Amino acid composition and nutritional quality of potato leaf phloem sap for aphids
1 Department of Biology, University of York, Heslington, York, YO10 5YW,
UK
2 ADAS Woodthorne, Wergs Road, Wolverhampton, WV6 8TQ, UK
* Author for correspondence (e-mail: ajk9{at}york.ac.uk)
Accepted 24 June 2002
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Summary |
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Key words: aphid, amino acid, Myzus persicae, Macrosiphum euphorbiae, plant quality, phloem sap, potato
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Introduction |
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Here, we investigate the causal basis of the reduction in aphid abundance
on developmentally mature potato plants relative to young plants
(Taylor, 1955;
Mackauer and Way, 1976
;
Parker et al., 2000
). Field
experiments have previously demonstrated that plant factors contribute to
differences in aphid performance for the two principal aphid species infesting
potato plants in the UK, Macrosiphum euphorbiae and Myzus
persicae (A. J. Karley, A. E. Douglas, W. E. Parker and J. J. Howard,
manuscript in preparation). The present study aimed to establish the
suitability (i.e. `quality') for aphids of young and mature potato plants in
terms of their phloem nutrient composition, focusing on the principal phloem
nutrients: sucrose and amino acids
(Fisher, 2000
). Extensive
studies of aphid physiology, mainly using chemically defined diets, have
revealed the central role of sucrose concentration, amino acid concentration
and composition, and sucrose:amino acid ratio in shaping aphid performance
(Auclair, 1963
;
Dadd, 1985
;
Douglas, 1998
).
The specific objectives were: (1) to quantify the nutritional characteristics of, and aphid performance on, potato plants of different developmental age under controlled conditions; (2) to correlate aphid performance with the principal plant nutrient factors of phloem sap carbon and nitrogen; and (3) to identify the causal basis of the link between phloem nutrient composition and aphid performance using chemically defined diets that mimic phloem nutrient profiles of plants at different developmental ages. By informing us of the plant nutritional factors that influence aphid performance, this approach might lead to the development of crop-management practices that exploit the `natural' variation in plant suitability for aphids, and thus aid aphid pest management.
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Materials and methods |
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Nutritional indices of plants
The analyses were conducted on potato plants (Solanum tuberosum)
cv Wilja (Edwin Tucker and Sons Ltd, Devon, UK) grown from tubers in 101 pots
of John Innes No. 3 compost, under glass (15-24°C) between June and August
1999. The plants were watered daily and sprayed weekly with nicotine as insect
control. Sampling commenced when the plants reached the emergence and
shoot-expansion stage (Jefferies and
Lawson, 1991), at 3 weeks after planting, and continued until
tuber filling had become the principal dry matter sink at 10.5 weeks after
planting.
There were three elements to each harvest: (1) Phloem sap analysis by the
EDTA exudation technique of King and Zeevart
(1974). Briefly, the terminal
leaflet was excised from a compound leaf midway along the shoot axis and
inserted immediately into 0.2 ml 5 mmol l-1 EDTA solution, pH 7.5.
The samples were incubated for 90 min in the dark in a sealed chamber
equilibrated at 25°C with a dish of saturated KH2PO4
to maintain high humidity. The EDTA samples were frozen at -20°C until
ready for analysis of sugar and amino acid content (see chemical analyses
below). Parallel experiments confirmed that the same array of amino acids
detected in the EDTA phloem exudates was also detected in phloem exudates
obtained from severed stylets of aphids feeding from potato plants (K. V.
Pescod, A. J. Karley and A. E. Douglas, unpublished data). (2) Leaf
carbon:nitrogen (C:N) content. The leaflet used for phloem exudation was
weighed, dried at 60-75°C for 48 h and re-weighed, then ball-milled to a
fine powder. The carbon and nitrogen contents were determined by gas
chromatography using a C,H,N NA2100 Brewanalyser (CE Instruments, Wigan, UK)
with urea standards. (3) Dry matter accumulation. The above-ground (shoots,
leaves and flowers/berries) and below-ground (roots, stolons and tubers) parts
of each plant were cleaned of soil, dried at 60-75°C for 48 h and weighed
to an accuracy of 0.01 g.
