Other meanings of Phytoalexin
Plant Biology
Phytoalexins are antimicrobial compounds produced de novo by plants in response to pathogen attack or stress. These low-molecular-weight secondary metabolites accumulate rapidly at infection sites and are a key component of plant innate immunity, often exhibiting broad-spectrum toxicity toward fungi, bacteria, and nematodes.
The term phytoalexin was coined in 1960 by I. A. M. Cruickshank to describe antimicrobial compounds that are synthesized de novo and accumulate in plants after exposure to microorganisms1. This distinguished them from preformed antimicrobial substances (phytoanticipins). The concept emerged from earlier work on disease resistance, notably the observation by Müller and Börger in 1940 that potato tubers developed resistance to Phytophthora infestans after prior inoculation with an incompatible race. Phytoalexins are chemically diverse, including terpenoids, alkaloids, flavonoids, and stilbenes, and their production is triggered by elicitors such as fungal cell wall fragments, plant signaling molecules, or abiotic stress.
Phytoalexin biosynthesis is induced at the transcriptional level, often within hours of pathogen recognition. Key regulatory pathways involve salicylic acid, jasmonic acid, and ethylene signaling, which converge on transcription factors such as WRKY and MYB families. For example, in Arabidopsis, the phytoalexin camalexin is synthesized from tryptophan via a pathway requiring the cytochrome P450 enzyme CYP71B15 (PAD3)2. In grapevine, the stilbene resveratrol is produced by stilbene synthase from malonyl-CoA and p-coumaroyl-CoA. The genes encoding these enzymes are often clustered and co-regulated, allowing rapid, coordinated accumulation of antimicrobial metabolites at infection sites.
Phytoalexins function primarily as nonspecific antimicrobial agents, disrupting pathogen membranes, inhibiting enzymes, or interfering with DNA synthesis. Their toxicity is concentration-dependent, and they often act synergistically with other defense proteins such as pathogenesis-related (PR) proteins. In addition to direct antimicrobial activity, some phytoalexins serve as signaling molecules that prime neighboring cells for enhanced defense. For instance, resveratrol in grapevine not only inhibits fungal growth but also contributes to the plant's oxidative stress response. Phytoalexins also play roles in plant–microbe symbiosis, as seen in legumes where they modulate rhizobial infection.
Beyond their classic role in defense, phytoalexins have surprising ecological and biotechnological dimensions. Some phytoalexins, such as the peanut stilbene arachidin-1, exhibit potent anti-inflammatory and anticancer activities in mammalian cells, making them candidates for nutraceutical development. In rice, the diterpenoid phytoalexins momilactones are also released into the soil and act as allelochemicals, inhibiting the growth of competing weeds. Certain phytoalexins are induced by abiotic stresses like UV light or heavy metals, blurring the line between biotic and abiotic stress responses. Moreover, some pathogens have evolved detoxification enzymes that degrade phytoalexins, illustrating an ongoing evolutionary arms race. The study of phytoalexins has also inspired the design of synthetic antimicrobial peptides that mimic their amphipathic properties.
Phytoalexins are a cornerstone of plant chemical defense, with applications ranging from crop protection to human health.
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