Suppression of sucrose synthase affects auxin signaling and leaf morphology in tomato.

Metabolic enzymes have been found to play roles in plant development. Sucrose synthase (SUS) is one of the two enzyme families involved in sucrose cleavage in plants. In tomato, six SUS genes have been found. We generated transgenic tomato plants with RNAi suppression of SlSUS1, SlSUS3 and SlSUS4 genes. Independent transgenic lines with RNAi suppression of more than one SUS gene exhibited morphological effects on their cotyledons and leaf structure, but there were no significant effects on their carbohydrate levels, demonstrating that SUS has a developmental function, in addition to its metabolic function. Shoot apices of the transgenic lines showed elevated expression of JAGGED (JAG) and the auxin transporter PIN1. In a PIN1-GFP fusion reporter/SUS-RNAi hybrid, PIN1-GFP patterns were altered in developing leaves (as compared to control plants), indicating that SlSUS suppression alters auxin signaling. These results suggest possible roles for SUS in the regulation of plant growth and leaf morphology, in association with the auxin-signaling pathway.

Comparison of a novel tomato sucrose synthase, SlSUS4, with previously described SlSUS isoforms reveals distinct sequence features and differential expression patterns in association with stem maturation.

Sucrose synthase (SUS) plays a role in many contexts of sugar metabolism, including low-oxygen and low-ATP respiration and the synthesis of cellulose. In tomato (Solanum lycopersicum), as in many plants, SUS is encoded by genes at several independent loci. Here, we report the isolation of a novel tomato SUS (SlSUS) isoform, SlSUS4, that is homologous to potato SUS isoform 1 (StSUS1) and also shows greater homology to SUS isoforms of other plants than to the other tomato SUS isoforms. All three tomato isoforms are very similar in genomic structure and sequence, yet each is located on a separate chromosome. Real-time expression analysis of the three distinct isoforms revealed widely varying patterns of expression, in terms of both tissue specificity and overall magnitude of expression. Analysis of SlSUS expression along the tomato stem revealed opposing expression gradients for two of the SlSUS isoforms, in apparent correlation with vascular tissue maturation. Western-blot analysis of SlSUS protein showed an increasing SlSUS concentration gradient along the developmental axis of the tomato stem, with the protein concentrated mainly in the vascular tissue of the stem. These gene expression and protein accumulation patterns indicate that each isoform may play a discrete role in the development of tomato plants, most notably in the development of vascular tissue in the stem.

Differences in neuronal differentiation between the transient cranial (Frorieps') and normal dorsal root ganglia.

The Frorieps' ganglia are dorsal root ganglia (DRG) that form, grow slowly compared to normal DRG, and then degenerate during normal embryonic development of amniotes. Their fate has been shown to be regulated by the Hox family and other genes involved in pattern formation, and by the mesodermal microenvironment of the cranial somites in which they develop. Neurons are known to differentiate within the Frorieps' DRG in the course of their short life span. In this study, we compared several aspects of neural differentiation between the longest-lived Frorieps' ganglia, DRG2, and normal trunk DRG in chick embryos. The expression of transcription factor Islet-1 and the RNA-binding protein Hu differ between normal and Frorieps' DRG at St. 23 (embryonic day 4). Islet-1 is expressed in a higher proportion of cells in DRG2 than DRG5, suggesting that cells in DRG2 differentiate more rapidly into neurons than in normal DRG. This molecular difference appears at the same developmental stage as overt morphological differences between DRG2 and normal DRG. Other LIM-homeobox proteins (Lim-1, -2 and -3 and Islet-2) are not expressed either in normal or Frorieps' DRG at early stages of development. Somite-grafting experiments reveal that the increased proportion of Isl-1(+) in DRG2 is due to the special microenvironment of the cranial somites. In contrast to Islet-1, the Hu antigen is expressed in a slightly lower proportion of cells in DRG2 at St. 23. At St. 28, there is a significant population of neurons in DRG2 that are Isl-1(+)/Hu(-). Since Islet-1 is normally expressed before Hu in DRG cells, it is possible that the reduced growth rate of the DRG2 ganglion may partially result from the inability of precociously differentiating Islet-1(+) neurons to further mature, as reflected by Hu antigen expression. In contrast to the neuronal markers examined, the satellite cell marker 7B3 is expressed at similar levels in DRG2 and DRG5.