K.C. Harrington

Mechanisms of glyphosate resistance in two perennial ryegrass (Lolium perenne) populations.

BACKGROUND Perennial ryegrass (Lolium perenne) has developed resistance to glyphosate within New Zealand vineyards following many years of herbicide application. The objectives of this work were to confirm resistance within two populations obtained from affected vineyards, and to determine the mechanism of resistance to glyphosate. RESULTS Population O was confirmed to have a 25-fold resistance to glyphosate, whereas population J had a sevenfold resistance. Results of genotyping assays demonstrated a single nucleotide substitution at codon 106 of 5-enolpyruvylshikimate-3-phosphate synthase in population O but not population J. Glyphosate-resistant and glyphosate-susceptible populations did not differ in glyphosate absorption. However, in both resistant populations, much more of the absorbed (14) C-glyphosate remained in the treated leaf than occurred in the susceptible population. Significantly more glyphosate was found in the pseudostem region of susceptible plants compared with resistant plants. CONCLUSION Both target-site and non-target-site mechanisms of glyphosate resistance were found in the perennial ryegrass population with 25-fold resistance, whereas only the non-target-site mechanism of resistance was found in the population with sevenfold resistance. This is the first study of the mechanism of glyphosate resistance in perennial ryegrass.

Non-target Site Mechanisms of Resistance to Herbicides

ABSTRACT Evolution of resistance in weeds to herbicides is threatening world agricultural production. Weed management has become more complicated with the development of non-target site resistance (NTSR) to herbicides in weeds. The NTSR mechanisms can be caused by herbicide metabolism, altered patterns of translocation, and herbicide absorption/penetration. Metabolism of herbicides consists of an activation phase and then a conjugation phase, though in some cases no activation phase is needed. The activation phase mainly involves the addition of functional groups to herbicide molecules using enzymes such as cytochrome P-450. Transcriptome-wide gene expression has shown that genes which encode for several cytochrome P-450s are upregulated in weeds resistant to ACCase-inhibitor and ALS-inhibitor herbicides. In the conjugated phase, several studies have shown that two important types of enzyme, glutathione S-transferases and glucosyltransferases, play crucial roles in conferring resistance to herbicides. An altered pattern of translocation can also play a crucial role in conferring NTSR to weeds. With glyphosate and paraquat, it has been shown that altered patterns of translocation are due to sequestration of the herbicide into vacuoles. However, some other little known mechanisms such as “hyper-sensitive” reactions, impaired translocation, and root exudation can affect the patterns of herbicide translocation within resistant weeds. The molecular understanding of NTSR mechanisms is still in its infancy. Recent developments in techniques such as high-throughput DNA/RNA sequencing technologies will soon allow further insights into the NTSR mechanisms in weeds. This information is essential for developing strategies to overcome weeds with the NTSR mechanisms.

Quick tests for detecting glyphosate-resistant Italian and perennial ryegrass

Three quick tests were evaluated for detecting glyphosate-resistant biotypes of Italian ryegrass and perennial ryegrass. Biotypes of these two species were used that were 13.4 and 7.3 times more resistant to glyphosate, respectively, than susceptible biotypes when assessed using a sprayed pot study. One assay exposed germinating seeds of resistant and susceptible populations for 8 days to different concentrations of glyphosate. Measurements of reduction in seedling root growth gave an estimate of resistance magnitude of 10.8 and 8.9 for Italian ryegrass and perennial ryegrass, respectively, similar to that from the sprayed pot study. Two other assays were tested and were able to differentiate resistant and susceptible biotypes. One measured the relative levels of shikimic acid in leaves following exposure to glyphosate; four to seven times more shikimic acid accumulated in glyphosate-susceptible populations of Italian and perennial ryegrass than in resistant populations. The other assay involved growing tillers of the plants in glyphosate solutions and the results showed that at glyphosate concentrations of 10 and 40 mg/L, the plantlets of resistant plants showed negligible visual injury and growth reduction compared with susceptible ones. Results obtained from these assays were not affected by whether the glyphosate formulation used was an isopropylamine salt or potassium salt.

Investigations of how phenoxy‐resistant Carduus nutans biotypes survive herbicide spraying

Abstract Biotypes of nodding thistle (Carduus nutans) have developed within New Zealand that require over six times the normal rate of MCPA, MCPB or 2,4‐D to kill them. Radiolabelled 2,4‐D was applied to both resistant and susceptible nodding thistle plants to investigate the mechanism of resistance. There was no difference in penetration of the 2,4‐D into the foliage of the resistant and susceptible plants. However some minor differences in translocation were detected, with the 2,4‐D apparently less mobile within the resistant plants. These differences were variable and did not fully explain the resistance. Significantly more radioactivity was released from the roots of susceptible plants. There were also small but significant differences in the quantities of radioactivity that could be removed from within the treated plants by ethanol, suggesting more binding within susceptible plants. Significant differences were detected in rates of metabolism of 2,4‐D within the root systems. More radioactivity was present as water‐soluble compounds in the resistant plants than the susceptible ones. Of the ether‐soluble components extracted, about 14 times more 2,4‐D was estimated to be present in susceptible plants than in resistant plants, with chromatography suggesting that compounds in the resistant plants were metabolites of 2,4‐D.

