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Jeanine M. Davis and Wayne H. Loescher

Diurnal fluctuations in soluble carbohydrates and starch were monitored in young (expanding), mature (first fully expanded), and old (nearing senescence) celery (Apium graveolens L.) leaves. In all tissues, mannitol and sucrose were the carbohydrates present in the highest concentrations. In old and young leaflets and their petioles, there was little change in levels of mannitol and sucrose in 26 hours. In mature leaflets, sucrose accumulated in the light and decreased in the dark; mannitol increased slightly in late afternoon. Starch concentration, although quite low, showed definite diurnal fluctuations in mature leaflets, but only small changes in young and old leaflets. Both sucrose and mannitol were present in mature petiole phloem tissues. Mannitol concentrations were high in the adjacent storage parenchyma tissue, but sucrose was almost undetectable. These data support earlier findings that sucrose is produced, translocated, and metabolized throughout the celery plant. Mannitol is also translocated, but also serves as a major storage carbohydrate in leaf tissues, especially petiole parenchyma. Starch serves as a minor short-term storage compound in leaflets.

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Guo-qing Song, Aaron E. Walworth, and Wayne H. Loescher

Grafting is a well-established agricultural practice, and it now has implications for the commercialization of transgenic plants. In transgrafted plants, only one part (scion or rootstock) is transgenic with the other part untransformed. However, transgenes may affect both mobile and immobile endogenous metabolites (e.g., RNAs, proteins, and phytohormones) and mobility has implications for transgrafting. In the phloem, long-distance transport of mobile metabolites can play important roles in plant development and signaling. In a transgrafted plant, an immobile transgene product (ITP) is not likely to be translocated across the graft union. In contrast, mobile transgene products (MTP) may be translocated across the graft. Regardless of the mobility of transgene products (TP), interaction of transgenic and nontransgenic parts in transgrafted plants through either the MTP or ITP has been demonstrated to be effective in facilitating changes in nontransgenic portions of the plant. Consequently, and particularly in fruit crops, transgrafting provides the potential for improving products from their nontransgenic parts with the possibility of minimizing the controversy over transgenic crops. This review focuses mainly on the mobility of TP and effects on the whole transgrafted plant.

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Wayne H. Loescher, Thaddeus McCamant, and John D. Keller

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Wayne H. Loescher, Paolo Sabbatini, Guo-Qing Song, Kenneth Sink, and James Flore

Mannitol, a sugar alcohol that appears to serve as an osmoprotectant/compatible solute to cope with salt stress, is synthesized in celery (Apium graveolens L.) via the action of a NADPH dependent mannose-6-phosphate reductase (M6PR). To evaluate the abiotic stress effects of mannitol biosynthesis, we transformed celery with an antisense construct of the celery leaf M6PR gene under control of the CaMV 35S promoter. Unlike wild type (WT) celery, independent antisense M6PR transformants did not accumulate significant amounts of mannitol in any tissue, with or without salt stress. In the absence of NaCl, and despite the lack of any significant accumulation of mannitol that is normally the major photosynthetic product, antisense transformants were mostly phenotypically similar to the WT celery. However, in the presence of NaCl, mature antisense transgenic plants were significantly less salt-tolerant, with reduced growth and photosynthetic rates, and some transformant lines were killed at 200 mM NaCl, a concentration that WT celery can normally withstand. Although mannitol biosynthesis is normally enhanced in salt-treated WT celery, no such increase was observed in the antisense transformants. Like our previous gain of function results showing enhanced salt tolerance in Arabidopsis plants transgenic for a sense M6PR construct, these loss of function results, using an antisense construct in celery, demonstrate a major role for mannitol biosynthesis in developing salt-tolerant plants.

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Sang-Dong Yoo, Zhifang Gao, Claudio Cantini, Wayne H. Loescher, and Steven van Nocker

A preliminary understanding of developmental processes among divergent species is essential to evaluate the applicability of information from model species to plants of agricultural importance. In tomato (Lycopersicon esculentum Mill.), where the molecular biology associated with fruit ripening has been studied most extensively, tissue softening is due at least in part to the activity of proteins called expansins, in concert with enzymatic activities that modify the pectin and xyloglucan components of the cell wall. We evaluated the potential for the concerted action of expansins and other cell wall-modifying enzymes during ripening in a highly divergent fruit species, sour cherry (Prunus cerasus L.). We identified a family of four expansin genes that was strongly upregulated at the advent of ripening. Activation of these genes was accompanied by strong upregulation of gene(s) encoding potential pectin methylesterases, pectate lyase(s), and xyloglucan endotransglycosylase(s). Initiation of ripening and gene induction were also associated with a rapid decrease in cell wall weight. These results suggest that expansin and several other distinct activities could be involved in ripening-associated cell wall modification in cherries.

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Gerardine Mukeshimana, Amy L. Lasley, Wayne H. Loescher, and James D. Kelly

Drought is an important abiotic stress that limits common bean (Phaseolus vulgaris) productivity. The objective of this study was to determine shoot traits that are associated with drought tolerance in common bean seedlings. Ten common bean genotypes consisting mainly of cultivars and breeding lines from the Mesoamerican race of the Middle American gene pool were first evaluated in the greenhouse. Genotypes were grown in a shallow soil profile to limit root growth and assess shoot phenotypes under stress. Water stress was imposed by withholding watering for 24 days after planting. Traits evaluated included wilting, unifoliate senescence, stem greenness, and recovery from drought. Biomass and number of pods/plant produced after drought recovery were evaluated to quantify the effect of early drought stress on bean growth and reproduction. A second group of 94 common bean genotypes from the Bean Coordinated Agricultural Project (BeanCAP) were evaluated using the same protocol to determine the genetic variability for the same traits in a wider range of genotypes. In general, genotypes known to possess drought avoidance in the field conferred by deep rooting traits performed poorly in these conditions suggesting that the assay could be used to identify seedling shoot traits that contribute to drought tolerance. Genotypes from race Mesoamerica showed the greatest range in wilting. Genotypes that showed a slow rate of wilting maintained a green stem and had a higher recovery rate after watering. Importantly, these genotypes demonstrated a smaller reduction in biomass and pod number under stress compared with non-stress treatments. A few genotypes recovered completely despite expressing severe wilting, whereas the majority of genotypes with high wilting rates did not recover. Among the BeanCAP materials, genotypes bred in the rainfed midwestern United States showed overall better recovery than those bred under the irrigated production system used in the western United States. Because recovery from drought is a prerequisite to plant regrowth, biomass, and pod production after drought stress, factors that contribute to recovery were studied. Stem greenness was highly positively correlated to the recovery, whereas wilting was negatively correlated to the recovery. In a regression analysis, stem greenness and slow wilting were found to be important contributors to the variability of recovery. In addition, photosynthetic rate and stomatal conductance (g S) explained variation in wilting and stem greenness. These results suggest that wilting and stem greenness might be useful traits to screen for drought tolerance in seedlings of common bean.