Biology Reference
In-Depth Information
10
o
C
15
o
C
20
o
C
25
o
C
6
5
4
3
2
1
0
amino acid
FIGURE 2.5
Essential amino acid content of phloem sap exudates from
Vicia faba
plants (cv. The Sutton)
reared at different temperatures. The EDTA exudates were taken from the top fully expanded leaf of pre-
Þowering plants at 3 weeks post sowing by the protocol of Douglas (1993). [Previously unpublished data
from C.R. Tosh and A.E. Douglas.]
of these very large cells is not thrown into extensive folds or underlain by large numbers of
mitochondria as would be expected if mycetocytes had a major function in the secretion of
essential amino acids.
A second issue relates to the response of the symbiosis to varying dietary amino acid supply.
The composition of phloem sap varies among plant species and with developmental age and envi-
ronmental conditions (Blackmer and Byrne, 1999; Karley et al., in press). This is illustrated by the
data in Figure 2.5; the total essential amino cid concentration in the phloem sap of
Vicia faba
declines
with temperature from 24 3.5% of the total amino acid content at 10
p
C to 16 1.2% at 25
p
C
(mean s.e., 8 reps), and this general pattern is followed by all individual essential amino acids
except histidine, which shows the reverse relationship. How does the symbiosis respond to such
variability? Febvay et al. (1999) and Douglas et al. (2001) have shown that the incorporation of
radioactivity from
14
C-sucrose and
14
C-glutamate, respectively, into essential amino acids is elevated
in aphids reared on diets of low essential amino acid content and interpreted these data as elevated
Buchnera
provisioning of these nutrients under conditions of increased aphid demand. This conclu-
sion is, however, premature because the elevated speciÝc activity of
14
C in the essential amino acids
probably reÞected, in part or wholly, the decreased total concentration of these compounds. It will
be necessary to explore the total Þux of metabolites, not just radiotracer Þux, to establish deÝnitively
whether
Buchnera
metabolism responds to variation in aphid nutritional demand.
A Ýnal unresolved issue relates to the developmental biology of the symbiosis. Aphid symbiosis
is developmentally complex, with multiple associations of different ages (see the section entitled
Modes of Transmission of Bacteria), and the embryos are particularly dependent on the
Buchnera
(Figure 2.4)
.
Does this mean that the symbiosis in the embryos is more active than that in the
maternal tissues? Or, alternatively, that the maternal symbiosis supplements the function of the
symbiosis in the embryos? The Ýnding that radioactivity from
14
C-labeled
essential amino acids
injected into aphids is recovered from embryos (Wilkinson and Ishikawa, 1999) is consistent with
the notion of net transfer of
Buchnera
-derived nutrients from the maternal symbiosis to embryos
but also open to the interpretation that the injected amino acids are catabolized to products
