Assimilate Transport and Partitioning in Plants: Approaches,Methods, and Facets of Research

This review, dedicated to the 100th anniversary of A.L. Kursanov's date of birth, considers the development of phloem transport studies since his book, Assimilate Transport in the Plant, was published in 1976. This book and several other fundamental publications on phloem structure and functions basically shaped this physiological issue; as a result, several international meetings by scientists working in the area were induced, and the proceedings of these meetings were published at regular intervals. Six conferences have been held to date, and six corresponding collections of papers have been published and are reviewed here along with other experimental communications and reviews. This review considers the following topics: (1) the phloem structure and the ultrastructure of specialized phloem cells, (2) the physiological functions of phloem and their regulation, (3) photosynthesis and phloem loading with assimilates, (4) phloem unloading and the related processes of plant growth and development, (5) the mechanisms of sugar and amino acid transport, (6) the levels of transport, (7) transport compartments; (8) xylem–phloem and symplast–apoplast communication; (9) phloem transport vs. the integral plant physiology, (10) transport of xenobiotics, and (11) the trophic transport networks in symbionts.     

Hexoses as phloem transport sugars: the end of a dogma?

According to most textbooks, only non-reducing carbohydrate species such as sucrose, sugar alcohols, and raffinose-family sugars function as phloem translocates. Occasional abundance of reducing sugar species (such as hexoses) in sieve-tube sap has been discarded as an experimental artefact. This study, however, discloses a widespread occurrence of hexoses in the sieve-tube sap. Phloem exudation facilitated by EDTA provided evidence that many of the members of two plant families (Ranunculaceae and Papaveraceae) investigated translocate >80% of carbohydrates in the form of hexoses. Representatives of other families also appear to translocate appreciable amounts of hexoses in the sieve tubes. Promoting effects of EDTA, activities of sucrose-degrading enzymes, and sugar uptake by micro-organisms on hexose contents of phloem exudates were checked. The rate of sucrose degradation is far too low to explain the large proportions of hexoses measured in phloem exudates; nor did other factors tested seem to stimulate the occurrence of hexoses. The validity of the approach is further supported by the virtual absence of hexoses in exudates from species that were known as exclusive sucrose transporters. This study urges a rethink of the existing views on carbohydrate transport species in the phloem stream. Hexose translocation is to be regarded as a normal mode of carbohydrate transfer by the phloem equivalent to that of sucrose, raffinose-family sugars, or sugar alcohols.     

Evidence on the mechanism of enhanced sucrose uptake at low cell turgor in leaf discs of Phaseolus coccinius

Plants often tolerate water deficits by lowering the osmotic potential of their cell sap. This may be achieved by accumulation of solutes which results in the maintenance of a positive turgor potential. In this study, the effect of water deficit on sugar uptake was investigated in leaf discs of Phaseolus coccinius L. (cv. Scarlet). Evidence is presented that cell turgor affects the kinetics of sugar transport at the membrane level. Uptake kinetics of sucrose, glucose and 3-O-methyl glucose by tissues equilibrated in solutions of relatively high (200–400 mOsm) osmotic concentration consisted of a sat-urable and a linear component. Low external osmotic concentration i.e., high cellular turgor inhibited the saturating component of sucrose uptake, resulting in a linear uptake profile. However, high cell turgor had no effect on glucose or 3-O-methyl glucose uptake kinetics. The effect of turgor versus osmotic component of water potential was differentiated by comparing responses to non-penetrating (manmtol) or polyethylene glycol, (3350) and penetrating (ethylene glycal) osmotica. Changes in sucrose uptake rates and kinetics were due to changes in cellular turgor and not osmotic potential. Furthermore, at low cellular turgor, a net increase in sucrose uptake occurred as a consequence of enhanced influx rates and not as a result of reduced efflux rates. The data are consistent with previous findings that sugar uptake rates are enhanced under low turgor. We present first evidence indicating that the mechanism by which higher rates of sucrose uptake are maintained underwater deficit conditions is by the activation of the saturable transport system. This mechanism supports previous suggestions that changes in cell turgor are sensed and manifested at the membrane level.     

