Friedrich Jacob

Mechanism of sucrose retrieval along the phloem path — a kinetic approach

In contrast to hexoses (fructose, glucose, 3-oxymethylglucose) sucrose is preferentially taken up by isolated conducting tissue ofCyclamen persicum Mill. Sucrose absorption follows a biphasic kinetic, with a saturable component (Km=5.2 mM; Vmax = 4.2 μmol · g FW-1 · h-1) in the lower range (<20 mM) and a linear component in the upper range (20–400 mM). The saturable, carrier-mediated uptake operates against the concentration gradient without attaining any equilibrium over 6 h. It is characterized by a pH optimum at 4.5 and sensitivity to the protonophore CCCP (carbonylcyanidem-chlorophenylhydrazone). The activity of the saturable uptake increases with decreasing turgor and it is independent of the presence of hexoses. There is no effect of pH or CCCP on linear sucrose uptake at high external concentrations. Hexose uptake between 0.25 and 20 mM shows a entirely linear kinetic. Leakage after preincubation with 1 mM sucrose exhibits a minimum at pH 5.0 and enters a steady state within 100 min. Furthermore, it was found to be independent of the osmolarity of the external solution. Results are discussed in terms of apoplastic leakage and reabsorption along the phloem path: efflux of sucrose is compensated by a specific, carrier-mediated influx, which is qualitatively similar to apoplastic phloem loading accomplished by proton cotransport. In according with the balanced efflux and influx it should be considered rather as a retrieval than a loading mechanism.

Studien an Cuscuta reflexa Roxb: VII. Zum Abwehrmechanismus von Lycopersicon esculentum Mill.

Summary By twining on the stem or petiole of Lycopersicon esculentum, Cuscuta reflexa was able to produce prehaustoria. However, the parasite was incapable of forming functional haustoria from these organs. The reason is a hypersensitive reaction of the external cell layers of the stem or petiole of Lycopersicon after contact with the adhesive secretory epithelium of the prehaustoria. As a result of this reaction a layer of dead cells between host and parasite is formed. The layer prevents the intrusion of the initial haustoria, developed within the prehaustoria into the host tissue. The hypersensitive reaction is restricted to the external cell layers of Lycopersicon, because when this layer was partially removed, a formation of functional haustoria was possible on this specific area. In this case an intensive growth of the parasite on Lycopersicon was also observed.

Transport of xenobiotics in higher plants. I: Structural prerequisites for translocation in the phloem

Summary The correlation between the mobility of xenobiotics in the phloem and their chemical structure was investigated using the following substances: phloem-mobile 2,4-D, xylem-mobile 2,4-dichloro-anisole derived from the elimination of the carboxyl group, xylem-mobile defenuron and atrazine, and their ambimobile derivatives N-(p-carboxyphenyl)-N′-methylurea, phenylureidoacetic acid and 2-chloro-4,6-bis-( β -alanino)-l,3,5-triazine. Mobility was characterized quantitatively by the translocation rates and the translocation quotient Q tr . The Q tr was determined by the Sinapis test exhibiting the ratio of mobility in phloem following leaf application and mobility in xylem following root application. Analysis of exudates from detached Yucca inflorescence stalks revealed the presence of phloem- and ambimobile compounds without any metabolic conversions in the phloem sap exclusively. The partition coefficients of substances mobile in the phloem were higher at p H 8 than at p H 5. The corresponding coefficients of xylem-mobile chemicals were of the same value. These results suggest that xenobiotics enter the sieve element-companion cell complex by diffusion. Phloem- and ambimobile compounds are accumulated by an ion trapping mechanism causing their retention in the sieve tube and preventing a complete escape along the path. Consequently, acidic compounds with pK a lower than 7 are expected to be phloem- or ambimobile.

