Biology Reference
In-Depth Information
Table 3.
Defence genes upregulated during nectary development.
Protein/cDNA
Function
Reference
Snakin 1
antimicrobial peptide
Segura et al., 1999
γ-thionin
antifungal peptides
Pelegrini & Franco, 2005
PR1
antifungal protein
Stintzi et al., 1993
PR5
antifungal protein
Stintzi et al., 1993
Chalcone synthase
flavonoid biochemistry
Bell et al., 1986
Wound-induced win1
antifungal protein
Ponstein et al., 1994
Wound-induced pin1
antiherbivore protein
Green & Ryan, 1972
Wound-induced pin2
antiherbivore protein
Bryant et al., 1976
Chamnongpol et al., 1998; Sasabe et al., 2000). Other downstream signalling
events mediated by hydrogen peroxide include calcium mobilization and
protein phosphorylation (Lecourieux et al., 2002; Neill et al., 2002).
The nectar of ornamental tobacco contains very high levels of hydrogen
peroxide (up to 4 mM); it is therefore not surprising that a large number of
defence genes are expressed in the nectary. We are currently working to
knock out the components of the nectar redox cycle in an attempt to produce
plants that lack these high levels of reactive oxygen species. It will be of
great interest to evaluate the expression of defence genes in such plants.
Nitric oxide also appears to function in this process (de Pinto et al., 2002)
but, to date, the presence of nitric oxide in the nectary has not been examined.
5.1.2
Role of ascorbate in plant stress and defence
Another biological compound that has potential to affect the defence path-
ways in the nectary is the antioxidant ascorbate. While it is not clear what
mechanisms are responsible for gene regulation by ascorbate, ascorbate
knockout plants do indeed show significant levels of altered gene expression
(Kiddle et al., 2003; Pastori et al., 2003). In the absence of ascorbate, many
defence genes are activated. These include the two pathogenesis-related pro-
teins glucanase and chitinase. These genes appear to be responsive to
ascorbate because they are downregulated in response to added ascorbate
(Pastori et al., 2003). Genes that increase following application of ascorbate
to ascorbate-depleted plants include metallothionein and superoxide dismu-
tase.