Aphid performance
The performance of M. persicae ADAS 99/12 and M.
euphorbiae ADAS 99/11 on potato plants cv. Wilja was quantified under
glasshouse conditions (described above). Newborn nymphs were confined in
mesh-covered clip-on leaf cages of 2.5 cm internal diameter (clip-cages)
attached to the abaxial surface of a leaf midway along the shoot axis of
`pretuber-filling' (3-5 weeks after planting) or `tuber-filling' (9-11 weeks
after planting) plants. Aphids were transferred to fresh plants (3-weeks-old
and 9-weeks-old) at fortnightly intervals throughout the experiment. All six
aphid clones were used for the performance analysis on chemically defined
diets because of considerable interclonal variation in the performance of
various aphid species on diets (Srivastava
et al., 1985; Sandström
and Pettersson, 1994
). 2-day-old nymphs from aphid cultures on
plants were transferred to diet sachets, prepared according to Prosser and
Douglas (1992
). Diets
contained 0.5 mol l-1 sucrose, 0.15 mol l-1 amino acids
and vitamins, mineral ions and organic acids (as described by
Prosser and Douglas, 1992
).
The diet solution was buffered with KH2PO4 to give a
final pH of 7.5. The diet amino acid compositions representative of the phloem
sap of pre-tuber-filling and tuber-filling potato plants are shown in
Table 1, and the diets
containing these amino acids are described as `young' and `old', respectively.
The relative composition of the amino acids of phloem exudates sampled from
plants at five and 10.5 weeks after planting was used as the basis for diet
construction (Table 1). These
mol% values were adjusted to allow inclusion of cysteine and proline, which
were not detected by our high-performance liquid chromatography (hplc) method.
Additionally, the percentage of essential amino acids was increased by 55% in
both diets because aphids do not settle readily or thrive on diets that mimic
the low essential amino acid contents of phloem sap (A. E. Douglas,
unpublished data). Taking into account these two adjustments, the mol% and
mmol l-1 values for each amino acid used in the diets are shown in
Table 1.
|
For aphids on plants or diets, the experiments were monitored daily. The
dates when they reached adulthood, initiated reproduction and died were
scored, and any offspring produced were counted each day and removed. Daily
fecundity was quantified as (total number of offspring/adult lifespan in
days). Rm (estimated intrinsic rate of population
increase) was calculated as 0.738(lnN)/Td, where
N is the number of offspring produced by an aphid in the time period
equivalent to the pre-reproductive development period (Td)
(Wyatt and White, 1977). For
the diet-reared aphids, the relative growth rate (RGR) was also determined as
ln(f/i)/t, where i and f are the fresh
masses of 2-day-old nymphs and teneral adults, respectively, and t is
the development time. Aphids were weighed to the nearest µg on a Mettler
MT5 microbalance. Any alate adults generated were excluded from performance
analyses.
Nutrient uptake and assimilation
The radiolabelled inulin technique
(Wright et al., 1985;
Wilkinson and Ishikawa, 1999
;
Douglas et al., 2001
) was used
to quantify aphid feeding and the uptake and assimilation of dietary glutamate
and glutamine by diet-reared aphids. The purity of radiochemicals was
confirmed by thin-layer chromatography (tlc) and hplc. 2-day-old nymphs were
reared to the final instar (6-day-old nymphs for M. persicae;
8-day-old nymphs for M. euphorbiae) on the `young' and `old' diets.
Ten replicate aphid pairs were then allowed to feed for 48 h from the same
diet formulation but supplemented with [3H]inulin (Sigma, 1 mCi
ml-1) at 0.3 MBq ml-1 diet solution and either
L-[U-14C]glutamate or L-[U-14C]glutamine (Amersham
Pharmacia Biotech UK Ltd, 50 µCi ml-1) at 0.3 MBq
ml-1 diet. The radioactive diet sachets were administered on a
Perspex ring (2.5 cm diameter, 0.7 cm height) above a GF/C glass-fibre filter
(2.5 cm diameter, Whatman), so that honeydew produced by the aphids
accumulated on the filter. Aphids feeding from replicate non-radioactive
sachets were included as controls. Each aphid pair was homogenised in 0.2 ml
ice-cold 0.05 mol l-1 Tris-HCl, pH 7.5. To quantify the
radioactivity, the filters and samples of the aphid homogenates were shaken
with Ultima-GoldTM XR scintillation fluid (Packard Bioscience B.V.,
Gröningen, The Netherlands) and counted in a Packard Tri-Carb Liquid
Scintillation Analyzer using pre-set 3H/14C dual
windows. The volume of diet ingested was calculated from the recovery of
[3H]inulin in honeydew; inulin is not transported across aphid guts
nor metabolised by these aphids (A. E. Douglas, unpublished data). The amount
of dietary [14C]amino acid assimilated by the aphids was calculated
from the difference between the amount ingested (as determined from feeding
rate) and the amount recovered as 14C in the honeydew. The
respiratory loss was estimated from the difference between the amount
assimilated (obtained as described above) and the 14C content of
the aphid tissues (see Wilkinson et al.,
2001
).
