Allelochemicals Biologica... - Name

CHUIHUA KONG
ALLELOCHEMICALS FROM Ageratum conyzoides L.
AND Oryza sativa L. AND THEIR EFFECTS
ON RELATED PATHOGENS
Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110016,
China, and South China Agricultural University, Guangzhou 510642, China.
E-mail: kongch@mail.edu.cn
Abstract. Allelochemicals play an important role in biological control of plant pathogens and diseases. Weed
Ageratum conyzoides L. and food crop Oryza sativa L. can produce and release many kinds of allelochemicals
participating in their defense against pathogens. The essential oil from A. conyzoides has been found to have
significant negative effects on several plant pathogens. In the A. conyzoides intercropped citrus orchard, A.
conyzoides released allelopathic flavones and agreatochromene into the soil to reduce the populations of soil
pathogenic fungi Phytophthora citrophthora, Pythium aphanidermatum and Fusarium solani. Further research
revealed that ageratochromene underwent a reversible transformation in the soils, that is, ageratochromene
released from A. conyzoides plants was transformed into its dimers, and the dimers can be remonomerized in the
soils. The reversible transformation between ageratochromene and its dimers in the A. conyzoides intercropped
citrus orchard soil can be an important mechanism maintaining bioactive allelochemicals at an effective
concentration, thus, sustaining the inhibition of pathogenic fungi in soil. Many kinds of allelochemicals in rice
were identified. Among them, alkylresorcinols, flavone and cyclohexenone had high antifungal activities on
Pyricularia oryzae and Rhizoctonia solani. Furthermore, these antifungal allelochemicals formed by rice can
be triggered by a large number of abiotic and biotic factors. Antifungal allelochemicals from rice mainly involved
two types of diterpenes and flavones, including momilactones A and B, oryzalexins A-F and S, phytocassanes
A-E and sakuranetin. These compounds help rice establishing its own pathogen defense mechanism. However,
it remains obscure which allelochemicals in rice are predominantly involved in defense mechanisms against the
pathogens. Therefore, further clarification of the resistance mechanism and multiple functions of these compounds
on rice are warranted.
1. INTRODUCTION
Plants produce many kinds of low-molecular-mass secondary metabolites that are
generally non-essential for the basic metabolic processes of the plant. Among these
secondary plant metabolites, some are known as allelochemicals that improved defense
against other plant competition, microbial attack or insect/animal predation. Plants
cannot move to escape pathogens. However, plants have evolved to successfully
withstand infection by a vast majority of pathogens that attack them (Stuiver and
Custers, 2001). It was found that plants biosynthesize phytoalexins as soon after
pathogenic attack (Dangl and Jones, 2001). The concept of phytoalexins as induced
anti-microbial allelochemicals in plant was first developed in 1940 (Muller and Borger,
193
Inderjit and K.G. Mukerji (eds.),
Allelochemicals: Biological Control of Plant Pathogens and Diseases, 193– 206.
© 2006 Springer. Printed in the Netherlands.