Chlorophyll Fluorescence Quenching During Photosynthetic Induction in Leaves of Abutilon striatum Dicks. Infected with Abutilon Mosaic Virus,Observed with a Field-Portable Imaging System

A portable Chi fluorescence imaging system was used to characterise nonuniform Chi fluorescence quenching in Abutilon striatum leaves infected with phloem-localised abutilon mosaic virus. The instrument was used to observe fluorescence emission at intervals during induction transients, and to map nonphotochemical quenching during saturating pulses applied in the course of these transients. Two symptom types were distinguished: yellow vein-associated motifs that showed lower maximum Chi fluorescence than nearby green tissues, but virtually zero nonphotochemical quenching, and vein-defined mosaics (pale green) that initially showed normal maximum Chi fluorescence but strongly impaired nonphotochemical quenching. Mature vein-defined mosaics (yellow to white areas) resembled vein-associated symptoms with zero nonphotochemical quenching. Islands of apparently healthy green tissue enclosed by mosaic symptoms showed slower nonphotochemical quenching than controls. Possible effects of localised carbohydrate accumulation, thought to follow from infection by the phloem-limited virus, on photosynthetic processes as well as the synthesis and stability of chloroplast protein complexes, are discussed in the context of symptom development.     

Genetic variability associated with photosynthetic pigment concentration, and photochemical and nonphotochemical quenching, in strains of the cyanobacterium Microcystis aeruginosa

Although populations of cyanobacteria are usually considered to be clonal, their capacity to survive environmental changes suggests intrapopulation genetic variation. We therefore estimated the genetic variability on the basis of two processes important for any photoautotroph - photochemical and nonphotochemical quenching - as well as photosynthetic pigment concentrations. For this purpose, two parameters related to photochemical and nonphotochemical quenching were measured using specific experimental and statistical procedures, in 25 strains of the cyanobacterium Microcystis aeruginosa, along with their contents of chlorophyll a, total carotenoids and phycocyanin. The experimental procedure allowed discrimination between genetic and nongenetic (or residual) variability among strains. The high genetic variability found in photosynthetic pigments and both photosynthetic parameters denotes large differences even among strains isolated from the same community. The high genetic diversity within a population could be important for the evolutionary success of cyanobacteria.     

红肉火龙果糖转运蛋白基因HpSWEET7的表达和转运活性鉴定

【目的】克隆红肉火龙果糖转运蛋白SWEET家族HpSWEET7基因,探讨其在果实糖积累中的生理作用,为改良果实风味和品质提供靶基因。【方法】从‘紫红龙’克隆HpSWEET7基因,开展序列比对和系统进化分析,利用qRT-PCR检测基因表达,融合荧光蛋白检测亚细胞定位,利用酵母生长互补和酸化检测糖转运活性。【结果】HpSWEET7的开放读码阅读框长度为771 bp,编码256个氨基酸残基。HpSWEET7含有2个“MtN3/_saliva”（Pfam编号PF03083）和7个跨膜结构域。HpSWEET7与拟南芥AtSWEET6和AtSWEET7具有最近的亲缘关系,属于SWEET II类。HpSWEET7在茎的表达微弱;果实授粉后10 d向20 d发育,其表达升高并达到最高;随后下降并在成熟（授粉后30 d）时表达微弱 。HpSWEET7在茎的表达量显著低于果实6个发育时期;在授粉后20 d的果实表达量最高,随果实发育逐渐下降并在成熟果实（授粉后30 d）的表达量最低。亚细胞定位检测证明HpSWEET7位于细胞膜。HpSWEET7具有葡萄糖、果糖、甘露糖、山梨醇和半乳糖转运活性,酸化检测进一步证明其转运葡萄糖和果糖。【结论】HpSWEET7跨细胞膜转运己糖,在红肉火龙果果实发育期间参与糖积累。     

Soluble acid invertase activity in leaves is independent of species differences in leaf carbohydrates, diurnal sugar profiles and paths of phloem loading

Leaf sucrose, starch, hexose and maximum extractable soluble acid invertase activity were compared throughout the day in source leaves of 13 plant species chosen for their putative phloem-loading type (apoplastic or symplastic). Four species which represent the different phloem-loading types (tomato, barley, maize and Fuchsia ) were studied in detail. Using this information we wished to determine whether a positive correlation between foliar carbohydrates and acid invertase activity exists in leaves from different species and, furthermore, whether this relationship is determined by phloem-loading type. Acid invertase activity was relatively constant throughout the day in all species. The extent of sucrose, hexose and starch accumulation and the sucrose: starch ratio measured at a given time were species-dependent. No correlations were found between foliar soluble acid invertase activity and the hexose, sucrose or starch content of the leaves in any of the species, regardless of phloem-loading type. The species examined could be divided into three distinct groups: (1) high sucrose, low invertase; (2) low sucrose, low invertase; and (3) low sucrose, high invertase. The absence of an inverse relationship between leaf sucrose, hexose or starch contents and endogenous soluble acid invertase suggests that this enzyme is not directly involved in carbon partitioning in leaves but serves an auxiliary function.     

