Activation of MAP kinases by green leaf volatiles in grasses

Green leaf volatiles co-opt proteins involved in molecular pattern signalling in plant cells

The green leaf volatiles (GLVs) Z-3-hexen-1-ol (Z3-HOL) and Z-3-hexenyl acetate (Z3-HAC) are airborne infochemicals released from damaged plant tissues that induce defenses and developmental responses in receiver plants, but little is known about their mechanism of action. We found that Z3-HOL and Z3-HAC induce similar but distinctive physiological and signaling responses in tomato seedlings and cell cultures. In seedlings, Z3-HAC showed a stronger root growth inhibition effect than Z3-HOL. In cell cultures, the two GLVs induced distinct changes in MAP kinase (MAPK) activity and proton fluxes as well as rapid and massive changes in the phosphorylation status of proteins within 5 min. Many of these phosphoproteins are involved in reprogramming the proteome from cellular homoeostasis to stress and include pattern recognition receptors, a receptor-like cytoplasmic kinase, MAPK cascade components, calcium signaling proteins and transcriptional regulators. These are well-known components of damage-associated molecular pattern (DAMP) signaling pathways. These rapid changes in the phosphoproteome may underly the activation of defense and developmental responses to GLVs. Our data provide further evidence that GLVs function like DAMPs and indicate that GLVs coopt DAMP signaling pathways.

Volatile uptake, transport, perception, and signaling shape a plant's nose

Herbivore-induced plant volatiles regulate defenses in undamaged neighboring plants. Understanding the mechanisms by which plant volatiles are taken up, perceived, and translated into canonical defense signaling pathways is an important frontier of knowledge. Volatiles can enter plants through stomata and the cuticle. They are likely perceived by membrane-associated receptors as well as intracellular receptors. The latter likely involves metabolization and transport across cell membranes by volatile transporters. Translation of volatiles into defense priming and induction typically involves mitogen-activated protein kinases (MAPKs), WRKY transcription factors, and jasmonates. We propose that the broad range of molecular processes involved in volatile signaling will likely result in substantial spatiotemporal and ontogenetic variation in plant responsiveness to volatiles, with important consequences for plant–environment interactions.

Volatile-mediated plant-plant interactions: volatile organic compounds as modulators of receiver plant defence, growth, and reproduction

It is firmly established that plants respond to biotic and abiotic stimuli by emitting volatile organic compounds (VOCs). These VOCs provide information on the physiological status of the emitter plant and are available for detection by the whole community. In the context of plant-plant interactions, research has focused mostly on the defence-related responses of receiver plants. However, responses may span hormone signalling and both primary and secondary metabolism, and ultimately affect plant fitness. Here we present a synthesis of plant-plant interactions, focusing on the effects of VOC exposure on receiver plants. An overview of the important chemical cues, the uptake and conversion of VOCs, and the adsorption of VOCs to plant surfaces is presented. This is followed by a review of the substantial VOC-induced changes to receiver plants affecting both primary and secondary metabolism and influencing plant growth and reproduction. Further research should consider whole-plant responses for the effective evaluation of the mechanisms and fitness consequences of exposure of the receiver plant to VOCs.

Processing of Airborne Green Leaf Volatiles for Their Glycosylation in the Exposed Plants

Green leaf volatiles (GLVs), the common constituents of herbivore-infested plant volatiles (HIPVs), play an important role in plant defense and function as chemical cues to communicate with other individuals in nature. Reportedly, in addition to endogenous GLVs, the absorbance of airborne GLVs emitted by infested neighboring plants also play a major role in plant defense. For example, the exclusive accumulation of (Z)-3-hexenyl vicianoside in the HIPV-exposed tomato plants occurs by the glycosylation of airborne (Z)-3-hexenol (Z3HOL); however, it is unclear how plants process the other absorbed GLVs. This study demonstrates that tomato plants dominantly accumulated GLV-glycosides after exposure to green leaf alcohols [Z3HOL, (E)-2-hexenol, and n-hexanol] using non-targeted LC-MS analysis. Three types of green leaf alcohols were independently glycosylated without isomerization or saturation/desaturation. Airborne green leaf aldehydes and esters were also glycosylated, probably through converting aldehydes and esters into alcohols. Further, we validated these findings in Arabidopsis mutants- (Z)-3-hexenal (Z3HAL) reductase (chr) mutant that inhibits the conversion of Z3HAL to Z3HOL and the acetyl-CoA:(Z)-3-hexen-1-ol acetyltransferase (chat) mutant that impairs the conversion of Z3HOL to (Z)-3-hexenyl acetate. Exposure of the chr and chat mutants to Z3HAL accumulated lower and higher amounts of glycosides than their corresponding wild types (Col-0 and Ler), respectively. These findings suggest that plants process the exogenous GLVs by the reductase(s) and the esterase(s), and a part of the processed GLVs contribute to glycoside accumulation. Overall, the study provides insights into the understanding of the communication of the plants within their ecosystem, which could help develop strategies to protect the crops and maintain a balanced ecosystem.

Social networking in crop plants: Wired and wireless cross-plant communications

The plant-associated microbial community (microbiome) has an important role in plant-plant communications. Plants decipher their complex habitat situations by sensing the environmental stimuli and molecular patterns and associated with microbes, herbivores and dangers. Perception of these cues generates inter/intracellular signals that induce modifications of plant metabolism and physiology. Signals can also be transferred between plants via different mechanisms, which we classify as wired- and wireless communications. Wired communications involve direct signal transfers between plants mediated by mycorrhizal hyphae and parasitic plant stems. Wireless communications involve plant volatile emissions and root exudates elicited by microbes/insects, which enable inter-plant signalling without physical contact. These producer-plant signals induce microbiome adaptation in receiver plants via facilitative or competitive mechanisms. Receiver plants eavesdrop to anticipate responses to improve fitness against stresses. An emerging body of information in plant-plant communication can be leveraged to improve integrated crop management under field conditions.

