Biology Reference
In-Depth Information
Ziegler, 1989). No single enzyme has been shown to completely convert
starch to simple sugars, so multiple enzymes most likely are involved. Phos-
phorolytic enzymes such as starch phosphorylase and R1 enzyme (starch
water dikinase) are expressed throughout nectary development. Genes that
are specifically turned on during the time just prior to anthesis include
α-
Thus, starch metabolism plays a central role in plant life, allowing for effi-
cient storage and utilization of carbohydrates, but also functions in floral
biology by storing carbohydrate equivalents for nectar production.
amylase and β-amylase (Ren and Thornburg, unpublished observations).
4
PROTECTION OF THE GYNOECIUM
Nectar is a metabolite-rich fluid freely offered to visiting pollinators to
maximize rates of pollen transfer. These visiting pollinators often harbour
micro-organisms that have the potential to infect the gynoecium. Because the
composition of nectar compares favourably with many bacterial growth me-
dia, the potential for deleterious infection of the gynoecial environment is
high. Indeed, in spite of the fact that insects are non-sterile and are often
promiscuous, floral infections are rare in plants.
We have hypothesized that this lack of infection is due to the presence in
ornamental tobacco nectar of a series of enzymes that contsitute a novel bio-
chemical pathway, the Carter-Thornburg nectar redox cycle (for details see
Nicolson & Thornburg, 2007, Chapter 5 in this volume; Carter et al., 1999;
Carter & Thornburg, 2000, 2004a, b, c). Briefly, this pathway functions to
generate very high levels of hydrogen peroxide, up to 4 mM (Carter &
Thornburg, 2000). This is 40 times the level produced by human neutrophils
in response to microbial attack (Prince & Gunson, 1987) and these levels are
indeed toxic to micro-organisms (Thornburg et al., 2003). We propose that the
enzymes in the nectar redox cycle, together with other factors (Naqvi et al.,
2005), function to maintain nectar in an axenic state, which then protects
the gynoecial environment from infection by pollinator-borne microbes
(Thornburg et al., 2003). Our laboratory is currently testing this hypothesis
directly by knocking out each of the components of the nectar redox cycle.
5
GENE EXPRESSION
Changes in phenotype are modulated by changes in gene expression. To
evaluate the changes in gene expression in the nectary we have used several
