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Carbohydrates, mainly sucrose, that are synthesized in source organs are transported to sink organs to support growth and development. Phloem loading of sucrose is a crucial step that drives long-distance transport by elevating hydrostatic pressure in the phloem. Three phloem loading strategies have been identified, two active mechanisms, apoplastic loading via sucrose transporters and symplastic polymer trapping, and one passive mechanism. The first two active loading mechanisms require metabolic energy, carbohydrate is loaded into the phloem against a concentration gradient. The passive process, diffusion, involves equilibration of sucrose and other metabolites between cells through plasmodesmata. Many higher plant species including Arabidopsis utilize the active loading mechanisms to increase carbohydrate in the phloem to higher concentrations than that in mesophyll cells. In contrast, recent data revealed that a large number of plants, especially woody species, load sucrose passively by maintaining a high concentration in mesophyll cells. However, it still remains to be determined how the worldwide important cereal crop, rice, loads sucrose into the phloem in source organs. Based on the literature and our results, we propose a potential strategy of phloem loading in rice. Elucidation of the phloem loading mechanism should improve our understanding of rice development and facilitate its manipulation towards the increase of crop productivity.

The gibberellin class of plant hormones has been implicated in the control of flowering in several species. In Arabidopsis, severe reduction of endogenous gibberellins delays flowering in long days and prevents flowering in short days. We have investigated how the differential effects of gibberellins on flowering correlate with expression of LEAFY, a floral meristem identity gene. We have found that the failure of gibberellin-deficient ga1-3 mutants to flower in short days was paralleled by the absence of LEAFY promoter induction. A causal connection between these two events was confirmed by the ability of a constitutively expressed LEAFY transgene to restore flowering to ga1-3 mutants in short days. In contrast to short days, impairment of gibberellin biosynthesis caused merely a reduction of LEAFY expression when plants were grown in long days or with sucrose in the dark. As a first step toward identifying other small molecules that might regulate flowering, we have developed a rapid in vitro assay for LEAFY promoter activity

During oil deposition in developing seeds of Arabidopsis, photosynthate is imported in the form of carbohydrates into the embryo and converted to triacylglycerols. To identify genes essential for this process and to investigate the molecular basis for the developmental regulation of oil accumulation, mutants producing wrinkled, incompletely filled seeds were isolated. A novel mutant locus, wrinkled1 (wri1), which maps to the bottom of chromosome 3 and causes an 80% reduction in seed oil content, was identified. Wild-type and homozygous wri1 mutant plantlets or mature plants were indistinguishable. However, developing homozygous wri1 seeds were impaired in the incorporation of sucrose and glucose into triacylglycerols, but incorporated pyruvate and acetate at an increased rate. Because the activities of several glycolytic enzymes, in particular hexokinase and pyrophosphate-dependent phosphofructokinase, are reduced in developing homozygous wri1 seeds, it is suggested that WRI1 is involved in the developmental regulation of carbohydrate metabolism during seed filling

The effects of plant hormones and sucrose (Suc) on potato (Solanum tuberosum L.) tuberization were studied using in vitro cultured single-node cuttings. Tuber-inducing (high Suc) and-noninducing (low Suc or high Suc plus gibberellin [GA]) media were tested. Tuberization frequencies, tuber widths, and stolon lengths were measured during successive stages of development. Endogenous GAs and abscisic acid (ABA) were identified and quantified by high-performance liquid chromatography and gas chromatography-mass spectrometry. Exogenous GA4/7 promoted stolon elongation and inhibited tuber formation, whereas exogenous ABA stimulated tuberization and reduced stolon length. Indoleacetic acid-containing media severely inhibited elongation of stolons and smaller sessile tubers were formed. Exogenous cytokinins did not affect stolon elongation and tuber formation. Endogenous GA1 level was high during stolon elongation and decreased when stolon tips started to swell under inducing conditions, whereas it remained high under noninducing conditions. GA1 levels were negatively correlated with Suc concentration in the medium. We conclude that GA1 is likely to be the active GA during tuber formation. Endogenous ABA levels decreased during stolon and tuber development, and ABA levels were similar under inducing and noninducing conditions. Our results indicate that GA is a dominant regulator in tuber formation: ABA stimulates tuberization by counteracting GA, and Suc regulates tuber formation by influencing GA levels

Many lines of evidence indicate that the increased activity of enzymes involved in phenolic compound biosynthesis and the subsequent accumulation of these compounds correlate with plant resistance to abiotic and biotic stresses. The production of anthocyanin pigment gene (PAP1) was previously identified using an activation tagging approach in Arabidopsis thaliana. A. thaliana PAP1-Dominant (pap1-D) mutant was generated by activation tagging. Throughout development, the pap1-D plant exhibited massive and widespread activation of genes encoding enzymes involved in phenylpropanoid natural product biosynthesis. Due to increased anthocyanin levels, pap1-D showed enhanced tolerance to salt, an important abiotic stress imposed on plants. In pap1-D plants, PAP1 transcript accumulation increased significantly in response to salt stress and abscisic acid. Enhanced anthocyanin levels induced by sucrose treatment increased the chance of survival of pap1-D and wild-type plants exposed to a high salt medium. Taken together, results of the present study indicate that genetic engineering of the flavonoid biosynthetic pathway can be used for successful improvement of plant survival in arid areas.

