Chemoreception (the perception of chemical molecules) is the most "primitive" and most widespread sense in biology: even bacteria perceive chemical gradients and move toward them (chemotaxis). In plants, chemoreception is highly developed both in the air (perception of VOCs) and in the soil (perception of ions and organic molecules through roots). Unlike animal olfaction (centralized in a dedicated organ like the nose, with receptors coupled to G-proteins specific to each odor molecule), plant "chemoreception" is distributed across every leaf and root cell.

How plants "smell" the air: VOC reception

Plants perceive environmental VOCs through various cellular mechanisms. Ethylene: the gaseous hormone produced by ripening fruit, damaged tissues, and plants under stress. Ethylene diffuses through the air and is perceived by specific membrane receptors (ETR1: Ethylene receptor 1 in Arabidopsis, a protein with an ethylene-binding domain on the endoplasmic reticulum membrane) in all cells of the plant and neighboring plants. Responses to ethylene include: fruit ripening (ethylene from a ripe apple diffuses to nearby apples in a fruit bowl: the "contagion" of ripening), leaf senescence (leaf yellowing), response to tissue damage (increased chemical defenses). Gaseous methyl jasmonate (MeJA): when a plant produces jasmonate in response to herbivore attack, part of the jasmonate is methylated (MeJA) and becomes volatile. Diffusing through the air, it is perceived by nearby plants through specific esterases that de-methylate it, converting it back to jasmonic acid → activation of defense genes in receiving plants. Gaseous methyl salicylate (MeSA): analogous to MeJA but for response to fungal and bacterial infections. SA (salicylic acid: the active ingredient in aspirin) produced during infection is methylated into volatile MeSA → diffuses to nearby plants → activation of SAR (Systemic Acquired Resistance). Every plant that has experienced an infection "informs" its neighbors through hormonal VOCs. Other VOCs as signals: plants perceive GLVs (Green Leaf Volatiles) from damaged nearby plants as a signal of herbivores in the area, specific sesquiterpenes as markers of specific herbivore activity, VOCs that attract roots of parasitic plants (Striga: germinates in response to strigolactones).

How plants "smell" the soil: root chemoreception

Roots are the most developed chemical sensory organs of plants: they constantly detect soil chemical composition and regulate directional growth (chemotropism), symbiosis (recognition of microbial partners), and stress response. Nutrient detection: nitrogen (nitrate NO3- and ammonium NH4+): specific nitrate receptors (NRT1.1: Nitrate transporter 1.1 in Arabidopsis, which also functions as a nitrate sensor) detect NO3- concentration and activate lateral root growth in nitrate-rich zones. Phosphorus (Pi): PHO genes detect phosphorus availability. Phosphorus deficiency activates strigolactone production (to attract mycorrhizae), organic acid production (to solubilize bound phosphate in soil), and changes in root architecture (more fine and capillary roots, fewer thick roots). Potassium: detected by specific ion channels (AKT1, HAK5) that change their expression based on K+ availability. Symbiosis detection: legume flavonoids detect Nod factors from Rhizobium (already discussed): a molecular chemical dialogue leading to nitrogen-fixing nodulation. Roots detect strigolactones from mycorrhizae (and vice versa: roots produce strigolactones to attract fungal hyphae). Detection of competitor allelochemicals: roots perceive allelochemicals produced by competitors (walnut juglone, corn DIMBOA) through mechanisms still partially unknown and modify root growth to avoid zones rich in these toxic substances.

Chemical perception in carnivorous plants: recognizing prey

Carnivorous plants have developed sophisticated chemoreception systems to recognize and "choose" prey. Prey selection in Dionaea: Dionaea closes in response to mechanical touch of sensitive hairs, but enzyme release requires an additional chemical signal: the presence of nitrogen (amino acids, urea) in prey secretions. Prey that doesn't "smell" of protein (e.g., a grain of sand) doesn't activate digestion even after trap closure. Soil acid chemoreception in Heliamphora (pitcher plants): the acidic pH of water in the pitcher (produced by bacterial and enzymatic digestion of previous prey) is perceived by glandular tissues as a signal that conditions are suitable for continued digestion. In the presence of fresh prey that raises pH, enzyme production increases. Utricularia (bladderwort): an aquatic carnivorous plant with small bladders that capture microorganisms through depression (the bladder suddenly deflates when sensitive hairs at the entrance are touched, sucking in water with prey). Sensitive hairs react to specific amino acids and polysaccharides of prey (chemical signals indicating that the touching organism is biological, not inorganic). A prey chemical recognition system that reduces false captures.

Chemical pollution and plant chemoreception: disruptions

Chemical pollution of air and soil can disrupt plant chemoreception in ways that have consequences for ecosystems. Ozone pollution (O3): tropospheric ozone (an oxidant produced by the reaction between NOx and VOCs in sunlight) reacts with many plant VOCs (terpenes, ethylene) altering their structure. VOCs oxidized by ozone may have different receptor properties from the original: plants "receive" altered or non-existent messages. Studies show that ozone pollution reduces the efficiency of pollinating insects in finding flowers (floral scents are degraded before reaching receivers). Pesticides and ethylene receptors: some fungicides and herbicides interfere with ethylene receptors in plants (ETR1 is a protein with copper ion in the binding site: copper-chelating agents interfere with the receptor). Alterations in ethylene response can disrupt fruit ripening, damage response, and leaf senescence. Heavy metals in soil: lead, cadmium, and mercury in soil interfere with ion receptors in roots (masking or mimicking nutrient ions like Ca2+, K+, Mg2+). They disrupt root chemoreception of nutrients and can induce nutrient deficiency responses even in adequately nourished soils.

The scent of pathogenic fungi: do plants "sense" them coming?

Pathogenic fungi (Botrytis cinerea, Phytophthora infestans, Fusarium oxysporum) produce characteristic VOCs during growth and sporulation. The question is whether host plants can detect these VOCs before the fungus establishes infection and respond with preventive defenses. Evidence: studies by Quintana-Rodriguez et al. (2018) showed that tomato plants exposed to VOCs from Botrytis cinerea increased production of antioxidant defenses and defense enzymes (chitinase, glucanase) in not-yet-infected leaves. Fungal VOCs as early warning signals. VOCs of identified fungal pathogens: fungi produce 1-octen-3-ol (a sesquiterpene with fresh mushroom odor), 3-octanone, and other characteristic compounds during mycelium growth. These compounds could be detected by volatile receptors in plant leaves and roots. The PAMP system (Pathogen-Associated Molecular Patterns) in plant defense: pathogen recognition in plants traditionally occurs through direct contact (PRR receptors: Pattern Recognition Receptors detect pathogen-specific molecules like fungal chitin or bacterial flagellin). Chemoreception of fungal VOCs at a distance would be an early addition to this system: a form of "early warning" before contact. Potential application: spray of fungal VOCs in sub-inhibitory concentrations on crops as a "primer" of defenses before actual infection. Still experimental.