Chemical analyses
The sucrose content of phloem exudates was quantifed by the method of
Dahlqvist (1984). Each 10
µl exudate sample was hydrolysed to completion with 10 U invertase (Cat.
no. I-4504, Sigma-Aldrich, Gillingham, Dorset) per ml in 50 mmol
l-1 sodium acetate buffer, pH 4.5 at 37°C for 30 min, and the
glucose produced was determined by the Sigma Diagnostics glucose assay kit
(GAGO-20) using glucose standards, following manufacturer's instructions but
with o-dianisidine concentration increased to 100 µg
ml-1.
Amino acids in phloem exudates and aphid honeydew were separated by
reverse-phase hplc, following derivatization with o-phthaldialdehyde
(Jones et al., 1981), using a
Hewlett-Packard HP1 100 Series autosampling LC system with C18
ZORBAXTM Eclipse XDB-C8 column and fluorescence detection. Amino acids
were quantified by comparison with the AA-S-18 (Sigma) reference amino acid
mixture, supplemented with asparagine, glutamine and tryptophan. All protein
amino acids, except proline and cysteine, could be detected using this method,
with a detection limit of approximately 0.5 pmol.
The protein content of aphid homogenates was quantified by the microassay method of BioRad, following manufacturer's instructions, with bovine serum albumin as standard.
Statistical analyses
Parametric statistical tests were applied to datasets confirmed to be
normally distributed (RyanJoiner one-sample test) with homogeneous
variances (Bartlett's test), which required logarithmic transformation where
indicated. Analysis of variance (ANOVA) or t-tests were applied to
test the impact of diet composition on aphid RGR, Rm and
feeding rate. Where appropriate, `clone' was nested as a random factor within
`species' in the ANOVA model, and aphid protein content was included as a
covariate. In the multivariate analysis of variance (MANOVA) analyses of
phloem exudate amino acids, normality or homogeneity of datasets could not be
achieved for all variates (even following exclusion of outlier values
identified using critical values for Dixon's test) and, therefore, replicate
MANOVA tests were used with and without the data for these variates. Principal
components analysis (PCA) with a correlation matrix to standardise variables
(Randerath, 1996) was used to
explore the impact of plant age on the amino acid composition of phloem
exudates. The non-parametric tests used were KruskallWallis and
Spearman Rank tests.
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Results |
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The C:N ratio in oven-dried leaf tissue increased steadily throughout the first 9 weeks of growth, then increased rapidly over the remainder of the experimental period (Fig. 1B). The C:N ratio data did not conform to the criteria of normality of error distribution and homogeneity of variance required for parametric statistical testing. The ratio of sucrose:amino acid (mol:mol) in phloem exudates varied significantly during the experiment (KruskalWallis: H6= 21.65, P<0.01). Spearman rank correlation analysis demonstrated no significant correlation between leaf C:N and exudate sucrose:amino acid over time nor for individual sampling dates (Table 2).
|
The amino acids in phloem exudates were dominated by the non-essential amino acids glutamate and aspartate and their amides, accounting for 28-56% of the total amino acids. The amino acid composition of exudates varied significantly with time (MANOVA of log-transformed nmol exuded data, Wilk's test: F108,225=4.549, P<0.01). To explore the variation in phloem amino acid composition, PCA was applied to compare data for `pre-tuber-filling' dates and `tuber-filling' dates, i.e. the dates before and after the switch in dry matter allocation at 9-9.5 weeks after planting (see Fig. 1A).
