Capsidiol is a

terpenoid The terpenoids, also known as isoprenoids, are a class of naturally occurring organic chemicals derived from the 5-carbon compound isoprene and its derivatives called terpenes, diterpenes, etc. While sometimes used interchangeably with "terpenes" ...

compound that accumulates in tobacco ''Nicotiana tabacum'' and chili pepper ''Capsicum annuum'' in response to fungal infection.Maldonado-Bonilla LD, Betancourt-Jiménez M,Lozoya-Gloria E (2008) "Local and systemic gene expression of sesquiterpene phytoalexin biosynthetic enzymes in plant leaves". ''European J. Plant Path.'' 121(4), 439-449. Capsidiol is categorized under the broad term of

phytoalexin Phytoalexins are antimicrobial substances, some of which are antioxidative as well. They are defined, not by their having any particular chemical structure or character, but by the fact that they are defensively synthesized ''de novo'' by plants th ...

, a class of low molecular weight plant secondary metabolites that are produced during infection.

Phytoalexins Phytoalexins are antimicrobial substances, some of which are antioxidative as well. They are defined, not by their having any particular chemical structure or character, but by the fact that they are defensively synthesized ''de novo'' by plants th ...

are also characterized as a part of a two pronged response to infection which involves a short term response consisting of production of free radicals near the site of infection and a long term response involving the production of hormones and an increase in enzymes to biosynthesize phyoalexins such as capsidiol.

Mechanism of production

Capsidiol is produced in the pepper fruit ''

Capsicum annuum ''Capsicum annuum'' is a species of the plant genus ''Capsicum'' native to southern North America, the Caribbean, and northern South America. This species is the most common and extensively cultivated of the five domesticated capsicums. The spec ...

'' or

tobacco Tobacco is the common name of several plants in the genus '' Nicotiana'' of the family Solanaceae, and the general term for any product prepared from the cured leaves of these plants. More than 70 species of tobacco are known, but the ...

Nicotiana tabacum ''Nicotiana tabacum'', or cultivated tobacco, is an Annual plant, annually grown herbaceous plant of the ''Nicotiana'' genus. The plant is tropical in origin, is commonly grown throughout the world, and is often found in Cultivation of tobacco, ...

'' after infection by the oomycete water-mold ''

Phytophthora capsici ''Phytophthora capsici'' is an oomycete plant pathogen that causes blight and fruit rot of peppers and other important commercial crops. It was first described by L. Leonian at the New Mexico State University Agricultural Experiment Station in ...

''. In pepper or tobacco fields, the oomycete is soil-borne and initially infects roots, collars and lower leaves. Sporangia are moved within fields by contact with field equipment, clothing, gloves, tools etc. and initial infections spread

zoospore A zoospore is a motile asexual spore that uses a flagellum for locomotion. Also called a swarm spore, these spores are created by some protists, bacteria, and fungi to propagate themselves. Diversity Flagella types Zoospores may possess one or ...

s through splashing of water from irrigation or rain.Uchida J.Y.; Aragaki M. (1980). "Chemical Stimulation of oospore formation in ''Phytophthoracapsici''". ''Mycologia'' 72, 1103-1108. Initial responses to infection include production of radical oxygen species, including H₂O₂.Arreola-Cortes A.; Castro-Mercado E.; Lozoya-Gloria E.; Garcia-Pineda E. (2007). "Capsidiol production in pepper fruits (''Capsicum annuum'') induced by arachidonic acid is dependent of an oxidative burst". ''Physiol. Mol. Plant Pathol.'' 70(1-3): 69-76. Exogenous treatment with H₂O₂ alone has been shown to induce capsidiol production. Capsidiol production is then increased in response to radical oxygen species production.

Biosynthesis

Capsidiol is a bicyclic terpene that is biosynthetically derived from the

mevalonate pathway The mevalonate pathway, also known as the isoprenoid pathway or HMG-CoA reductase pathway is an essential metabolic pathway present in eukaryotes, archaea, and some bacteria. The pathway produces two five-carbon building blocks called isopentenyl ...

via farnesyl pyrophosphate (FPP). The ''E'',''E''-farnesyl cation is first created by the loss of

pyrophosphate In chemistry, pyrophosphates are phosphorus oxyanions that contain two phosphorus atoms in a P–O–P linkage. A number of pyrophosphate salts exist, such as disodium pyrophosphate (Na2H2P2O7) and tetrasodium pyrophosphate (Na4P2O7), among othe ...

. Kinetic studies have indicated that turnover appears to be limited by a chemical step after the initial loss of pyrophosphate.Dewick P.M. (2002)."The biosynthesis of C5-C25 terpenoid compounds". ''Nat. Prod. Rep.'' 16, 97-130 Crystal structures of recombinant tobacco 5-epi- aristolochene synthase (TEAS), alone and also complexed with two FPP analogues have been reported and analyzed to suggest the following mechanism of biosynthesis.Starks C.M.; Back K.; Chappell J.; Noel J.P.; (1997) Structural Basis for Cyclic Terpene Biosynthesis by Tobacco 5-Epi-Aristolochene Synthase. ''Science'' 277, 1815-1820 The ''E'',''E''-farnesyl cation undergoes cyclization to form the germacryl cation. The second ring closure gives the bicyclic eudesmyl cation, which is stabilized by various dipole interactions, then H-2 migrates to C-3 producing a tertiary cation at C-2 (farnesyl numbering). Production of 5-epi-aristolochene from FPP by 5-epi-aristolochene 3-hydroxylase, a sesquiterpene cyclase, is considered the critical step in capsidiol biosynthesis. Aristolochene synthase enzymes from '' Penicillium roqueforti'', ''

'' have been purified and their crystal structures have been reported suggesting different steriochemistries for aristolochene. '' Penicillium roquefortis enzyme appears to synthesizes aristolochene by way of (''S'')-

germacrene Germacrenes are a class of volatile organic hydrocarbons, specifically, sesquiterpenes. Germacrenes are typically produced in a number of plant species for their antimicrobial and insecticidal properties, though they also play a role as insect ph ...

A , however, the ''

'' enzyme 5-epi-aristolochene synthase produces the diastereoisomeric product by way of (''R'')-

Notes and references

{{reflist Phytoalexins Diols Decalins Sesquiterpenes Terpenes found in Solanaceae