No abstract

Evidence is presented for the association of a phytoplasma, provisionally named sugarcane yellows phytoplasma (ScYP), in sugarcane affected by a yellow leaf syndrome. The phytoplasma was consistently detected in leaves of more than 40 varieties from eight African countries. It was present in all symptomatic as well as some asymptomatic field grown cane samples but not in plants grown from true seed, and it was also observed in phloem sieve tubes by transmission electron microscopy. Phytoplasma 16s rDNA was confirmed by PCR, and restriction fragment analysis using RsaI and HaeIII confirmed that PCR-amplified products were of phytoplasma rather than of plant or of other pathogen origin. Sequences obtained from the intergenic spacer region, between the 16s and 23s rDNA genes, confirmed the identity of the phytoplasma as belonging to the western X group of phytoplasmas.

Ascorbate (AsA) plays a fundamental role in redox homeostasis in plants and animals, primarily by scavenging reactive oxygen species. Three genes, representing diverse steps putatively involved in plant AsA biosynthesis pathways, were cloned and independently expressed in Solanum lycopersicum (tomato) under the control of the CaMV 35S promoter. Yeast-derived GDP-mannose pyrophosphorylase (GMPase) and arabinono-1,4-lactone oxidase (ALO), as well as myo-inositol oxygenase 2 (MIOX2) from Arabidopsis thaliana, were targeted. Increases in GMPase activity were concomitant with increased AsA levels of up to 70% in leaves, 50% in green fruit, and 35% in red fruit. Expression of ALO significantly pulled biosynthetic flux towards AsA in leaves and green fruit by up to 54 and 25%, respectively. Changes in AsA content in plants transcribing the MIOX2 gene were inconsistent in different tissue. On the other hand, MIOX activity was strongly correlated with cell wall uronic acid levels, suggesting that MIOX may be a useful tool for the manipulation of cell wall composition. In conclusion, the Smirnoff-Wheeler pathway showed great promise as a target for biotechnological manipulation of ascorbate levels in tomato.

No abstract

Pyrophosphate: fructose 6-phosphate 1-phosphotransferase (PFP) is a cytosolic enzyme catalyzing the first committed step in glycolysis by reversibly phosphorylating fructose-6-phosphate to fructose-1,6-bisphosphate. The position of PFP in glycolytic and gluconeogenic metabolism, as well as activity patterns in ripening strawberry, suggest that the enzyme may influence carbohydrate allocation to sugars and organic acids. Fructose-2,6-bisphosphate activates and tightly regulates PFP activity in plants and has hampered attempts to increase PFP activity through overexpression. Heterologous expression of a homodimeric isoform from Giardia lamblia, not regulated by fructose-2,6-bisphosphate, was therefore employed to ensure in vivo increases in PFP activity. The coding sequence was placed into a constitutive expression cassette under control of the cauliflower mosaic virus 35S promoter and introduced into strawberry by Agrobacterium tumefaciens-mediated transformation. Heterologous expression of PFP resulted in an up to eightfold increase in total activity in ripe berries collected over two consecutive growing seasons. Total sugar and organic acid content of transgenic berries harvested during the first season were not affected when compared to the wild type, however, fructose content increased at the expense of sucrose. In the second season, total sugar content and composition remained unchanged while the citrate content increased slightly. Considering that PFP catalyses a reversible reaction, PFP activity appears to shift between gluconeogenic and glycolytic metabolism, depending on the metabolic status of the cell.