The first two principal components (PC 1 and PC 2) accounted for 51% of the variation in the dataset (Fig. 2). A plot of the amino acid attributes revealed that PC 1 tended to separate essential from non-essential amino acids (see Table 1 for list of essential and non-essential amino acids), while the amino acids glutamate and aspartate were separated from their amides, glutamine and asparagine, by PC 2 (Fig. 2A). Other non-essential amino acids (e.g. glycine, serine and alanine) were also separated along PC 2 (Fig. 2A). These two axes achieved good, but not perfect, separation of the `pre-tuber-filling' and `tuber-filling' samples (Fig. 2B), indicating a shift during plant development in the amino acid composition of the exudates. The key shift was from pre-tuber-filling plants with exudates dominated by non-essential amino acids, particularly glutamine, asparagine, serine and threonine (with an average of 22.4% essential amino acids in week 5), to tuber-filling plants with exudates dominated by the essential amino acids (with an average of 34% essential amino acids in week 10.5) and by variation in the non-essential amino acids glutamate, glycine and alanine.
|
Similar results were obtained in a separate experiment on the amino acid content of phloem exudates of potato plants reared in JuneAugust 2001 under glasshouse conditions and infested with aphids (data not shown). This indicated that the time-dependent shift in phloem amino acid composition of potato plants was linked to plant development and occurred independently of aphid infestation.
Aphid performance
Aphids of M. persicae ADAS 99/12 and M. euphorbiae ADAS
99/11 performed less well on tuber-filling plants (9-11 weeks) than on
pre-tuber-filling plants (3-5 weeks) by the indices of development time and
survival to adulthood, mean daily fecundity and Rm
(Table 3).
|
The chemically defined diets were acceptable to all clones of both M. persicae and M. euphorbiae, as indicated by the ready settling of the 2-day-old nymphs onto the diet sachets, by low pre-adult mortality (<20% for M. persicae clones and <27% for M. euphorbiae clones) and by a low incidence of stillborn offspring (0.1-4.5% of offspring produced by each clone). The mean age at death did not vary significantly between those feeding on `young' and `old' diets for any clone (date not shown), with the exception of M. persicae clone ADAS 99/12, for which the aphids feeding on the `old' diet formulation died before those on `young' diets (mean age at death of 34.5 days and 50 days, respectively).
The mean RGR of nymphs varied from 0.23 g-1 g-1 d to 0.32 g-1 g-1 d for M. euphorbiae clones and from 0.32 g-1 g-1 d to 0.37 g-1 g-1 d for M. persicae clones and, for all clones except M. euphorbiae clone ADAS 99/11, the mean RGR was greater on the `young' diet than on the `old' diet (Fig. 3A). ANOVA revealed that the effects of both diet formulation and aphid species, but not the interaction term, were statistically significant; the term `aphid clone' was also significant (Fig. 3A). Rm displayed a similar pattern to RGR (Fig. 3B). The mean Rm was significantly higher for M. persicae than for M. euphorbiae and also for aphids on `young' diet rather than on `old' diet, with significant interclonal variation but non-significant interaction term (Fig. 3B). Inspection of the data revealed that the higher Rm of aphids on the `young' diet compared with the `old' diet mainly resulted from their greater fecundity rather than a difference in the pre-reproductive development period (data not shown).
|
The Rm values for M. persicae ADAS 99/12 and M. euphorbiae ADAS 99/11 on pre-tuber-filling and tuber-filling plants are included on Fig. 3B for comparison. With the exception of M. euphorbiae ADAS 99/11 on pre-tuber-filling plants, values of Rm on plants were lower than those on diets.
Aphid feeding and nutrient assimilation
Aphid feeding rate of final instar M. persicae aphids on the
`young' and `old' diet formulations is shown in
Fig. 4. ANCOVA (shown in
Fig. 4) revealed that diet
uptake by M. persicae was significantly depressed on the `old' diet,
relative to the `young' diet, even after the effect of body size (measured as
aphid protein content, which also varied significantly) on feeding rate was
taken into account as the covariate; i.e. the low feeding rates on `old' diet
could not be contributed entirely to small aphid size. The feeding rate of
M. persicae also varied significantly between clones, but this
variation could not be related to interclonal differences in size (aphid
protein content) or any other aspect of aphid performance, and its basis was
not investigated here.
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Radiolabelled glutamate or glutamine was included in the diet sachets used
for analysis of aphid feeding rates displayed in
Fig. 4, so that aphid
metabolism of dietary glutamate and glutamine, the dominant amino acids in the
diets (and in potato phloem sap) could be monitored. Less than 10% of the
radioactivity from both [14C]glutamine and
[14C]glutamate was recovered from the honeydew of aphids (M.
persicae ADAS 99/12, ADAS 99/13 and RB/4158, M. euphorbiae ADAS
99/11) feeding from both diet formulations, and, consistent with previous
studies on other aphid species (Febvay et
al., 1995; Wilkinson et al.,
2001
), <80% of the assimilated radioactivity was lost from the
aphids under all treatments, presumably by respiration. No substantial
difference was identified in the metabolism of these amino acids by aphids on
the two diets.
