No abstract

Plant volatiles play important roles in signalling between plants and insects, but their role in communication among plants remains controversial. Previous research on plant-plant communication has focused on interactions between neighbouring plants, largely overlooking the possibility that volatiles function as signals within plants. Here, we show that volatiles released by herbivore-wounded leaves of hybrid poplar (Populus deltoides x nigra) prime defences in adjacent leaves with little or no vascular connection to the wounded leaves. Undamaged leaves exposed to volatiles from wounded leaves on the same stem had elevated defensive responses to feeding by gypsy moth larvae (Lymantria dispar L.) compared with leaves that did not receive volatiles. Volatile signals may facilitate systemic responses to localized herbivory even when the transmission of internal signals is constrained by vascular connectivity. Self-signalling via volatiles is consistent with the short distances over which plant response to airborne cues has been observed to occur and has apparent benefits for emitting plants, suggesting that within-plant signalling may have equal or greater ecological significance than signalling between plants.

Summary• Herbivore-induced plant volatiles (HIPVs), in addition to attracting natural enemies of herbivores, can serve a signaling function within plants to induce or prime defenses. However, it is largely unknown, particularly in woody plants, which volatile compounds within HIPV blends can act as signaling molecules.• Leaves of hybrid poplar saplings were exposed in vivo to naturally wound-emitted concentrations of the green leaf volatile (GLV) cis-3-hexenyl acetate (z3HAC) and then subsequently fed upon by gypsy moth larvae. Volatiles were collected throughout the experiments, and leaf tissue was collected to measure phytohormone concentrations and expression of defense-related genes.• Relative to controls, z3HAC-exposed leaves had higher concentrations of jasmonic acid and linolenic acid following gypsy moth feeding. Furthermore, z3HAC primed transcripts of genes that mediate oxylipin signaling and direct defenses, as determined by both qRT-PCR and microarray analysis using the AspenDB 7 K expressed sequence tags (EST) microarray containing c. 5400 unique gene models. Moreover, z3HAC primed the release of terpene volatiles.• The widespread priming response suggests an adaptive benefit to detecting z3HAC as a wound signal. Thus, woody plants can detect and use z3HAC as a signal to prime defenses before actually experiencing damage. GLVs may therefore have important ecological functions in arboreal ecosystems.

Increased nitrogen (N) mobilization and export from terrestrial forest ecosystems following canopy herbivory have been well documented, though the mechanism behind the loss is not clear. Because carbon (C) and N dynamics are closely linked, herbivore activity may also affect C distribution. We initiated a replicated mini-ecosystem experiment to test the hypothesis that insect frass (feces) influences soil C and N dynamics following insect defoliation. One hundred and sixty red oak (Quercus rubra) saplings were transplanted to seven-gallon (26.5-L) pots with soil and litter from the Coweeta Hydrologic Laboratory (CWT) (Otto, North Carolina, USA) and overwintered in experimental pot stands. During the 2002 growing season, trees were subjected to a 3 ϫ 2 factorial experimental design with three damage groups (herbivore, mechanical, ''undamaged'') and two frass depositions (frass, no frass).Frass deposition increased soil total C, total N, and the soil NH 4 ϩ pool. Leachate NO 3 Ϫ export also increased following frass additions. We suggest that herbivore frass mobilizes sufficient C and N to affect soil pools and N export, though abiotic factors may influence the ultimate fate of the nutrients in frass. In addition, herbivory increased soil respiration and decreased total soil N relative to ''undamaged'' controls independent of frass deposition. While we discuss four possible mechanisms for this observation, we hypothesize that the increased soil respiration results from enhanced root-exudate C and subsequent microbial oxidation. This mechanism has implications for C sequestration and N retention in forest soils. In addition, the effects of mechanical damage consistently did not match those of real herbivory, suggesting that differential responses of Q. rubra to damage types also may affect soil nutrient dynamics. Our results demonstrate that the feeding activity of insect herbivores can have direct and indirect effects on the cycling of C and N within the season of defoliation.

The Populus sucrose (Suc) transporter 4 (PtaSUT4), like its orthologs in other plant taxa, is tonoplast localized and thought to mediate Suc export from the vacuole into the cytosol. In source leaves of Populus, SUT4 is the predominantly expressed gene family member, with transcript levels several times higher than those of plasma membrane SUTs. A hypothesis is advanced that SUT4-mediated tonoplast sucrose fluxes contribute to the regulation of osmotic gradients between cellular compartments, with the potential to mediate both sink provisioning and drought tolerance in Populus. Here, we describe the effects of PtaSUT4-RNA interference (RNAi) on sucrose levels and raffinose family oligosaccharides (RFO) induction, photosynthesis, and water uptake, retention and loss during acute and chronic drought stresses. Under normal water-replete growing conditions, SUT4-RNAi plants had generally higher shoot water contents than wild-type plants. In response to soil drying during a short-term, acute drought, RNAi plants exhibited reduced rates of water uptake and delayed wilting relative to wild-type plants. SUT4-RNAi plants had larger leaf areas and lower photosynthesis rates than wild-type plants under well-watered, but not under chronic water-limiting conditions. Moreover, the magnitude of shoot water content, height growth, and photosynthesis responses to contrasting soil moisture regimes was greater in RNAi than wild-type plants. The concentrations of stress-responsive RFOs increased in wild-type plants but were unaffected in SUT4-RNAi plants under chronically dry conditions. We discuss a model in which the subcellular compartmentalization of sucrose mediated by PtaSUT4 is regulated in response to both sink demand and plant water status in Populus.