Fluctuations in the counting rate of photons originating from uncorrelated point sources become, within the coherently illuminated area, slightly enhanced compared to a random sequence of classical particles. This phenomenon, known in astronomy as the Hanbury Brown-Twiss effect, is a consequence of quantum interference between two indistinguishable photons and Bose Einstein statistics. The latter require that the composite bosonic wavefunction is a symmetric superposition of the two possible paths. For fermions, the corresponding two-particle wavefunction is antisymmetric: this excludes overlapping wave trains, which are forbidden by the Pauli exclusion principle. Here we use an electron field emitter to coherently illuminate two detectors, and find anticorrelations in the arrival times of the free electrons. The particle beam has low degeneracy (about 10(-4) electrons per cell in phase space); as such, our experiment represents the fermionic twin of the Hanbury Brown-Twiss effect for photons.

SummaryThe aim of this work was to establish whether plastidial phosphoglucomutase is involved in the starch biosynthetic pathway of potato tubers and thereby to determine the form in which carbon is imported into the potato amyloplast. For this purpose, we cloned the plastidial isoform of potato PGM (StpPGM), and using an antisense approach generated transgenic potato plants that exhibited decreased expression of the StpPGM gene and contained signi®cantly reduced total phosphoglucomutase activity. We con®rmed that this loss in activity was due speci®cally to a reduction in plastidial PGM activity. Potato lines with decreased activities of plastidial PGM exhibited no major changes in either whole-plant or tuber morphology. However, tubers from these lines exhibited a dramatic (up to 40%) decrease in the accumulation of starch, and signi®cant increases in the levels of sucrose and hexose phosphates. As tubers from these lines exhibited no changes in the maximal catalytic activities of other key enzymes of carbohydrate metabolism, we conclude that plastidial PGM forms part of the starch biosynthetic pathway of the potato tuber, and that glucose-6-phosphate is the major precursor taken up by amyloplasts in order to support starch synthesis.

Abstract. The substrate dependence and product inhibition of three different fructokinases and three different hexokinases from growing potato (Solanum tuberosum L.) tubers was investigated. The tubers contained three specific fructokinases (FK1, FK2, FK3) which had a high affinity for fructose (Km= 64, 90 and 100 laM) and effectively no activity with glucose or other hexose sugars. The affinity for ATP (Kin=26, 25 and 240 laM) was at least tenfold higher than for other nucleoside triphosphates. All three fructokinases showed product inhibition by high fructose (K i = 5.7, 6.0 and 21 mM) and were also inhibited by ADP competitively to ATP. Sensitivity to ADP was increased in the presence of high fructose, or fructose-6-phosphate. In certain conditions, the K i (ADP) was about threefold below the K m (ATP). All three fructokinase were also inhibited by fructose-6-phosphate acting non-competitively to fructose (K i = 1.3 mM for FK2). FK1 and FK2 showed very similar kinetic properties whereas FK3, which is only present at low activities in the tuber but high activities in the leaf, had a generally lower affinity for ATP, and lower sensitivity to inhibition by ADP and fructose. The tuber also contained three hexokinases (HK1, HK2, HK3) which had a high affinity for glucose (Km =41, 130 and 35 IJM) and mannose but a poor affinity for fructose (Km= 11, 22 and 9 mM). All three hexokinases had a tenfold higher affinity for ATP (Kin=90, 280 and 560 ~M) than for other nucleoside triphosphates. HK1 and HK2 were both inhibited by ADP (K i =40 and 108 [JM) acting competitively to ATP. HK1, but not HK2, was inhibited by glucose-6-phosphate, which acted non-competitively to glucose (Ki=4.1 mM). HK1 and HK2 differed, in that HK1 had a narrower pH optimum, a higher affinity for its substrate, and showed inhibition by glucose-6-phosphate.Abbreviations: FK=fructokinase; Fru6P=fructose-6-phosphate; Glc6P = glucose-6-phosphate; HK = hexokinase; NTP = nucleoside triphosphate; Pi=inorganic phosphate; UDPGlc=uridine-5'-diphosphoglucose Correspondence to: Mark Stitt; FAX: 49(6221) 565859The relevance of these properties for the regulation of hexose metabolism in vivo is discussed.

Potato (Solanum tuberosum L.) plants transformed with sense and antisense constructs of a cDNA encoding the potato hexokinase 1 (StHK1) exhibited altered enzyme activities and expression of StHK1 mRNA. Measurements of the maximum catalytic activity of hexokinase revealed a 22-fold variation in leaves (from 22% of the wild-type activity in antisense transformants to 485% activity in sense transformants) and a 7-fold variation in developing tubers (from 32% of the wild-type activity in antisense transformants to 222% activity in sense transformants). Despite the wide range of hexokinase activities, no change was found in the fresh weight yield, starch, sugar, or metabolite levels of transgenic tubers. However, there was a 3-fold increase in the starch content of leaves from the antisense transformants after the dark period. Starch accumulation at the end of the night period was correlated with a 2-fold increase of glucose and a decrease of sucrose content. These results provide strong support for the hypothesis that glucose is a primary product of transitory starch degradation and is the sugar that is exported to the cytosol at night to support sucrose biosynthesis.

Abstract.A combination of chromatography on DE-52 cellulose, Cibacron Blue agarose, Mono Q anion exchanger and gel filtration was used to resolve different hexose-phosphorylating enzymes from growing "sink" potato tubers (Solanum tuberosum L.). Three enzymes (fructokinases: FK1, FK2 and FK3) are active with fructose and inactive with glucose, and three (hexokinases: HK1, HK2 and HK3) are active with glucose but not with fructose. Elution from DE-52 columns showed that the relative abundance of the six activities changes, depending on the organ and on the developmental stage. FK1 and FK2 were present at high activities in tubers but at very low activity in leaves; conversely FK3 was present at very low activity in tubers but at high activity in leaves. During storage of potato tuber, and also during sprouting, there was a decrease of FK1 and FK2. In contrast, glucose-phosphorylating activity was very low in growing tubers. During storage and sprouting the activity of the glucose-phosphorylating enzymes rose, until they exceeded FK1 and FK2. This was due particularly to an increase of HK1, whereas HK2 declined relative to HK1, and HK3 was always negligible. These changes in the pattern of hexose-phosphorylating enzyme forms are compared with the changing metabolic fluxes and pools of hexose sugars in potato tubers. It is concluded that organ-and development-specific changes in the abundance of the various enzyme forms contribute to the regulation of hexose metabolism in the potato.