phytoalexin

Phytoalexins are low-molecular-weight antimicrobial secondary metabolites produced de novo by plants in response to pathogen attack or abiotic stress. They accumulate at the site of infection and contribute to defense by inhibiting the growth of fungi, bacteria, and other pathogens. The term distinguishes induced compounds from constitutively present defense chemicals, sometimes called phytoanticipins.

Phytoalexins belong to diverse chemical classes, including phenolics such as stilbenes and flavonoids, terpenoids, and indole alkaloids. Biosynthesis commonly starts from primary metabolism and is upregulated by defense signaling pathways, notably salicylic acid and jasmonic acid, often in response to elicitors derived from pathogens or damage. Enzymes such as phenylalanine ammonia-lyase and chalcone synthase in the phenylpropanoid pathway, as well as specific terpene synthases, are involved.

Indole phytoalexins example camalexin in Arabidopsis thaliana; resveratrol in grapevines; pisatin in peas; phaseollin in beans; glyceollins in soybean; capsidiol in pepper. These molecules often exert antimicrobial effects by disrupting cell membranes, inhibiting enzyme activity, or interfering with fungal development. Phytoalexin production can be rapidly triggered within hours to days after infection or stress.

While phytoalexins form part of plant innate immunity, resistance is typically polygenic and context-dependent; producers may vary among species and even cultivars. Because their synthesis entails energetic costs and can be toxic at high concentrations, regulation is tightly controlled and usually localized to infected tissues. Research into phytoalexin pathways informs crop protection and breeding for disease resistance.