The Silent Language of the Plants

 Introduction

Think of a world where communication goes beyond just words. In the highly dense forests, meadows, and gardens, plants are quietly exchanging messages in ways that are both sophisticated and surprising. Though stationary, these living trees, herbs, shrubs, vines, bushes and, grasses engage in a form of dialogue that challenges our understanding of intelligence. From releasing chemical signals for alerting neighbors of threats to forming vast underground networks for resource-sharing, plants exhibit a level of interaction that is both complex and awe-inspiring. Let’s unveil the hidden ways plants communicate in.

 

The History of Plant’s Communication

The first evidence of plant-plant communication was observed by the Scientists, namely, Vicherová, Glinwood, Hájek, Šmilauer, & Ninkovic in the Bryophytes.

They observed that Sphagnum flexuosum released certain type of compounds, by receiving which; the Hamatocaulis vernicosus altered its growth pattern and the release of β-cyclocitral to a degree six times higher.

 

The Molecules Plants Use for Communication

The molecules that plants use for communicating are called as Volatile Organic Compounds (VOCs).

These molecules can be released intentionally (as signals) or unintentionally (as cues) from one plant to another.

These molecules are transferred from the emitter plant to the responder plant mainly by direct touch or by wind.

Figure 1: The release of VOCs by the plants.

 The Common VOCs of the Plants

VOCs from the Leaves: Isoprene, α-pinene, β-pinene, β-caryophyllene, cis-3-hexenal, methyl jasmonate etc.

VOCs from the Flowers: β-caryophyllene, linalool, phenylacetaldehyde, methyl benzoate, hexana, nonana, geranyl acetate, bbenzyl acetate, methyl jasmonate, etc.

VOCs from the Fruits: ethyl acetate, butyl acetate, hexana, nonana, limonene, α-pinene, methyl jasmonate, acetic acid, citric acid, etc.

VOCs from the Stems: α-pinene, β-pinene, β-caryophyllene, phenol, guaiacol, vanillin, coniferaldehyde, acetic acid, formic acid, etc.

VOCs from the Barks: α-pinene, β-pinene, phenol, guaiacol, vanillin, coniferaldehyde, acetic acid, formic acid, β-caryophyllene, etc.

  

The Plants Structures That Receive the VOCs

Stomata (the small opening on the leaves), Trichomes (the hair like structures on leaves and stems), Cuticles of leaves, Epidermis of leaves, Petals of flowers, Sepals of flowers, Anther of flowers, Pollen of flowers, Stamen of flower, Stigma of flowers, Style of flowers, Ovules of flowers, etc.

 

 Figure 2: The stomata of plant leaf.

Figure 3: Trichomes, the leaf hair.

Figure 4: Different ultra-structures of the leaf.

Figure 5: Different parts of the flower.

The VOCs Perception and Downstream Pathways

The acceptance of the VOCs on the leaf surface is via stomata, or by glandular trichomes, which receives as well as decides, whether the VOC should be transferred or not based on its toxicity.

 

Figure 6: Plant perception mechanisms of VOCs: A, storage and release of VOCs via glandular trichomes; B, release and uptake of VOCs via stomata cells as part of perceptive and emission mechanisms; C, partitioning of VOCs via a bidirectional diffusion along a concentration gradient; D, nonvascular lipophilic transport via the endoplasmic reticulum (ER) during direct contact with the cell wall; E, VOCs or metabolite transport via specific transport proteins; F, perception of VOCs from specific receptors (extra or intracellular) with a resulting signaling cascade and gene expression; G, metabolic breakdown of VOCs resulting in fragments with signaling characteristics.

After plants received the VOCs, they often undergo Esterification or Reduction to be stabilized, used in plant’s defense and growth, alter the flavour and aroma profiles or facilitate other systemic signaling.

One example of it could be the Glycosylation and Glutathionylation of certain VOCs to produce a non-volatile compound, (Z)-3-hexenyl vicianoside, which gives the signal to the uninjured plants to develop the ability to defend themselves against future herbivore attacks.

The species-specificity of the receiver in responding to the VOCs significantly minimizes the fitness costs and avoids unnecessary and costly responses.

  

The Conditions, Under Which the VOCs Are Released

The exposure to pressure due to touch, Infection by pathogens like bacteria, fungi, virus, Stress due to increased salinity, oxidative damage of proteins and DNA by ozone that enter through stomata, Stress due to Temperature, Stress due to Light, Stress due to Cold, Stress due to Droughts, Stress due to Parasitism, Stress due to Herbivory, Stress due to Soil borne pathogens, Stress due to Competitive neighbors.

 Figure 7: Plant under different stressful conditions.  

Even it is studied that some herbivorous insects can manipulate the VOCs signals and takes the advantage of the wrong response by the receiver.

For example, white flies after feeding on one plants, they manipulate the emitted VOCs.

In response to those manipulated VOCs, the nearby plants invest in defense against pathogens instead of against herbivores.

Figure 8: White fly ( Trialeurodes vaporariorum) on the leaf.

Mutualism- It is observed that the VOCs from the plants of barley attract the seven-spot ladybird, that feeds on the aphids, ultimately saves the plant’s life from aphids as they feed on the plant sap and can promote virus or fungal infection top the plant.

Figure 9: Seven-spot lady bird (Coccinella septempunctata) on the leaf.

Parasitism- Parasitic plants can explore new potent hosts by receiving the VOCs as cues.

 

Figure 10: Dodder (Cuscuta campestris), a parasitic plant on it's host.

Plants Strategies, That Are Dependent on the VOCs

To prevent these stressful situations, plants adapt to some changes like-

Kin selection- Plants have specific chemotypes of the VOCs, which they only release for their genetically related individuals. 

Sex of recipient- It is observed that even after having specific chemotypes for individuals of own species, some plants have more specific chemotypes that are exchanged between females only as they have to invest more resources in reproduction.

E.g.- A woody shrub, Baccharis salicifolia.

 

The Effects after Receiving the VOCs

Genetic regulation of other genes and TFs – Some of the VOCs have the ability to directly change the physiological status of the plant at its molecular level.

For example, Using a β-glucorunidase enzyme it has been reported that in Arabidopsis thaliana, certain VOCs can change the transcriptome (the entire RNAs of the responder plant).

Genetic evolution – Not only the RNAs, but also the permanent changes in the genes involved for certain transcription factors as well as the genes associated with stress.

 

Conclusion

Plants communicate using volatile organic compounds (VOCs) as a fascinating and complex mechanism to interact with their environment. Through the release of these chemical signals, plants can convey information about stressors such as herbivore attacks, diseases, or environmental changes. This communication can trigger defensive responses not only in the affected plant but also in neighboring plants, enhancing their survival chances.

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