Biology Reference
In-Depth Information
methods, including macroarray analysis and EST (expressed sequence tagged)
gene analysis. Current ongoing studies include microarray analysis.
5.1
Macroarray analysis identifies defence genes
We have previously utilized a tomato macroarray system to evaluate the ex-
pression of a limited number (~500) of genes (Thornburg et al., 2003). These
studies have led to a number of unique observations. Not only are the nec-
tarins (nectar-expressed proteins) expressed during nectary development but,
in addition, we have previously identified a number of defence-related pro-
teins that are upregulated during nectary development. Some of these, shown
in Table 3, include antimicrobial, antifungal, and antiherbivore proteins.
This broad spectrum of defence gene expression points to a unique and
previously unrecognized feature of the nectary gland—that the nectary has a
major defence function in addition to its function of secreting nectar. Be-
cause of the metabolically rich complexity of nectar and the non-sterile
nature of visiting pollinators, it is, in retrospect, not surprising that the nec-
tary should have an active function in defence.
The mechanisms that induce defence gene activation are receiving sig-
nificant attention in today's plant literature. There are a number of factors
known to affect defence gene function in plants. Two of the most important
may be hydrogen peroxide and ascorbate. Both of these small molecular weight
compounds have significant effects on plant defence gene expression. Both
compounds also have important roles in nectary biology. Because of their
presence in the nectary, it is not surprising that they may also affect defence
gene expression in the nectary.
5.1.1
Role of hydrogen peroxide in plant stress and defence
Reactive oxygen intermediates, including hydrogen peroxide, are triggered
by an oxidative burst in plant cells that occurs in response to a variety of bi-
otic and abiotic stresses (Levine et al., 1994; Desikan et al., 2000). These
reactive oxygen intermediates act as signalling molecules to initiate defence
gene expression. The production of the reactive oxygen species that proceed
hydrogen peroxide production appears to be mediated by a membrane-bound
NADPH oxidase that is responsible for the induction of a number of defence
genes (Orozco-Cardenas et al., 2001; Torres et al., 2002). One major response
to the production of these reactive oxygen species, especially hydrogen per-
oxide, is the activation of a number of defence genes (Alvarez et al., 1998;