Action potential opens access for the charged cofactor to the chloroplasts of <Emphasis Type="Italic">Chara corallina</Emphasis> cells

Effects of inhibitors and cofactors of cyclic and noncyclic electron transport on nonphotochemical quenching of chlorophyll fluorescence induced by action potential (AP) was investigated in Chara corallina cells. Under control conditions, energy-dependent quenching (qE) develops upon the increase in photosynthetically active radiation (PAR); it also arises and reversibly disappears after AP generation at moderate irradiances. The treatment of cells with diuron (DCMU) completely eliminated qE established at high irradiances and qE induced by AP generation. The activation of cyclic electron transport by DCMU in combination with phenazine methosulfate restored qE at high irradiances but did not restore qE imposed after AP generation. The presence in the medium of a PSI acceptor, methyl viologen at concentrations from 100 μM to 0.83 mM had no effect on fluorescence and photosynthetic activity of chloroplasts until the application of a single excitatory stimulus. Once a single AP was generated in the presence of methyl viologen, it induced irreversible qE at a wide range of irradiances, which indicates the AP-triggered redirection of a part of electron flow from the main pathway to the artificial acceptor. It is concluded that AP generation opens access for permeation of a divalent cation methyl viologen from the medium to the chloroplast stroma across two membrane barriers, the plasmalemma and the inner membrane of the chloroplast envelope.     

Uptake of mannitol from the media by in vitro grown plants

Higher plants grown in vitro are very seldom fully autotrophic. Therefore, such cultures are usually supplied with exogenous sugars. However, at higher sugar concentration a decrease in dry matter accumulation is observed which can be explained by a decrease in osmotic potential of the medium.To test this explanation a series of experiments with mannitol, a sugar alcohol often used for simulation of osmotic stress, were performed with excised wheat embryos, rape seedlings and potato stem segments grown in vitro. As the presence of mannitol in the medium caused a significant decrease in dry matter accumulation, the content of mannitol in the shoot tissues was determined using HPLC analysis to estimate the uptake and transport of mannitol from roots to shoots. Mannitol contents up to 30% of dry weight in wheat and 20% in rape and potato shoots were found, indicating that mannitol is easily taken up by in vitro plants and transported to shoots. There were no large changes in the content of glucose, fructose and sucrose caused by the presence of mannitol in the tissues. These data show that mannitol can not be used as an inert osmoticum in in vitro studies.     

Photoprotection in a zeaxanthin- and lutein-deficient double mutant of Arabidopsis

When light absorption by a plant exceeds its capacity for light utilization, photosynthetic light harvesting is rapidly downregulated by photoprotective thermal dissipation, which is measured as nonphotochemical quenching of chlorophyll fluorescence (NPQ). To address the involvement of specific xanthophyll pigments in NPQ, we have analyzed mutants affecting xanthophyll metabolism in Arabidopsis thaliana. An npq1 lut2 double mutant was constructed, which lacks both zeaxanthin and lutein due to defects in the violaxanthin de-epoxidase and lycopene -cyclase genes. The npq1 lut2 strain had normal Photosystem II efficiency and nearly wild-type concentrations of functional Photosystem II reaction centers, but the rapidly reversible component of NPQ was completely inhibited. Despite the defects in xanthophyll composition and NPQ, the npq1 lut2 mutant exhibited a remarkable ability to tolerate high light.This revised version was published online in October 2005 with corrections to the Cover Date.     

Quenching of the fluorescence emitted by P695-682 at room temperature in etiolated illuminated leaves

Chlorophyll(ide) fluorescence emission decreases at room temperature during completion of protochlorophyll(ide) reduction. The process responsible for this quenching is parallel to the P_688-676 P_695-682 transition. It proceeds equally well in darkness and in the light. It consists in a decrease of the fluorescence yield of chlorophyll(ide) in P_695-682. Apparently, room temperature P_695-682 fluorescence is regulated by a conjunction of factors such as energy transfers and photobiochemical activities.Abbreviations NADP nicotinamide-adenine dinucleotide phosphate - CPI chlorophyll-protein-complex I - CPII chlorophyll-protein-complex II Aspirant du Fond National de la Recherche Scientifique, Belgium