Root Exudation and Rhizosphere Biology: Multiple Functions of a Plant SecondaryMetabolite - Communication in Plants: Neuronal Aspects of Plant Life - page 413

Plant autoinhibition mediated by secondary metabolites has been the
subject of much research (Singh et al. 1999). However, (±)-catechin is one
of the first secondary metabolites determined to act as both an allelochem-
ical and an autoinhibitor. Autoinhibition of seedling establishment may
influence plant populations in several ways. First, autoinhibition may be
a mechanism through which adults avoid establishment of intraspecific
competitors in dense populations (Schenk et al. 1999). Second, seedlings
that produce autoinhibitors may reduce intraspecific competition by pre-
venting the establishment of their siblings and unrelated seedlings (Dyer
2004). Third, autoinhibitors that delay germination, rather than killing
seeds or seedlings, may postpone germination in areas where intraspe-
cific competition from adults would prevent seedling survival (Picman
and Picman 1984). Determining the specific effects of autoinhibition on
C. maculosa populations will require further research to determine whether
(±)-catechin reduces C. maculosa seedling root elongation or delays ger-
mination under field conditions.
Autoinhibition by C. maculosa is probably most important in well-
established stands, where soil (±)-catechin is likely to be concentrated
and widespread. (±)-Catechin concentrations lower than 0.5 mg ml −1 have
little effect on C. maculosa root elongation, and no effect on germination
(Perry et al. 2005b). However, much lower (±)-catechin concentrations have
large effects on other, more (±)-catechin-sensitive species (Bais et al. 2003;
Weir et al. 2003; Perry et al. 2005a). Thus, C. maculosa seedlings should
have an advantage over more (±)-catechin-sensitive species in soils with
moderate (±)-catechin concentrations, while very high (±)-catechin con-
centrations may prevent establishment of C. maculosa seedlings as well
as other species. In well-established C. maculosa populations with high
soil (±)-catechin concentrations, autoinhibition, water stress (Jacobs and
Sheley 1998), and seed predation (Muller-Schärer and Schroeder 1993)
probably operate together to limit C. maculosa seedling establishment.
27.5
(±)-Catechin Effects on Soil Communities
In addition to the inhibitory effects of (±)-catechin on plants, (+)-catechin
and (-)-catechin both reduce survival of some soil organisms. However,
the two enantiomers appear to affect different organisms. Bais et al. (2002)
found that (+)-catechin, but not (-)-catechin, inhibited growth of several
soil bacteria. Six bacterial strains were exposed to each of the enantiomers
or the racemic mixture, applied to filter discs. Xanthomonas campestris
ssp. vesicatoria , Erwinia amylovora , Pseudomonas fluorescens ,and E. caro-
tovora were each susceptible to 100 µg ml −1 (+)-catechin or to 200 µg ml −1