Multiple effects of a naturally occurring proline to threonine substitution within acetolactate synthase in two herbicide-resistant populations of Lactuca serriola

Two populations of Lactuca serriola L. with resistance to acetolactate synthase (ALS)-inhibiting herbicides were discovered in wheat fields at two locations more than 25 km apart in South Australia. Both resistant populations carried a single base change within a highly conserved coding region of the ALS gene that coded for a single amino acid modification within ALS. The modification of proline 197 to threonine resulted in an enzyme that was highly resistant (>200-fold) to inhibition by sulfonylurea herbicides and moderately resistant to triazolopyrimidine and imidazolinone herbicides. The herbicide-resistant ALS was also less sensitive to inhibition by the branched-chain amino acids valine and leucine. In addition, the resistant enzyme had a lower K_m for pyruvate. However, extractable ALS activity was similar between resistant and susceptible plants. The substitution of threonine for proline 197 within ALS has multiple impacts on ALS enzyme activity in L. serriola that may influence the frequency of this resistant allele in the environment.     

Managing the risk of glyphosate resistance in Australian glyphosate- resistant cotton production systems

BACKGROUND: Glyphosate-resistant cotton varieties are an important tool for weed control in Australian cotton production systems. To increase the sustainability of this technology and to minimise the likelihood of resistance evolving through its use, weed scientists, together with herbicide regulators, industry representatives and the technology owners, have developed a framework that guides the use of the technology. Central to this framework is a crop management plan (CMP) and grower accreditation course. A simulation model that takes into account the characteristics of the weed species, initial gene frequencies and any associated fitness penalties was developed to ensure that the CMP was sufficiently robust to minimise resistance risks. RESULTS: The simulations showed that, when a combination of weed control options was employed in addition to glyphosate, resistance did not evolve over the 30 year period of the simulation. CONCLUSION: These simulations underline the importance of maintaining an integrated system for weed management to prevent the evolution of glyphosate resistance, prolonging the use of glyphosate-resistant cotton. Copyright (c) 2007 Society of Chemical Industry.     

Antagonistic Interactions Between Strains of Xanthomonas oryzae pv. oryzae

ABSTRACT The ability of some phytopathogenic bacterial strains to inhibit the growth of others in mixed infections has been well documented. Here we report that such antagonistic interactions occur between several wild-type strains of the rice bacterial blight pathogen Xanthomonas oryzae pv. oryzae. In mixed inoculations, a wild-type Philippine strain was found to inhibit the growth of a wild-type Korean strain. Furthermore, a nonpathogenic mutant of the Philippine strain maintained these antagonistic properties. Growth curve analysis indicated that both the wild-type Philippine strain and its nonpathogenic mutant inhibited the growth of the Korean strain 2 days after infection and prior to the onset of disease symptoms. When mixed with the nonpathogenic mutant, 10 out of 18 diverse wild-type X. oryzae pv. oryzae strains did not cause disease. Conversely, three of the strains that were not affected by the nonpathogenic mutant were found to inhibit the growth of both the wild-type and mutant Philippine strains, indicating that antagonism is widespread and strain specific. The observed growth inhibition occurred only in planta and did not correlate with bacteriocin activity in vitro. Antagonistic interactions also were found to affect resistance (R) gene-mediated resistance. The R gene Xa21 was capable of protecting rice plants coinoculated with nonantagonistic virulent and avirulent strains; however, when avirulent strains were coinoculated with virulent antagonistic strains, disease ensued. Taken together, these results indicate that X. oryzae pv. oryzae has evolved strategies to compete with rival strains in a fashion that allows virulent strains to evade R gene-mediated protection even when avirulent strains are present in the inoculum.     

Influences of dissolved oxygen on juvenile largemouth bass foraging behaviour

Summer stratification often leads to large areas of hypolimnetic hypoxia in lakes and reservoirs. These areas of hypoxia alter fish behaviours and distributions as well as restrict access to valuable prey resources, yet few studies have examined foraging behaviour of fish in response to low dissolved oxygen (DO) concentrations. We observed foraging behaviour of juvenile largemouth bass, Micropterus salmoides, in response to varying DO concentrations in tanks that simulated a stratified lake water column during the summer: 28°C oxygenated epilimnion, 15°C deoxygenated hypolimnion. Compared with saturated concentrations (8.0–9.0 mg/L), hypolimnetic oxygen concentrations of 3.0 mg/L and 1.5 mg/L resulted in a drastic decrease in prey consumption, handling efficiency and time spent below the thermocline mainly due to avoidance behaviour of hypoxic conditions. However, we found at high hypolimnetic prey densities, individual fish were more willing to venture into reduced oxygen concentrations. Several unique behaviours including transporting prey above the oxycline for consumption, aquatic surface respiration and gill flaring were employed by largemouth bass foraging in low oxygen environments. Reduced hypolimnetic oxygen concentrations may influence and alter feeding strategies, especially for fish that rely on benthic prey resources.     

An olfactometer for measuring the repellent effect of chemicals on the stable fly Stomoxys calcitrans (L.)

An olfactometer is described that permits the evaluation of chemicals acting in the vapour phase against the stable fly Stomoxys calcitrans (L.). Hungry flies, held in a metal frame cage covered with soft gauze, are attracted to a target held adjacent to one side of the cage, and emitting an attractive airflow. Flies landing on the target are counted at intervals of 15s. Flies starved for 24h were tested in the olfactometer when 2, 3 and 4 days old. With the exception of the first exposure of the 4-day-old flies, all the flies of the three ages showed greater than 80% attraction to the olfactometer target after exposure for 2 min, for each of four successive exposures at 10-min intervals. For all ages there was a significant increase in attraction at the second exposure but no further increase with subsequent exposures. It is likely that the sensillae, stimulated by the first exposure, subsequently responded faster. Eight chemicals were incorporated into the airflow behind the target to test their effect on flies. Permethrin (a low vapour pressure pyrethroid) and crotoxyphos (a low vapour pressure organophosphate) did not act in the vapour phase. Empenthrin (a high vapour pressure pyrethroid) and dichlorvos (a high vapour pressure organophosphate) both exhibited the toxic effect of knock-down on the flies without repellency. Oil of citronella and citronellol, two known fly repellents, gave a general reduction of attraction that was dose dependent; fewer flies found the target and those that did stayed a shorter length of time. Natural pyrethrum gave a similar effect to these two at lower concentrations, but at higher concentrations, it also showed a repellent effect. N,N-Diethyl-m-toluamide gave a low reduction in attraction. The olfactometer shows potential for use with other flying insects, both to observe the effects of an insecticide and also to study the behavioural responses in the absence of insecticides.