QUANTITATIVE DETERMINATION OF MOBILITY OF XENOBIOTICS AND CRITERIA OF THEIR PHLOEM AND XYLEM MOBILITY

Abstract The general criteria for establishing the relationship between chemical structures of xenobiotics and the properties of mobility in higher plants were investigated. A translocation test with seedlings of Sinapis alba was developed where the ratio of phloem and xylem mobility of exogenously applied compounds can be expressed be means of the translocation quotient Qtr. It represents a physiological constant of the substance under investigation. Phloem as xylem mobility requires substances which are neither extremely lipophilic nor extremely hydrophilic. Favourable prerequisites for phloem mobility are given by polar acidic compounds which have a clearly different degree of dissociation at pH 5 and pH 8. The absorption into the sieve tube-companion cell complex is effected passively by the principle of an ion trap. Xylem mobility is favoured by the apolar character of a compound and - in the case of polar substances - by anions with no difference in dissociation at pH 5 and pH 8. The presence of such a difference in addition to xylem-mobile properties defines ambimobility.

Transport of Xenobiotics in Higher Plants III. Absorption of 2,4-D and 2,4-Dichloroanisole by Isolated Conducting Tissue of Cyclamen

Summary Uptake of phloem-mobile 2,4-D and xylem-mobile 2,4-dichloroanisole (2,4-DCA) into isolated conducting tissue was investigated. 2,4-D absorption exhibited a pH optimum at 4.0 and attained sixfold accumulation within 1 h. It was sensitive to metabolic inhibitors. A Q10 value of 1.2 indicates a passive entrance mechanism. These results confirm the hypothesis that ion trapping causes retention in the alkaline sieve tube sap. However the concentration response of 2,4-D uptake tended to saturate even at concentrations higher than 10−4 M. Monitoring sucrose uptake after 2,4-D preincubation revealed an injuring effect on the plasmalemma, which seems to be responsible for the observed pattern of concentration dependent uptake. The absorption of xylem-mobile 2,4-DCA deriving from phloem-mobile 2,4-D by elimination of the carboxyl group was not affected by the external pH or the presence of inhibitors in the medium. The Q10 amounted to 1.2. Although this corresponds to previous investigations of xylemmobile substances, additional results disagree with past experience. 2,4-DCA was accumulated in the conducting tissue. Influx and efflux showed a slow initial phase and a slight escape respectively. The concentration kinetic was hyperbolic. This behaviour seems to be caused by unspecific binding to different cell constituents. The sites involved are still uncertain, but they are obviously not concerned with retention and transport in sieve tubes.

Studies on Cuscuta reflexa Roxb. VI. Is there an autoparasitic withdrawal of nutrients

Summary The phenomenon of autoparasitism in the genus Cuscuta has been reported on a number of occasions. Attempts to prove the transport of radiolabelled sucrose from a host filament of Cuscuta reflexa to a parasitizing shoot of the same species were not successful. The growth of parasitizing Cuscuta stems separated from the stock culture was not stronger than the growth of isolated shoots of the same length. Light and electron microscopic pictures of cross-sections of parasitized Cuscuta stems showed an isolating layer of cells in front of the penetrating haustoria formed in the cortex of the infested shoot. This way the haustorial cells cannot reach the vascular bundles of the parasitized Cuscuta stem. In spite of the presence of haustoria, Cuscuta reflexa cannot absorb food substances autoparasitically.

Studies on Cuscuta reflexa Roxb. IV. Evidence of a high abscisic acid content in the parasite in comparison with its host Vicia faba L.

Summary Investigations on the endogenous level of growth affecting substances, which have proved to be growth inhibitors of wheat seedlings, have been carried out in the parasite/host system of Cuscuta reflexa Roxb. Vicia faba L. The parasite Cuscuta contained a high level of endogenous growth inhibitors. By using gas chromatography, abscisic acid was shown to be the most important inhibitor within Cuscuta - The maximum content of abscisic acid within the parasite was found in its haustorial region, where the level was much higher than that of hosts inhibitor. A possible participation of abscisic acid in the process of transferring nutrients from host to parasite is discussed.