Maize (Zea mays L.) plants were grown to the nine-leaf stage. Despite a saturating N supply, the youngest mature leaves (seventh position on the stem) contained little NO3- reserve. Droughted plants (deprived of nutrient solution) showed changes in foliar enzyme activities, mRNA accumulation, photosynthesis, and carbohydrate and amino acid contents. Total leaf water potential and CO2 assimilation rates, measured 3 h into the photoperiod, decreased 3 d after the onset of drought. Starch, glucose, fructose, and amino acids, but not sucrose (Suc), accumulated in the leaves of droughted plants. Maximal extractable phosphoenolpyruvate carboxylase activities increased slightly during water deficit, whereas the sensitivity of this enzyme to the inhibitor malate decreased. Maximal extractable Suc phosphate synthase activities decreased as a result of water stress, and there was an increase in the sensitivity to the inhibitor orthophosphate. A correlation between maximal extractable foliar nitrate reductase (NR) activity and the rate of CO2 assimilation was observed. The NR activation state and maximal extractable NR activity declined rapidly in response to drought. Photosynthesis and NR activity recovered rapidly when nutrient solution was restored at this point. The decrease in maximal extractable NR activity was accompanied by a decrease in NR transcripts, whereas Suc phosphate synthase and phosphoenolpyruvate carboxylase mRNAs were much less affected. The coordination of N and C metabolism is retained during drought conditions via modulation of the activities of Suc phosphate synthase and NR commensurate with the prevailing rate of photosynthesis

Separation of bacterial cells from soil is a key step in the construction of metagenomic BAC libraries with large DNA inserts. Our results showed that when combined with sodium pyro-phosphate and homogenization for soil dispersion, sucrose density gradient centrifugation (SDGC) was more effective at separating bacteria from soil than was low speed centrifugation (LSC). More than 70% of the cells, along with some soil colloids, were recovered with one round of centrifugation. A solution of 0.8% NaCl was used to resuspend these cell and soil pellets for purification with nycodenz density gradient centrifugation (NDGC). After purification, more than 30% of the bacterial cells in the primary soil were extracted. This procedure effectively removed soil contamination and yielded sufficient cells for high molecular weight (HMW) DNA isolation. Ribosomal intergenic spacer analysis (RISA) showed that the microbial community structure of the extracted cells was similar to that of the primary soil, suggesting that this extraction procedure did not significantly change the the soil bacteria community structure. HMW DNA was isolated from bacterial cells extracted from red soil for metagenomic BAC library construction. This library contained DNA inserts of more than 200 Mb with an average size of 75 kb.

The fruit of apricot has been used as food in FYR Macedonia since a long time ago. The results based on fruit physical and chemical analyses clearly showed that different apricot genotypes have very important contents of soluble solids, individual sugars, and titratable acidity. In addition, the contents of these chemical compounds in some genotypes were higher than those in the control cultivar Hungarian Best. Using the PC analysis (PC1 = 32.13%, PC2 = 22.86%, and PC3 = 18.32%), apricot genotypes were separated into groups with similar physical and chemical attributes. These relationships may help to select a set of genotypes with better fruit quality performances which, in our study, might be indicated as DL-1/1/04, DL-1/2/03, D-1/04 and K-5/04.

We evaluated the influence of different types of pectin on colour and textural properties of strawberry jams and low-calorie strawberry jams containing fructose and aspartame or fructose syrup and aspartame. The highest anthocyanin and total phenol concentrations were detected in strawberry jam samples prepared with low methoxy amidated pectin. After 4 and 6 weeks of storage at two storage temperatures, room temperature and 4 deg C, anthocyanin and total phenol concentrations decreased. Free radical scavenging activity also decreased during storage. The highest values of texture characteristics, namely firmness, consistency and cohesiveness, were found in strawberry jam samples prepared with high methoxyl pectin.

In many species translocation of sucrose from the mesophyll to the phloem is carrier mediated. A sucrose/H+-symporter cDNA, NtSUT1, was isolated from tobacco (Nicotiana tabacum) and shown to be highly expressed in mature leaves and at low levels in other tissues, including floral organs. To study the in vivo function of NtSUT1, tobacco plants were transformed with a SUT1 antisense construct under control of the cauliflower mosaic virus 355 promoter. Upon maturation, leaves of transformants expressing reduced amounts of SUT1 mRNA curled downward, and strongly affected plants developed chloroses and necroses that led to death. The leaves exhibited impaired ability to export recently fixed 14CO2 and were unable to export transient starch during extended periods of darkness. As a consequence, soluble carbohydrates accumulated and photosynthesis was reduced. Autoradiographs of leaves show a heterogenous pattern of CO2 fixation even after a 24-h chase. The 14C pattern does not change with time, suggesting that movement of photosynthate between mesophyll cells may also be impaired. The affected lines show a reduction in the development of the root system and delayed or impaired flowering. Taken together, the effects observed in a seed plant (tobacco) demonstrate the importance of SUT1 for sucrose loading into the phloem via an apoplastic route and possibly for intermesophyll transport as well