did not differ between treatments, soil C/N ratio was significantly higher in the soil of communicated plants. Thus, the aboveground plant-to-plant communication led to reductions in root nodule symbiosis and soil nutrient concentrations. Our results suggest that there are broader effects of induced chemical defenses in aboveground plant organs upon belowground microbial interactions and soil nutrients, and emphasize that plant response based on plant-to-plant communications are a bridge between above- and below-ground ecosystems.

Evidence that ERF transcriptional regulators serve as possible key molecules for natural variation in defense against herbivores in tall goldenrod.

We collected Solidago altissima clones to explore their leaf damage resistance, and as a result identified five accessions that exhibited variable defense abilities against the generalist herbivore Spodoptera litura. In order to characterize molecules involved in such natural variation, we focused on ethylene response factors (ERFs) that exhibited distinct transcription patterns in the leaves of the five accessions (e.g., S1 and S2) after wounding: the transcript of SaERF1 and SaERF2 was induced in wounded S1 and S2 leaves, respectively. Although transcription levels of SaERFs in leaves of the five accessions did not correlate with the accessions' phytohormone levels, these transcription levels accorded with the possibility that ethylene and jasmonate signaling play crucial roles in wound-induced transcription of SaERF1 in S1 leaves, and SaERF2 in S2 leaves, respectively. SaERF1 was found to be a positive regulator of the GCC box and DRE element in the upstream regions of promoters of defense genes, whereas SaERF2 served as a negative regulator of genes controlled through the GCC box. Transgenic Arabidopsis plants expressing SaERF1 or SaERF2 showed enhanced and suppressed transcript levels, respectively, of a defensin gene, indicating that ERFs may be partly responsible for herbivore resistance properties of S. altissima accessions.

Insect Herbivory Selects for Volatile-Mediated Plant-Plant Communication.

Plant volatile organic compounds (VOCs) are major vehicles of information transfer between organisms and mediate many ecological interactions [1-3]. Altering VOC emission in response to herbivore damage has been hypothesized to be adaptive, as it can deter subsequent herbivores [4], attract natural enemies of herbivores [5], or transmit information about attacks between distant parts of the same plant [6-9]. Neighboring plants may also respond to these VOC cues by priming their own defenses against oncoming herbivory, thereby reducing future damage [10-12]. However, under which conditions such information sharing provides fitness benefits to emitter plants, and, therefore, whether selection by herbivores affects the evolution of such VOC signaling, is still unclear [13]. Here, we test the predictions of two alternative hypotheses, the kin selection and mutual benefits hypotheses [14], to uncover the selective environment that may favor information sharing in plants. Measuring the response to natural selection in Solidago altissima, we found strong effects of herbivory on the way plants communicated with neighbors. Plants from populations that experienced selection by insect herbivory induced resistance in all neighboring conspecifics by airborne cues, whereas those from populations experiencing herbivore exclusion induced resistance only in neighbors of the same genotype. Furthermore, the information-sharing plants converged on a common, airborne VOC signal upon damage. We demonstrate that herbivory can drive the evolution of plant-plant communication via induction of airborne cues and suggest plants as a model system for understanding information sharing and communication among organisms in general.

Geographic dialects in volatile communication between sagebrush individuals.

Plants respond to volatile cues emitted by damaged neighbors to increase their defenses against herbivores. We examined whether plants communicated more effectively with local neighbors than distant neighbors in a reciprocal experiment at two sites. Three branches on focal plants were incubated with air from (1) a control, (2) an experimentally clipped "foreign" plant from 230 km away, or (3) an experimentally clipped "local" plant from the same population as the focal plant. Branches incubated with air from the controls experienced 50-80% more leaf damage than those receiving air from experimentally clipped plants. Of more interest, branches receiving volatiles from experimentally clipped "local" plants received 50-65% of the leaf damage as those receiving volatiles from experimentally clipped "foreign" plants. Sabinyl compounds and related terpinenes were found to differ consistently for plants from southern and northern sites. These results indicate that cues vary geographically in their effectiveness and suggest that sagebrush responds more strongly to local than foreign dialects.

Seasonal variation of responses to herbivory and volatile communication in sagebrush (Artemisia tridentata) (Asteraceae).

