The acyclic polyol sorbitol is a primary photosynthetic product and the principal photosynthetic transport substance in many economically important members of the family Rosaceace (e.g. almond [Prunus dulcis (P. Mill.) [Pyrus communis]). To understand key steps in long-distance transport and particularly partitioning and accumulation of sorbitol in sink tissues, we have cloned two sorbitol transporter genes (PcSOT1 and PcSOT2) from sour cherry (Prunus cerasus) fruit tissues that accumulate large quantities of sorbitol. Sorbitol uptake activities and other characteristics were measured by heterologous expression of PcSOT1 and PcSOT2 in yeast (Saccharomyces cerevisiae). Both genes encode proton-dependent, sorbitol-specific transporters with similar affinities (K m sorbitol of 0.81 mm for PcSOT1 and 0.64 mm for PcSOT2). Analyses of gene expression of these transporters, however, suggest different roles during leaf and fruit development. PcSOT1 is expressed throughout fruit development, but especially when growth and sorbitol accumulation rates are highest. In leaves, PcSOT1 expression is highest in young, expanding tissues, but substantially less in mature leaves. In contrast, PcSOT2 is mainly expressed only early in fruit development and not in leaves. Compositional analyses suggest that transport mediated by PcSOT1 and PcSOT2 plays a major role in sorbitol and dry matter accumulation in sour cherry fruits. Presence of these transporters and the high fruit sorbitol concentrations suggest that there is an apoplastic step during phloem unloading and accumulation in these sink tissues. Expression of PcSOT1 in young leaves before completion of the transition from sink to source is further evidence for a role in determining sink activity.Sorbitol (an acyclic polyol) and Suc are the primary photosynthetic products and the major phloemtranslocated components in a number of economically important taxa in the family Rosaceae, in particular in the subfamilies Pomoideae (e.g. (Loescher and Everard, 1996). Sorbitol is often the dominant translocated photosynthetic product. In mature apricot (Prunus armeniaca) leaves, for example, 65% to 75% of the translocated carbon is sorbitol (Bieleski and Redgwell, 1985). In these species, understanding the factors involved in facilitating and regulating sorbitol transport, including export from the leaf, long-distance distribution via the phloem network, and import into various sink tissues, is at least as important as those of Suc and other sugars.In the past 10 years, sugar transporters have been extensively studied in various sink and source tissues with the isolation of two distinct families of sugar carriers: the disaccharide transporters that primarily catalyze Suc transport and the monosaccharide transporters that mediate transport of the hexose sugars (for review, see Weise et al., 2000; Williams et al., 2000). Active uptake of Suc across the plasma membrane in all known cases involves an H ϩ -Suc symporter (Lemoine, 2000). The biochemical properties of the Suc transporters ...

SummaryRaf®nose and stachyose are ubiquitous galactosyl-sucrose oligosaccharides in the plant kingdom which play major roles, second only to sucrose, in photoassimilate translocation and seed carbohydrate storage. These sugars are initially metabolised by a-galactosidases (a-gal). We report the cloning and functional expression of the ®rst genes, CmAGA1 and CmAGA2, encoding for plant a-gals with alkaline pH optima from melon fruit (Cucumis melo L.), a raf®nose and stachyose translocating species. The alkaline a-gal genes show very high sequence homology with a family of unde®ned`seed imbibition proteins' (SIPs) which are present in a wide range of plant families. In order to con®rm the function of SIP proteins, a representative SIP gene, from tomato, was expressed and shown to have alkaline a-gal activity. Phylogenetic analysis based on amino acid sequences shows that the family of alkaline a-gals shares little homology with the known prokaryotic and eukaryotic a-gals of glycosyl hydrolase families 27 and 36, with the exception of two cross-family conserved sequences containing aspartates which probably function in the catalytic step. This previously uncharacterised, plant-speci®c a-gal family of glycosyl hydrolases, with optimal activity at neutral-alkaline pH likely functions in key processes of galactosyl-oligosaccharide metabolism, such as during seed germination and translocation of RFO photosynthate.

SummaryThe Populus shoot undergoes primary growth (longitudinal growth) followed by secondary growth (radial growth), which produces biomass that is an important source of energy worldwide. We adopted joint PacBio Iso-Seq and RNA-seq analysis to identify differentially expressed transcripts along a developmental gradient from the shoot apex to the fifth internode of Populus Nanlin895. We obtained 87 150 full-length transcripts, including 2081 new isoforms and 62 058 new alternatively spliced isoforms, most of which were produced by intron retention, that were used to update the Populus annotation. Among these novel isoforms, there are 1187 long non-coding RNAs and 356 fusion genes. Using this annotation, we found 15 838 differentially expressed transcripts along the shoot developmental gradient, of which 1216 were transcription factors (TFs). Only a few of these genes were reported previously. The differential expression of these TFs suggests that they may play important roles in primary and secondary growth. AP2, ARF, YABBY and GRF TFs are highly expressed in the apex, whereas NAC, bZIP, PLATZ and HSF TFs are likely to be important for secondary growth. Overall, our findings provide evidence that long-read sequencing can complement short-read sequencing for cataloguing and quantifying eukaryotic transcripts and increase our understanding of the vital and dynamic process of shoot development.