Description: This report describes the findings of a study conducted on the watersheds of "Lake Lewisville, Lake Ray Roberts, Lake Grapevine and the Elm Fork of the Trinity River between Lake Lewisville and Frazier Dam," which are all part of the upper Trinity drainage basin (p. 31). The study examines the potential benefit of "remote sensing and geographic information systems (GIS) for watershed management" in and around Dallas, Texas (p. i).

Description: This article discusses the design of highly specific cytotoxins by using trans-splicing ribozymes. Abstract: We have designed ribozymes based on a self-splicing group I intron that can trans-splice exon sequences into a chosen RNA target to create a functional chimeric mRNA and provide a highly specific trigger for gene expression. We have targeted ribozymes against the coat protein mRNA of a widespread plant pathogen, cucumber mosaic virus. The ribozymes were designed to trans-splice the coding sequence of the diphtheria toxin A chain in frame with the viral initiation codon of the target sequence. Diphtheria toxin A chain catalyzes the ADP ribosylation of elongation factor 2 and can cause the cessation of protein translation. In a Saccharomyces cerevisiae model system, ribozyme expression was shown to specifically inhibit the growth of cells expressing the virus mRNA. A point mutation at the target splice site alleviated this ribozyme-mediated toxicity. Increasing the extent of base pairing between the ribozyme and target dramatically increased specific expression of the cytotoxin and reduced illegitimate toxicity in vivo. Trans-splicing ribozymes may provide a new class of agents for engineering virus resistance and therapeutic cytotoxins.

Description: This article discusses symplastic continuity between companion cells and the translocation stream. Substantial symplastic continuity appears to exist between companion cells (CCs) and sieve elements of the phloem, which suggests that small solutes within the CC are subject to indiscriminate long-distance transport via the translocation stream. To test this hypothesis, the distributions of exotic and endogenous solutes synthesized in the CCs of minor veins were studied. Octopine, a charged molecule derived from arginine and pyruvate, was efficiently transported through the phloem but was also transferred in substantial amounts to the apoplast, and presumably other non-phloem compartments. The disaccharide galactinol also accumulated in non-phloem compartments, but long-distance transport was limited. Conversely, sucrose, raffinose, and especially stachyose demonstrated reduced accumulation and efficient transport out of the leaf. The authors conclude that small metabolites in the cytosol of CCs do enter the translocation stream indiscriminately but are also subject to distributive forces, such as nonselective and carrier-mediated membrane transport and symplastic dispersal, that may effectively clear a compound from the phloem or retain it for long-distance transport. A model is proposed in which the transport or oligosaccharides is an adaptive strategy to improve photoassimilate retention, and consequently translocation efficiency, the phloem.

Description: This article discusses genome-wide mutagenesis of Zea mays L. using RescueMu transposons. Abstract: Derived from the maize Mu I transposon, RescueMu provides strategies for maize gene discovery and mutant phenotypic analysis. 9.92 Mb of gene-enriched sequences next to RescueMu insertion sites were co-assembled with expressed sequence tags and analyzed. Multiple plasmid recoveries identified probable germinal insertions and screening of RescueMu plasmid libraries identified plants containing probable germinal insertions. Although frequently recovered parental insertions and insertion hotspots reduce the efficiency of gene discovery per plasmid, RescueMu targets a large variety of genes and produces knockout mutants.

Description: This article discusses graft transmission of a floral stimulant derived from CONSTANS. Abstract: Photoperiod in plants is perceived by leaves and in many species influences the transition to reproductive growth through long-distance signaling. CONSTANS (CO) is implicated as a mediator between photoperiod perception and the transition to flowering in Arabidopsis. To test the role of CO in long-distance signaling, CO was expressed from a promoter specific to the companion cells of the smallest veins of mature leaves. This expression in tissues at the inception of the phloem translocation stream was sufficient to accelerate flowering at the apical meristem under noninductive (short-day) conditions. Grafts that conjoined the vegetative stems of plants with different flower-timing phenotypes demonstrated that minor-vein expression of CO is able to substitute for photoperiod in generating a mobile flowering signal. The authors' results suggest that a CO-derived signal(s), or possibly CO itself, fits the definition of the hypothetical flowering stimulant, florigen.