Transcriptome Analysis of Wounding in the Model Grass Lolium temulentum

For forage and turf grasses, wounding is a predominant stress that often results in extensive loss of vegetative tissues followed by rapid regrowth. Currently, little is known concerning the perception, signaling, or molecular responses associated with wound stress in forage- and turf-related grasses. A transcriptome analysis of Lolium temulentum plants subjected to severe wounding revealed 9413 upregulated and 7704 downregulated, distinct, differentially expressed genes (DEGs). Categories related to signaling, transcription, and response to stimuli were enriched in the upregulated DEGs. Specifically, sequences annotated as enzymes involved in hormone biosynthesis/action and cell wall modifications, mitogen-activated protein kinases, WRKY transcription factors, proteinase inhibitors, and pathogen defense-related DEGs were identified. Surprisingly, DEGs related to heat shock and chaperones were more prevalent in the downregulated DEGs when compared with the upregulated DEGs. This wound transcriptome analysis is the first step in identifying the molecular components and pathways used by grasses in response to wounding. The information gained from the analysis will provide a valuable molecular resource that will be used to develop approaches that can improve the recovery, regrowth, and long-term fitness of forage and turf grasses before/after cutting or grazing.

Metabolomics Reveal Induction of ROS Production and Glycosylation Events in Wheat Upon Exposure to the Green Leaf Volatile Z-3-Hexenyl Acetate

The activation and priming of plant defense upon perception of green leaf volatiles (GLVs) have often been reported. However, information as to which metabolic pathways in plants are affected by GLVs remains elusive. We report the production of reactive oxygen species in the tip of young wheat leaves followed by activation of antioxidant-related enzyme activity. In this study, we aimed to uncover metabolic signatures upon exposure to the GLV Z-3-hexenyl acetate (Z-3-HAC). By using an untargeted metabolomics approach, we observed changes in the phenylpropanoid pathways which yield metabolites that are involved in many anti-oxidative processes. Furthermore, exposure to GLV, followed by infection with Fusarium graminearum (Fg), induced significantly greater changes in the phenylpropanoid pathway compared to a sole Z-3-HAC treatment. Fragmentation of a selection of metabolites, which are significantly more upregulated in the Z-3-HAC + Fg treatment, showed D-glucose to be present as a substructure. This suggests that Z-3-HAC induces early glycosylation processes in plants. Additionally, we identified the presence of hexenyl diglycosides, which indicates that aerial Z-3-HAC is metabolized in the leaves by glycosyltransferases. Together these data indicate that GLV Z-3-HAC is taken up by leaves and incites oxidative stress. This subsequently results in the modulation of the phenylpropanoid pathway and an induction of glycosylation processes.

Transcriptome analysis of the model grass Lolium temulentum exposed to green leaf volatiles

BACKGROUND

Forage and turf grasses are routinely cut and grazed upon throughout their lifecycle. When grasses are cut or damaged, they rapidly release a volatile chemical cocktail called green leaf volatiles (GLV). Previously we have shown that mechanical wounding or exposure to GLV released from cut grass, activated a Lt 46 kDa mitogen-activated protein kinase (MAPK) within 3 min and a 44 kDa MAPK within 15-20 min in the model grass species Lolium temulentum (Lt). Currently very little is known concerning the perception, signaling or molecular responses associated with wound stress in grasses. Since GLV are released during wounding, we wanted to investigate what genes and signaling pathways would be induced in undamaged plants exposed to GLV.

RESULTS

RNA-Seq generated transcriptome of Lolium plants exposed to GLV identified 4308 up- and 2794 down-regulated distinct differentially-expressed sequences (DES). Gene Ontology analysis revealed a strong emphasis on signaling, response to stimulus and stress related categories. Transcription factors and kinases comprise over 13% of the total DES found in the up-regulated dataset. The analysis showed a strong initial burst within the first hour of GLV exposure with over 60% of the up-regulated DES being induced. Specifically sequences annotated for enzymes involved in the biosynthesis of jasmonic acid and other plant hormones, mitogen-activated protein kinases and WRKY transcription factors were identified. Interestingly, eleven DES for ferric reductase oxidase, an enzyme involved in iron uptake and transport, were exclusively found in the down-regulated dataset. Twelve DES of interest were selected for qRT-PCR analysis; all displayed a rapid induction one hour after GLV exposure and were also strongly induced by mechanical wounding.

CONCLUSION

The information gained from the analysis of this transcriptome and previous studies suggests that GLV released from cut grasses transiently primes an undamaged plant's wound stress pathways for potential oncoming damage, and may have a dual role for inter- as well as intra-plant signaling.

From Acetoin to ( Z)-3-Hexen-1-ol: The Diversity of Volatile Organic Compounds that Induce Plant Responses

Evidence that plants can respond to volatile organic compounds (VOCs) was first presented 35 years ago. Since then, over 40 VOCs have been found to induce plant responses. These include VOCs that are produced not only by plants but also by microbes and insects. Here, we summarize what is known about how these VOCs are produced and how plants detect and respond to them. In doing so, we highlight notable observations we believe are worth greater consideration. For example, the VOCs that induce plant responses appear to have little in common. They are derived from many different biosynthetic pathways and have few distinguishing chemical or structural features. Likewise, plants appear to use several mechanisms to detect VOCs rather than a single dedicated "olfactory" system. Considering these observations, we advocate for more discovery-oriented experiments and propose that future research take a fresh look at the ways plants detect and respond to VOCs.