Allelochemicals Biologica... - Name

ALLELOCHEMICALS FROM AGERATUM CONYZOIDES AND ORYZA SATIVA 203
(Figure 8). They were produced in rice leaves and stems with M. grisea and R. solani.
Phytocassanes A-E had high antifungal activity against the pathogenic fungi, M.
grisea and R. solani (Koga et al., 1995; 1997). Their ED 50 values in prevention of
spore germination and germ tube growth of M. grisea were very low (Figure 8).
Sakuranetin is a flavonoid-type phytoalexin that was identified from rice plants. It
had high antifungal activity and a large amount of accumulation in rice leaves
(Nakazato et al., 2000).
It has been shown in host-pathogen interactions that resistance reactions can be
triggered by a large number of abiotic and biotic factors. Among the chemical factors,
macromolecules of microbial origin are very important. Plant defense responses are
stimulated by very low concentrations of these molecules (Darvill and Albersheim,
1984). It was confirmed that a chemical for plant disease control might function by
activating the natural resistance mechanisms of the host, for example, 2,2-dichloro-
3,3-dimethyl cyclopropane carboxylic acid may exert its systemic fungicidal activity
against the rice blast disease caused by P. oryzae (Cartwright et al., 1977). The
application of methionine on wounded rice leaves induced the production of rice
phytoalexins, sakuranetin and momilactone A. In the paddy field, methionine treatment
has been demonstrated to reduce rice blast (Nakazato et al., 2000).
An increasing number of studies have shown that rice phytoalexins are induced
by elicitor that are produced by pathogenic microorganisms and make field disease
control by inducing the pathogen defense mechanism in rice (Schaffrath et al., 1995;
Tamogami et al., 1997a,b). Elicitors have been investigated extensively. It has been
shown that jasmonic acid and its related compounds play important roles as the
signaling molecules that elicit the production of phytoalexins in rice. Sakuranetin
production may be elicited by exogenously applied jasmonic acid in rice leaves.
Furthermore, sakuranetin production by exogenously applied jasmonic acid was
significantly counteracted by amino acid, cytokinin, kinetin and zeatin (Tamogami et
al., 1997a,b).
4. CONCLUSION
Many interactions between plant pathogens and their hosts are allelopathic.
Allelochemicals can be applied in biological control of weeds and plant diseases (Rice,
1995). Our research suggested that allelochemicals produced and released from A.
conyzoides intercropping in citrus orchard did play important roles in integrated pest
management. Many kinds of allelochemicals in rice not only inhibit the germination
and growth of weeds, but also participate in the defense against pathogens. However,
it remains unclear which allelochemicals in rice are predominantly involved in defense
mechanisms against the pathogens. Therefore, further clarification of the resistance
mechanism and multiple functions of rice allelochemicals are warranted.
Acknowledgments : The author thanks Dr. Inderjit and anonymous reviewers for
thoughtful criticisms and correcting English on earlier versions of the manuscript. The
work was supported by National Natural Science Foundation of China (NSFC
No.30170182; 30430460) and Hundreds-Talent Program of Chinese Academy of Sciences.