P. Geigenberger et al., PHLOEM-SPECIFIC EXPRESSION OF PYROPHOSPHATASE INHIBITS LONG-DISTANCE TRANSPORT OF CARBOHYDRATES AND AMINO-ACIDS IN TOBACCO PLANTS, Plant, cell and environment, 19(1), 1996, pp. 43-55
As reported in a previous paper [Lerchl et al. (1995) Plant Cell, 7, 2
59-270], expression of Escherichia call inorganic pyrophosphatase in t
he cytosol under the control of the phloem-specific rolC promoter from
Agrobacterium rhizogenes results in decreased growth of transgenic to
bacco plants. In this paper we investigate the effect of the phloem-sp
ecific expression of pyrophosphatase on phloem metabolism, and on plan
t growth and allocation. A small decrease in the hexose phosphate/UDP-
glucose ratio, the ATP/ADP ratio and the respiration rate in the midri
bs of the transformants provides evidence that mobilization of sucrose
via pyrophosphate-dependent reactions is necessary for phloem energy
metabolism. The source leaves of the transformants had higher levels o
f carbohydrates and amino acids and a much higher glutamine/glutamate
ratio than the wild type, showing that export was inhibited and that t
he growth inhibition was not due to a lack of photoassimilates or orga
nic nitrogen in the leaves. The accumulation of photoassimilates was p
aralleled by a decrease in photosynthesis, chlorophyll content and rib
ulose bisphosphate carboxylase/oxygenase (Rubisco) activity, a small i
ncrease in hexose phosphates and triose phosphates and a decrease in g
lycerate 3-phosphate in the source leaves. There was a decrease of sol
uble sugars and amino acids in sink leaves of the transformants. In si
nk leaves amino acids decreased more than carbohydrates and a decrease
in the glutamine/ glutamate ratio was observed. This was accompanied
by a large decrease of nitrate. Sugars and amino acids were also reduc
ed in the root tips of the transformants. The carbohydrate /amino acid
ratio decreased 5-fold in the root tips, indicating a particularly ac
ute shortage of carbohydrates. Relatively high levels of sugars and am
ino acids in the basal regions of the root and the increase in sugars
in the midrib indicate that there is also increased leakage of assimil
ates out of the phloem during long-distance transport. Metabolism is r
equired to maintain phloem function along the transport route, as well
as for the initial step of loading. The transformants showed decrease
d stem and root growth. The growth inhibition was largest in condition
s allowing rapid growth of the wild type (high light and nitrogen supp
ly).