Allelochemicals Biologica... - Name

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REN-SEN ZENG
Glomus intraradices, Glomus mosseae, and Gigaspora rosea leads to the accumulation
of similar cyclohexenone derivatives (Vierheilig et al., 2000). However, no fungusspecific
induction of these compounds are known. Pathogens and endophyte did not
induce the formation of cyclohexenone derivatives in barley roots (Maier et al., 1997).
The role of cyclohexenone derivatives in disease resistance is unknown.
In response to pathogen attack, plants activate an array of inducible defense
reactions, many of which involve the transcriptional activation of the corresponding
defense genes, including genes that encode enzymes involved in the synthesis of lignin
and phytoalexins (Dixon et al., 1984; Dixon and Harrison 1990). Transcript levels of
some pivotal enzymes of defense response of plants significantly increase after
mycorrhizal fungal infection (Harrison and Dixon, 1993; 1994). Several inducible
defence-related genes, including those encoding isoflavonoid phytoalexins such as
phenylalanine ammonia lyase (PAL), chalcone synthase (CHS), chalcone isomerase
(CHI) and for the cell wall structural protein HRGP, have been reported to be induced
during mycorrhiz establishment (Tagu and Martin, 1996). Mycorrhization resulted
in a local and systemic induction of plant defence-related enzymes chitinase,
chitosanase and beta-1,3-glucanase, as well as superoxide dismutase in tomato plants
(Pozo et al., 2002). Chalcone isomerase (CHI) and chitinase activities increased in
inoculated roots prior to mycorrhizal colonization (Volpin et al., l994, Kapulink et
al., 1996), whereas the increase in PAL activity coincided with colonization. Production
of some new compounds, and increase in the activity of the enzymes peroxidase and
polyphenol oxidase, was observed following inoculation with AM fungi (Charitha
Devi and Reddy, 2002). Dumas-Gaudot et al. (1992a,b) found new chitinase isoforms
that were specifically induced in several AM associations and were different from
those elicited by root fungal pathogens, indicating a different pattern of plant response
to pathogenic and mutualistic fungi. Glomus mosseae induced new chitinase isoforms
in tomato roots (Pozo et al., 1996). Expression of genes encoding enzymes that
synthesize phenolpropanoid compounds has been detected in mycorrhizal roots (Garcia-
Garrido and Ocampo, 2002). Other defense related genes shown to be upregulated in
mycorrhizal symbioses include: genes involved in metabolism of reactive oxygen
species, chitinase and beta I ,3-glucanase, and genes involved in senescence, including
glutathione-S-transferase. Mycorrhiza also induced changes in PR protein expression
in tobacco leaves (Shaul et al., 1999).
Colonization of barley, wheat and maize and rice roots by Glomus intraradices
resulted in strong induction of transcript levels of the pivotal enzymes of
methylerythritol phosphate pathway of isoprenoid biosynthes i.e., 1 -deoxy-D-xylulose
5-phosphate synthase (DXS) and 1 -deoxy-D-xylulose 5-phosphate reductoisomerase
(DXR) (Walter et al., 2000). At the same time six cyclohexenone derivatives were
characterized from mycorrhizal wheat and maize roots. DXS2 transcript levels are
low in most tissues but are strongly stimulated in roots upon colonization by
mycorrhizal fungi, correlated with accumulation of carotenoids and apocarotenoids
(Walter et al., 2002). Some reports show that the AM symbiosis may cause an increase,
decrease, or no change in the plant defense reactions (Guenoune et al., 2001; Mohr et
al., 1998).