References

410 L.G. Perry et al.
monooxygenase, lipid transfer protein, heat shock protein, DNA-J protein,
and blue copper-binding protein (Bais et al. 2003). Phenylpropanoid and
terpenoid phytoalexin pathway genes, a number of which produce enzymes
that can act as antioxidants (Sticher et al. 1997), were also induced in the
roots 1 h after catechin treatment. However, the most interesting change
in gene expression that they observed was the upregulation of ten genes
in the first 10 min after catechin treatment. These included genes associated
with calcium signaling and oxidative stress, as well as four unknown
genes lacking homology with genes from other organisms. Identification
of the function of these unknown genes could provide insights into the
factors that cause plants to be susceptible to catechin, and into potential
mechanisms of resistance.
27.3.4
(±)-Catechin Is Present at Phytotoxic
Concentrations in C. maculosa Soils
While laboratory experiments demonstrated that (±)-catechin is a potent
phytotoxin with strong effects on plant biochemistry and gene expression,
field observations were required to gauge the importance of (±)-catechin
in C. maculosa competitive interactions under natural conditions. In particular,
for (±)-catechin to influence plant interactions, it must be present
at phytotoxic concentrations in C. maculosa field soils. Several studies
have reported exceptionally high soil (±)-catechin concentrations in North
American C. maculosa soils (Bais et al. 2002, 2003; Perry et al. 2005b), indicating
that soil (±)-catechin is present in sufficient quantities in C. maculosa
soil to inhibit plant neighbors. Bais et al. (2003) reported a mean soil
(±)-catechin concentration of 2.24±0.20 mg g −1 and Perry et al. (2005b)
reported a mean soil (±)-catechin concentration of 1. 55 ± 1.27 mg g −1 dry
soil. In addition, tests of the soil extracts from one site confirmed that the
(±)-catechin in C. maculosa soils has similar phytotoxicity to (±)-catechin
from commercial sources (Perry et al. 2005b). Bais et al. (2002) found that
(±)-catechin concentrations declined with distance from the C. maculosa
taproot, and with increasing soil depth. However, Perry et al. (2005b) found
that soil (±)-catechin concentrations did not change with distance from the
C. maculosa taproot but remained high as far as 25 cm from the taproot,
indicating that high soil (±)-catechin concentrations may be ubiquitous in
at least some well-established C. maculosa populations. The differences in
soil (±)-catechin concentrations among the studies are not surprising, since
the studies were conducted in different locations and at different times. The
effects of soil characteristics, climate, and season on (±)-catechin secretion,
stability, and soil absorption are not yet understood.