John P.H. Reade

A role for glutathione S-transferases in resistance to herbicides in grasses

Abstract Herbicide resistance is the heritable ability of a weed biotype or population to survive a herbicide application that would effectively kill a susceptible population of the weed. In the U.K. the most widespread and financially important herbicide-resistant weed is blackgrass. Investigations to elucidate the molecular mechanisms conferring herbicide resistance to blackgrass populations have been ongoing for two decades. Although the identification of target site–resistant populations has proved to be relatively straightforward (using, for example, target site assays in vitro), the study and understanding of resistance mechanisms involved in enhanced metabolism has proven to be more problematic. Research has focused on the cytochrome P450 monooxygenase and glutathione S-transferase (GST) enzyme families, both of which have been shown to be important in herbicide metabolism in many weed and crop species. GST activity and abundance are greater in a selection of herbicide-resistant blackgrass biotypes, and herbicide treatment of field populations of blackgrass results in the survival of the proportion of population possessing the greatest GST activity and abundance. In addition, GST activity in the field increases between winter and spring, and this coincides with reduced efficacy of important blackgrass herbicides. GST activities within field populations of blackgrass are highly varied, and this plasticity is discussed in relation to the development of resistant populations in field situations. This article describes research results in blackgrass and compares them with GST studies in other weed species as well as with other mechanisms for enhanced metabolism-based resistance. Nomenclature: Blackgrass, Alopecurus myosuroides Huds. ALOMY.

Purification, characterization and comparison of glutathione S-transferases from black-grass (Alopecurus myosuroides Huds) biotypes

In the UK biotypes of black-grass (Alopecurus myosuroides Huds) showing resistance to both chlorotoluron (CTU) and aryloxyphenoxypropionate graminicides are increasingly being observed. Although the precise mechanisms involved in this resistance have yet to be identified, increased herbicide metabolism has been implicated as being involved in at least some cases of resistance. Glutathione S-transferases (GSTs) are a group of enzymes which have been demonstrated to metabolise herbicides in some plants, and the resistant black-grass biotype Peldon contains approximately double the GST activity towards 1-chloro-2,4-dinitrobenzene (CDNB) of susceptible biotypes. To investigate further the possible role of GSTs in herbicide resistance in black-grass, a purification procedure has been developed for these enzymes. A 27.5 kDa polypeptide possessing GST activity was purified from the susceptible biotype Herbiseed. Purification of GSTs from the resistant biotype Peldon also identified this polypeptide along with an additional 30 kDa polypeptide. An in-vitro kinetic study of both crude and purified GST extracts, and western blot analysis using antisera raised against the 27.5 kDa polypeptide, suggest that the 30 kDa polypeptide may possess GST activity, and is not a precursor of the 27.5 kDa polypeptide. These results are discussed and compared to GST profiles for other weeds and crops demonstrating herbicide resistance or tolerance.

High- but not low-intensity light leads to oxidative stress and quality loss of cold-stored baby leaf spinach

BACKGROUND Quality management in the fresh produce industry is an important issue. Spinach is exposed to various adverse conditions (temperature, light, etc.) within the supply chain. The present experiments were conducted to investigate the effect of light conditions (dark, low-intensity light (LL) and high-intensity light (HL)) and photoperiod (6 h HL and 18 h dark) on the quality changes of cold-stored spinach. RESULTS HL exposure resulted in oxidative stress, causing tissue damage and quality loss as evidenced by increased membrane damage and water loss. The content of total ascorbic acid was reduced under HL conditions. On the other hand, storage of spinach under LL conditions gave promising results, as nutritional quality was not reduced, while texture maintenance was improved. No significant differences, with the exception of nutritional quality, were found between spinach leaves stored under continuous (24 h) low-intensity light (30-35 µmol m(-2) s(-1)) and their counterparts stored under the same light integral over 6 h (130-140 µmol m(-2) s(-1)). CONCLUSION LL extended the shelf-life of spinach. The amount of light received by the leaves was the key factor affecting produce quality. Light intensity, however, has to be low enough not to cause excess oxidative stress and lead to accelerated senescence.

Moderate water stress prevents the postharvest decline of ascorbic acid in spinach (Spinacia oleracea L.) but not in spinach beet (Beta vulgaris L.)

BACKGROUND Babyleaf salads such as spinach (Spinacia oleracea L.) and spinach beet (Beta vulgaris L. subsp. cicla var. cicla) are an important dietary source of antioxidants such as ascorbic acid (vitamin C). Such compounds may be important in disease prevention in consumers but the level of these compounds in leaves frequently declines after harvest. As such, methods to maintain antioxidant levels in fresh produce are being sought. RESULTS Irrigation deficits were used to apply water stress to S. oleracea and B. vulgaris plants. This treatment prevented postharvest decline of leaf ascorbic acid content in S. oleracea but not in B. vulgaris. Ascorbic acid levels in leaves at harvest were unaffected by the treatment in both species compared to well-watered controls. CONCLUSION We have shown that restricted irrigation provides a viable means to maintain leaf vitamin content after harvest in S. oleracea, an important finding for producers, retailers and consumers alike.