Zur Feinstruktur der endophytischen Cyanophyceen von Gunnera chilensis Lam.

Summary Nostoc punctiforme lives within the cells of Gunnera chilensis . The blue-green algae are contained in the cytoplasm of the host cells and are surrounded by the plasmalemma of the higher plant. The cyanophyceae are surrounded by a 0.1—l μ m layer of mucilage without structure in the electron microscope and a 4-layered cell wall. Inside the blue-green algae are to be seen the typical thylakoids, derived from the plasmalemma, osmiophilic globules and nucleus equivalents. Nostoc punctiforme isolated from Gunnera chilensis and growing in artificial medium has the same cell wall as the symbiontic living algae. Other structural changes were not observed in the electron microscopic level. The number of Nostoc punctiforme in a cell are increased with the age of the colony. In old colonies the cytoplasm of the host cell are reduced to a very small border. Beside the normal Nostoc celles are obvious ly dead cells, which are very black, without structures and of half-moon shape. The dead cells in cultures of isolated Nostoc punctiforme have their shape but the cytoplasm gives no reaction with OSO 4 and only the thin lined thylakoids are visible. In sterile growing Gunnera plants the infection with Nostoc punctiforme was studied. The blue-green algae penetrate the intercellulares and later the cell wall. A scheme of this process is given and discussed according to the endosymbiont-hypothesis.

Aufnahme von Saccharose und Xenobiotika in isoliertes Leitgewebe von Cyclamen

Summary The uptake of sucrose, MCPA and bromfeuufon into conducting tissue isolated from Cyclamen petiole has been investigated. In accordance with data published by other authors previously sucrose uptake into vascular bundels is characterized by features of phloem loading. Experiments with Cyclamen confirm these results (pH optimum 5.0; apparent km value 2.6 mM; Q10 value 1.9; sensitive to DNP and CCCP; high retention). DCCD does not exercise any influence on absorption during an incubation of 2 h. Since MCPA and bromfenuron differ extremly in their mobility, uptake into and release from conducting tissue of both substances were determined. MCPA is translocated in the phloem contrary to bromfenuron, which is mobile in the xylem only. There is no resistance during influx and efflux of bromfenuron. Because of its lipophilic nature, this substance should be able to enter apoplast and symplast. The absorption of MCPA is connected with accumulation and strong pH dependence (optimum 4.0). It can be inhibited by uncouplers. A Q10 value of 1.4 and a linear concentration dependence support the assumption, that the uptake of MCPA is accomplished by diffusion. pH dependence and sensitivity to uncouplers indicate either a binding activity in the symplast or membrane transfer by change of lipid solubility depending on pH regime in phloem (ion trapping).

Zur Mineralstoff-Konkurrenz zwischen Linum und Camelina I. Aufnahme von 35S-Sulfat

Summary Both in monoculture and mixed culture, the uptake of 35 S sulphate into 20-day-old plants of Linum usitatissimum L. und Camelina sativa (L.) CRANTZ as well as 35 S transport from the root into the shoot of these plants were examined. In equal periods the Camelina plants took up essentially more sulphate of the nutrient solution than the Linum plants did. From among the two species, when standing in mixed culture, the Linum plants took up less sulphate ions than those in monoculture, whilst Camelina plants in mixed culture absorbed more sulphate ions than they did in monoculture. The differences of absorbed sulphate between monocultures and mixed cultures can be interpreted as the result of an inter-species competition for these ions. These findings support the opinion that the diminution of the dry weight of Linum under the influence of Camelina is caused decisively by competition and not by allelopathic factors. A participation of such factors, however, cannot be excluded. A diminished absorption of sulphate also took place in Linum plants having grown in a nutrient solution, in which Camelina plants already had been cultivated, but which mineral substances a new had been given to. The transportation of 35 S off the roots into shoot occurred in either species under mixed culture conditions to a higher extent than in the respective monocultures.