In recent years, there has been increasing interest in how changes in agricultural practice associated with the introduction of particular genetically modified (GM) crops might indirectly impact the environment. There is also interest in any effects that might be associated with recombinant and novel combinations of DNA passing into the environment, and the possibility that they may be taken up by microorganisms or other live biological material. From the current state of knowledge, the impact of free DNA of transgenic origin is likely to be negligible compared with the large amount of total free DNA. We can find no compelling scientific arguments to demonstrate that GM crops are innately different from non-GM crops. The kinds of potential impacts of GM crops fall into classes familiar from the cultivation of non-GM crops (e.g., invasiveness, weediness, toxicity, or biodiversity). It is likely, however, that the novelty of some of the products of GM crop improvement will present new challenges and perhaps opportunities to manage particular crops in creative ways.

Antifungal saponins occur in many plant species and may provide a preformed chemical barrier to attack by phytopathogenic fungi. Some fungal pathogens can enzymatically detoxify host plant saponins, which suggests that saponin detoxification may determine the host range of these fungi. A gene encoding a saponin detoxifying enzyme was cloned from the cereal-infecting fungus Gaeumannomyces graminis. Fungal mutants generated by targeted gene disruption were no longer able to infect the saponin-containing host oats but retained full pathogenicity to wheat (which does not contain saponins). Thus, the ability of a phytopathogenic fungus to detoxify a plant saponin can determine its host range.

Summary The aim of this work was to investigate the conditions required for amylose synthesis in starch granules. Although the major granule‐bound isoform of starch synthase ‐ GBSSI ‐ catalyses the synthesis of amylose in vivo, 14C from ADP[14C]glucose was incorporated primarily into a specific subset of amylopectin chains when supplied to starch granules isolated from pea (Pisum sativum L.) embryos and potato (Solanum tuberosum L.) tubers. Incubation of granules with soluble extracts of these organs revealed that the extracts contained compounds that increased the incorporation of 14C into amylose. These compounds were rendered inactive by treatment of the extracts with α‐glucosidase, suggesting that they were malto‐oligosaccharides. Consistent with this idea, provision of pure malto‐oligosaccharides to isolated granules resulted in a dramatic shift in the pattern of incorporation of 14C, from amylopectin chains to amylose molecules. Comparison of the pattern of incorporation in granules from wild‐type peas and lam mutant peas which lack GBSSI showed that this effect of malto‐oligosaccharides was specifically on GBSSI. The significance of these results for understanding of the synthesis of amylose and amylopectin in storage organs is discussed.

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Two proteases (PRT1 and PRT2) were fractionated from culture supernatants of wild-type Xanthomonas campestris pv. campestris by cation-exchange chromatography on SP-5PW. Inhibitor experiments showed that PRT 1 was a serine protease which required calcium ions for activity or stability or both and that PRT 2 was a zinc-requiring metalloprotease. PRT 1 and PRT 2 showed different patterns of degradation of beta-casein. The two proteases comprised almost all of the extracellular proteolytic activity of the wild type. A protease-deficient mutant which lacked both PRT 1 and PRT 2 showed considerable loss of virulence in pathogenicity tests when bacteria were introduced into mature turnip leaves through cut vein endings. This suggests that PRT 1 and PRT 2 have a role in black rot pathogenesis.

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Mutations that reduced the rate of starch synthesis in pea (Pisum sativum L.) embryos through effects on enzymes on the pathway from sucrose to adenosine 5'-diphosphoglucose (ADPglucose) also led to a reduction in the amylose content of the starch of developing embryos. Evidence is presented that this relationship between rate of synthesis and the composition of starch is due to the fact that amylopectin-synthesising isoforms of starch synthase have higher affinities for ADPglucose than the amylose-synthesising isoform. First, developing mutant embryos (rb, rug3 and rug4 mutants) displayed both reduced amylose contents in their starches and reduced ADPglucose contents relative to wild-type embryos. Second, incubation of detached, wild-type embryos for 6 h at high and low glucose concentrations resulted in differences in both ADPglucose content and the relative rates of amylose and amylopectin synthesis. At 0.25 M glucose both ADPglucose content and the proportion of synthesised starch that was amylose were about twice as great as at 25 &mgr;M glucose. Third, S(0.5) values for soluble (amylopectin-synthesising) starch synthases in developing embryos were several-fold lower than that for granule-bound (amylose synthesising) starch synthase. Estimates of the expected amylose contents of the starch of the mutant embryos, based on the reduction in their ADPglucose contents and on the S(0.5) values of the starch synthases, were very similar to the measured amylose contents. The implications of these results for the determination of starch composition are discussed.