Allelochemicals Biologica... - Name

40
ANA LUISA ANAYA
tetrahydrobenzo[b]oxepins derivatives also showed apparent inhibition against both
cress and oat (Doi et al., 2004).
The tremendous impact of parasitic plants on world agriculture has prompted
much research aimed at preventing infestation. Orobanche and Striga spp. are two
examples of parasitic weeds that represent a serious threat to agriculture in large
parts of the world. The life cycle of these parasitic weeds is closely regulated by the
presence of their hosts, and secondary metabolites that are produced by host plants
play an important role in this interaction. A special interest has been arising on those
host-produced stimulants that induce the germination of parasite seeds. Three classes
of compounds have been described that have germination-stimulating activity:
dihydrosorgoleone, the strigolactones and SL. Keyes et al. (2001) suggest that
dihydrosorgoleone is the active stimulant in the root exudates of sorghum and other
monocotyledonous hosts. However, Butler et al. (1995) and Wigchert et al. (1999)
suggest that dihydrosorgoleone is less likely to be the germination stimulant in vivo
because of its low water solubility, and because no correlation between its production
and the germination of Striga has been found. To date, there is no definite proof that
the germination of parasitic weed seeds in the field is induced by one single signal
compound or class of compounds (and indeed such proof will be hard to obtain)
(Bouwmeester et al., 2003). The capacity of SL, which share some structural features
with the strigolactones, to induce the germination of S. asiatica has been reported
(Fischer et al., 1989, 1990). In addition, a decade after the results of Fischer studies,
Francisco Macías and his group (Pérez de Luque et al., 2000; Galindo et al., 2002)
performed some studies of the structure-activity relationship (SAR) directed to evaluate
the effect of several SL as germination stimulants of three Orobanche spp. (O. cumana,
O. crenata, and O. ramosa). Results are compared with those obtained in the same
bioassay with an internal standard, the synthetic analogue of strigol GR-24. A high
specificity in the germination activity of SL on the sunflower parasite O. cumana has
been observed, and a relationship between such activity and the high sunflower SL
content is postulated. Molecular properties of the natural and synthetic germination
stimulants (GR-24, GR-7, and Nijmegen-1) and SL have been studied using MMX
and PM3 calculations. Consequently, comparative studies among all of them and
their activities have been made. SL tested present similarities in molecular properties
such as the volume of the molecule and the spatial disposition of the carbon backbone
to the natural germination stimulant orobanchol. These properties could be related to
their biological activity. Considering that the sun-flower–O. cumana interaction is
highly specific and that sunflower contains many SL, it is tempting to speculate that
O. cumana has evolved to respond to sesquiterpene lactones (and not or less to
strigolactones) (Bouwmeester et al., 2003).
3.2. Phenolics
In relation with phenolics, Inderjit et al. (1997) conducted a study to understand the
effects of certain phenolics, terpenoides, and their equimolar mixture through agar
gel and soil growth bioassays and their recovery from soils. The eight compounds