Plants can respond to insect herbivory in various ways to avoid reductions in fitness. However, the effect of herbivory on plant performance can vary depending on the seasonal timing of herbivory. We investigated the effects of the seasonal timing of herbivory on the performance of sagebrush (Artemisia tridentata). Sagebrush is known to induce systemic resistance by receiving volatiles emitted from clipped leaves of the same or neighboring plants, which is called volatile communication. Resistance to leaf herbivory is known to be induced most effectively after volatile communication in spring. We experimentally clipped 25 % of leaves of sagebrush in May when leaves were expanding, or in July when inflorescences were forming. We measured the growth and flower production of clipped plants and neighboring plants which were exposed to volatiles emitted from clipped plants. The treatment conducted in spring reduced the growth of clipped plants. This suggests that early season leaf herbivory is detrimental because it reduces the opportunities for resource acquisition after herbivory, resulting in strong induction of resistance in leaves. On the other hand, the late season treatment increased flower production in plants exposed to volatiles, which was caused mainly by the increase in the number of inflorescences. Because the late season treatment occurred when sagebrush produces inflorescences, sagebrush may respond to late herbivory by increasing compensation ability and/or resistance in inflorescences rather than in leaves. Our results suggest that sagebrush can change responses to herbivory and subsequent volatile communication seasonally and that the seasonal variation in responses may reduce the cost of induced resistance.

Airborne signals of communication in sagebrush: a pharmacological approach.

When plants receive volatiles from a damaged plant, the receivers become more resistant to herbivory. This phenomenon has been reported in many plant species and called plant-plant communication. Lab experiments have suggested that several compounds may be functioning as airborne signals. The objective of this study is to identify potential airborne signals used in communication between sagebrush (Artemisia tridentata) individuals in the field. We collected volatiles of one branch from each of 99 sagebrush individual plants. Eighteen different volatiles were detected by GC-MS analysis. Among these, 4 compounds; 1.8-cineol, ß-caryophyllene, α-pinene and borneol, were investigated as signals of communication under natural conditions. The branches which received either 1,8-cineol or ß-caryophyllene tended to get less damage than controls. These results suggested that 1,8-cineol and ß-caryophyllene should be considered further as possible candidates for generalized airborne signals in sagebrush.

Deciphering the language of plant communication: volatile chemotypes of sagebrush.

Volatile communication between sagebrush (Artemisia tridentata) individuals has been found previously to reduce herbivory and to be more effective between individuals that are genetically identical or related relative to between strangers. The chemical nature of the cues involved in volatile communication remains unknown for this and other systems. We collected headspace volatiles from sagebrush plants in the field and analyzed these using GC-MS. Volatile profiles were highly variable among individuals, but most individuals could be characterized as belonging to one of two chemotypes, dominated by either thujone or camphor. Analyses of parents and offspring revealed that chemotypes were highly heritable. The ecological significance of chemotypes and the genetic mechanisms that control them remain poorly understood. However, we found that individuals of the same chemotype communicated more effectively and experienced less herbivory than individuals of differing chemotypes. Plants may use chemotypes to distinguish relatives from strangers.

Kin recognition affects plant communication and defence.

The ability of many animals to recognize kin has allowed them to evolve diverse cooperative behaviours; such ability is less well studied for plants. Many plants, including Artemisia tridentata, have been found to respond to volatile cues emitted by experimentally wounded neighbours to increase levels of resistance to herbivory. We report that this communication was more effective among A. tridentata plants that were more closely related based on microsatellite markers. Plants in the field that received cues from experimentally clipped close relatives experienced less leaf herbivory over the growing season than those that received cues from clipped neighbours that were more distantly related. These results indicate that plants can respond differently to cues from kin, making it less likely that emitters will aid strangers and making it more likely that receivers will respond to cues from relatives. More effective defence adds to a growing list of favourable consequences of kin recognition for plants.

An air transfer experiment confirms the role of volatile cues in communication between plants.

Previous studies reported that sagebrush plants near experimentally clipped neighbors experienced less herbivory than did plants near unclipped neighbors. Blocking air flow with plastic bags made this effect undetectable. However, some scientists remained skeptical about the possibility of volatile communication between plants since the existence and identity of a cue that operates in nature have never been demonstrated. We conducted an air transfer experiment that collected air from the headspace of an experimentally clipped donor plant and delivered it to the headspace of an unclipped assay plant. We found that assay plants treated with air from clipped donors were less likely to be damaged by naturally occurring herbivores in a field experiment. This simple air transfer experiment fulfills the most critical of Koch's postulates and provides more definitive evidence for volatile communication between plants. It also provides an inexpensive experimental protocol that can be used to screen plants for interplant communication in the field.