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Discussion |
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We have evidence that the correlation between aphid performance and potato
phloem amino acid composition is robust because consistent developmental
shifts in amino acid composition have been demonstrated for potato plants over
two consecutive years under glass (this study) and across three consecutive
years in the field (A. J. Karley et al., manuscript in preparation). In
addition, studies involving a number of plant species have confirmed that the
amino acid composition of phloem sap derived from EDTA-induced exudation
reflects that of stylectomy-derived phloem sap
(Weibull et al., 1990;
Sandström et al., 2000
),
including potato (K. V. Pescod, A. J. Karley and A. E. Douglas, unpublished
data).
A striking aspect to the phloem amino acid composition data is that the
dominant amino acid in the phloem sap of pre-tuber-filling plants is
glutamine, which declines dramatically as the plants develop, allowing
glutamate and aspartate to predominate in the phloem of tuber-filling plants.
High relative levels of glutamate have been implicated in reduced nutritional
quality of phloem sap for aphids (Douglas,
1993) and in reduced aphid performance on `resistant' cultivars of
some plant species (Chen et al.,
1997
; Weibull,
1988
). A second feature is that the phloem of tuber-filling plants
is relatively enriched in essential amino acids, nutrients that animals cannot
synthesise de novo. Thus, the phloem of pre-tuber-filling plants
could be regarded as less nutritious than that of tuber-filling plants, in
apparent contradiction with the poor performance of aphid clones on the latter
(and on `old' diet composition). However, aphids are nutritionally `buffered'
from variation in the dietary supply of these nutrients because they obtain
supplementary amino acids from their symbiotic bacteria, Buchnera
aphidicola (Douglas,
1998
; Shigenobu et al.,
2000
), and the rate of bacterial synthesis of amino acids
increases in response to low dietary supply
(Febvay et al., 1999
;
Douglas et al., 2001
). Thus,
the impact of the non-essential amino acid component of the diet on aphid
performance must outweigh any effects of the essential amino acids.
The nitrogen content of the diet is known to influence aphid performance
(Prosser et al., 1992;
Abisgold et al., 1994
;
Girousse and Bournoville,
1994
; Febvay et al.,
1988
) but it was not determined in this study because the EDTA
exudation technique does not allow quantification of the amount of phloem
exuded nor, therefore, of phloem nutrient concentrations
(Weibull et al., 1990
).
However, the absence of a clear developmental trend in phloem sucrose:amino
acid ratio (Fig. 1B), which can
be quantified reliably by the EDTA method, precluded diet construction to
examine the impact of developmental shifts in total nitrogen availability on
aphid performance.
In the absence of phloem analysis, the poor aphid performance on tuber-filling plants might have been attributed to elevated leaf C:N ratio (Fig. 1B), which is widely recognised as an indicator of low plant nutritional quality. However, the total C:N ratio of plant tissue did not covary with the sucrose:amino acid ratio of the phloem sap on which the aphids feed, suggesting that plant indices based on the elemental composition (e.g. C:N, N content per unit dry mass) of the total plant tissue are not necessarily an accurate index of the nutritional quality of the plant for phloem-feeding insects.
We reasoned that if the correlation between phloem amino acid composition
and aphid performance could be reproduced with chemically defined diets, then
phloem amino acid composition is a likely factor contributing to the
difference in aphid performance between the pre-tuber-filling and
tuber-filling plants. The results obtained across multiple clones provide
general qualitative support and contribute to a sparse literature that
attempts to translate correlative observations into dietary studies of the
causal basis for nutritional effects on aphid performance (Sandström,
1994, 2000;
Bolsinger and Flückiger,
1989
). Exact replication of performance between plant and diets
would not be expected because the diets, by definition, lack the mechanical
and olfactory cues of a plant. The performance of M. persicae clone
ADAS 99/12 on both diet formulations was superior to its performance on potato
plants (Fig. 3B), indicating
that potato is not an optimal host for this aphid species. In addition, the
Rm for M. euphorbiae clone ADAS 99/11 on
tuber-filling plants was lower than that on `old' diet
(Fig. 3B). These two
observations indicate that additional plant factors (allelochemical or
physical) not monitored in this study also contribute to the reduction in
aphid performance on tuber-filling plants. The performance of both aphid
species on the `young' diet was comparable with that on favoured plants such
as spring cabbage (M. persicae;
Jenkins, 2001
) and immature
potato plants (M. euphorbiae; Fig.