Self-recognition affects plant communication and defense.

Animals have the ability to distinguish self from non-self, which has allowed them to evolve immune systems and, in some instances, to act preferentially towards individuals that are genetically identical or related. Self-recognition is less well known for plants, although recent work indicates that physically connected roots recognize self and reduce competitive interactions. Sagebrush uses volatile cues emitted by clipped branches of self or different neighbours to increase resistance to herbivory. Here, we show that plants that received volatile cues from genetically identical cuttings accumulated less natural damage than plants that received cues from non-self cuttings. Volatile communication is required to coordinate systemic processes such as induced resistance and plants respond more effectively to self than non-self cues. This self/non-self discrimination did not require physical contact and is a necessary first step towards possible kin recognition and kin selection.

Plant age, communication, and resistance to herbivores: young sagebrush plants are better emitters and receivers.

Plants progress through a series of distinct stages during development, although the role of plant ontogeny in their defenses against herbivores is poorly understood. Recent work indicates that many plants activate systemic induced resistance after herbivore attack, although the relationship between resistance and ontogeny has not been a focus of this work. In addition, for sagebrush and a few other species, individuals near neighbors that experience simulated herbivory become more resistant to subsequent attack. Volatile, airborne cues are required for both systemic induced resistance among branches and for communication among individuals. We conducted experiments in stands of sagebrush of mixed ages to determine effects of plant age on volatile signaling between branches and individuals. Young and old control plants did not differ in levels of chewing damage that they experienced. Systemic induced resistance among branches was only observed for young plants. Young plants showed strong evidence of systemic resistance only if airflow was permitted among branches; plants with only vascular connections showed no systemic resistance. We also found evidence for volatile communication between individuals. For airborne communication, young plants were more effective emitters of cues as well as more responsive receivers of volatile cues.

Plant volatiles, rather than light, determine the nocturnal behavior of a caterpillar.

Although many organisms show daily rhythms in their activity patterns, the mechanistic causes of these patterns are poorly understood. Here we show that host plant volatiles affect the nocturnal behavior of the caterpillar Mythimna separata. Irrespective of light status, the caterpillars behaved as if they were in the dark when exposed to volatiles emitted from host plants (either uninfested or infested by conspecific larvae) in the dark. Likewise, irrespective of light status, the caterpillars behaved as if they were in the light when exposed to volatiles emitted from plants in the light. Caterpillars apparently utilize plant volatile information to sense their environment and modulate their daily activity patterns, thereby potentially avoiding the threat of parasitism.

Damage-induced resistance in sagebrush: volatiles are key to intra- and interplant communication.

Airborne communication between individuals, called "eavesdropping" in this paper, can cause plants to become more resistant to herbivores when a neighbor has been experimentally clipped. The ecological relevance of this result has been in question, since individuals may be too far apart for this interaction to affect many plants in natural populations. We investigated induced resistance to herbivory in sagebrush, Artemisia tridentata, caused by experimental clipping of the focal plant and its neighbors. We found no evidence for systemic induced resistance when one branch was clipped and another branch on the same plant was assayed for naturally occurring damage. In this experiment, air contact and plant age were not controlled. Previous work indicated that sagebrush received less damage when a neighboring upwind plant within 15 cm had been experimentally clipped. Here we found that pairs of sagebrush plants that were up to 60 cm apart were influenced by experimental clipping of a neighbor. Furthermore, we observed that most individuals had conspecific neighbors that were much closer than 60 cm. Air contact was essential for communication; treatments that reduced airflow between neighboring individuals, either because of wind direction or bagging, prevented induced resistance. Airflow was also necessary for systemic induced resistance among branches within an individual. Reports from the literature indicated that sagebrush is highly sectorial, as are many desert shrubs. Branches within a sagebrush plant do not freely exchange material via vascular connections and apparently cannot rely on an internal signaling pathway for coordinating induction of resistance to herbivores. Instead, they may use external, volatile cues. This hypothesis provides a proximal explanation for why sagebrush does not demonstrate systemic induced resistance without directed airflow, and why airborne communication between branches induces resistance.