3B) and, consequently, the results obtained with diet-reared
aphids cannot be attributed to a non-specific malaise on diets.
Although the physiological basis of the difference between aphid
performance on `young' and `old' diets remains to be resolved fully, the
results summarised in Fig. 4
indicate that it involves reduced feeding rate on the `old' diet. This is
despite the higher concentration of the amino acid methionine, a known
phagostimulant for aphids (Mittler,
1967; Srivastava and Auclair,
1974
), in the `old' diet than in the `young' diet. [However,
dietary glutamate inhibits feeding by the pea aphid Acyrthosiphon
pisum (Srivastava et al.,
1983
) and its effects might obscure those of methionine].
A potentially rewarding line of future investigation relates to the reduced
amide:acid ratio of the `old' diet. Much of the glutamine and glutamate
ingested by aphids is respired (Febvay et
al., 1995; Wilkinson et al.,
2001
), and the release of the carbon skeleton
(
-ketoglutarate) of these amino acids for entry into the tricarboxylic
acid (TCA) cycle involves removal of amino groups. De-amination releases two
amino groups per proton for glutamine but only one amino group per proton for
glutamate (this will also be true for asparagine and aspartate, respectively,
if these are de-aminated and transaminated to form glutamate); high relative
levels of glutamate and aspartate could increase the acid load, which would
have implications for pH homeostasis in the aphid tissues. The resultant
metabolic stress might contribute to the difference in aphid performance
between `young' and `old' diets and pre-tuber-filling and tuber-filling
plants. It might, however, be simplistic to attribute variation in aphid
performance on diets and plants of different amino acid composition to a
change in the concentration of single amino acids. The impact of dietary amino
acid composition on aphid nutrition and general physiology might be driven by
complex interactions between the various amino acids.
Why does the phloem nutrient profile vary with plant developmental
age?
When considering variation in phloem amino acid composition, it is
important to distinguish between the developmental age of plant parts and of
the entire plant. This study sampled from the fully expanded `source' leaf at
the node midway along the shoot axis and, thus, was considered to be
developmentally equivalent across the different plant ages studied.
Consequently, the variation in phloem amino acid composition obtained in
Fig. 2 relates exclusively to
developmental age of the plant. Complementary study of field-grown potato
plants has revealed that the shift in amino acid composition with plant
developmental age is broadly uniform across leaves from the apex to basal
position along the shoot axis (A. J. Karley, A. E. Douglas, W. E. Parker and
J. J. Howard, manuscript in preparation). In other words, the developmental
effects obtained are systemic, at least with respect to the shoot system.
Developmental changes in phloem amino acid composition have not been studied
widely; but, Corbesier et al.
(2001) reported high phloem
glutamine levels exported from leaves of white mustard Sinapis alba
and thale cress Arabidopsis thaliana at flowering induction (which is
broadly equivalent to the pre-tuber-filling plants of potato), and Boggio et
al. (2000
) linked high
glutamate levels in the pericarp of the tomato Lycopersicon
esculentum to phloem import of glutamate during fruit ripening (which is
equivalent to the tuber-filling stage in potato). These results raise the
possibility that the developmental shifts in phloem amino acid composition
described in this study commonly occur in other plant species and might not be
specific to potato.
The processes underlying developmental shifts in phloem amino acid
composition are anticipated to include developmental regulation of transporter
expression for phloem loading of amino acids in leaf vascular tissue, for
which there is sound evidence (Kwart et
al., 1993; Fischer et al.,
1995
). Other processes that might be involved include metabolism,
unloading, retrieval and xylemphloem transfer of amino acids along the
length of the translocation pathway
(Fischer et al., 1995
;
Rentsch and Frommer, 1996
;
Hirner et al., 1998
). The
relative abundance of glutamate, glutamine and aspartate, the dominant amino
acids in potato phloem sap, is probably shaped mostly by the activities of
glutamine synthetase (GS) and aspartate aminotransferase in the source tissues
and/or sieve elements (McGrath and
Coruzzi, 1991
; Lam et al.,
1996
; Vincent et al.,
1997
; Finnemann and
Schjoerring, 2000
). GS activity is regulated by 14-3-3 proteins
(Finnemann and Schjoerring
2000
), and certain 14-3-3 isoforms are known to be under
developmental regulation in potato
(Wilczynski et al., 1998
).
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Acknowledgments |
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References |
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