Relative persistence of commonly used forestry herbicides for preventing the establishment of broom (Cytisus scoparius) seedlings in New Zealand plantations

BackgroundMany of the herbicides used in site preparation of forests and during initial selective weed management can persist in the soil for several months, killing or stunting seedlings of broom (Cytisus scoparius (L.) Link) as they establish. The objective of this research was to determine the relative persistence and effect over time of the herbicides most widely applied within radiata pine (Pinus radiata D.Don) plantations in New Zealand on broom survival and growth.MethodsEleven herbicide treatments were applied in early summer to a Tokomaru silt loam soil. Soil samples were collected from each treatment at fortnightly intervals for up to a year. The samples were placed into pots and immediately sown with scarified viable broom seeds in a heated glasshouse. Relative growth and biomass of seedlings were determined as the ratio of growth and survival in each treatment relative to growth and survival in an untreated control. Logistic curves were fitted to recorded values of relative growth and survival over time and the time to reach 50 and 95% of the vigour of the plants in the untreated control was determined.ResultsThe treatments can be approximately ranked in the following order from most to least persistent: triclopyr/picloram > high rate of clopyralid > high rate of hexazinone > terbuthylazine/hexazinone > low rate of hexazinone > low rate of clopyralid > high rate of terbuthylazine > triclopyr > high rate of metsulfuron-methyl > low rate of terbuthylazine > low rate of metsulfuron-methyl.ConclusionAs the rate of triclopyr/picloram used in this work is too phytotoxic to be used selectively over newly planted radiata pines, this treatment would only be suitable prior to tree planting so residual activity would control seedlings that develop after scrub clearance. For post-plant weed management operations, results show hexazinone and high rates of clopyralid to be useful for long-term management of broom as both these herbicides have low phytotoxicity to radiata pine. When combined with low rates of triclopyr and picloram, clopyralid could provide an effective treatment that could be used in combination with oversown or naturally occurring grasses to manage broom.

Investigation of herbicides tolerated by Stevia rebaudiana crops

Abstract A pot trial and two field trials evaluate the safety to stevia plants of 25 herbicides, many of which have been used for selective control in other Asteraceae crops. Pre-emergence herbicides that show potential for use in transplanted stevia include trifluralin, pendimethalin, oryzalin, bromacil, terbacil, linuron, methabenzthiazuron and alachlor. Post-emergence treatments that are tolerated reasonably well by stevia include many of the herbicides listed above plus clethodim, haloxyfop, propyzamide, thifensulfuron, flumetsulam and pyridate. Other post-emergence herbicides that cause some crop damage but may merit further research include bentazone, ethofumesate, MCPB, picolinafen, chloridazon and metribuzin. Terbacil and bromacil are the most effective herbicides for weed control, although some crop damage did occur. Controlling weeds through frequent hand-weeding allowed a 30-fold increase in stevia production in one trial, although the best result obtained by herbicides was a 19-fold increase using bromacil immediately before transplanting stevia.

Genetic inheritance of restricted herbicide translocation in a glyphosate-resistant Lolium perenne population

ABSTRACT One of the crucial factors in the evolution of herbicide resistance in weeds is the mode of inheritance. Experiments were conducted to determine the inheritance of glyphosate resistance in a population of perennial ryegrass (Lolium perenne) caused by restricted glyphosate translocation. First, the degree of dominance of the glyphosate resistance trait was evaluated by generating first filial generation families through pair-crossing resistant with susceptible plants. Dose-response experiments showed that these first generation families had an intermediate level of glyphosate resistance compared with that of the parent plants. The phenotypic segregation of the resistance trait was then investigated by backcrossing the first generation offspring with individuals from the original susceptible population. Results obtained from spraying the resulting backcrossed families with two rates of glyphosate gave segregation values that best fitted the one-gene model. Hence, glyphosate resistance in perennial ryegrass caused by the restricted herbicide translocation trait is controlled by a single nuclear gene with incomplete dominance.

Glyphosate-resistant population of Lolium perenne loses resistance at winter temperatures

In New Zealand vineyards, evolution of glyphosate resistance in perennial ryegrass (Lolium perenne) has been reported recently, and restricted translocation of glyphosate has been found in these resistant plants. Past research with other plant species has found that restricted glyphosate translocation is temperature dependent. Glasshouse dose-response experiments were conducted with resistant perennial ryegrass at different temperatures to investigate if this population becomes susceptible to glyphosate under cooler conditions. Glyphosate-susceptible plants (population SP) grown in either warm (average of 20 °C) or cool (average of 9 °C) temperatures following treatment with several rates of glyphosate responded similarly to the herbicide regardless of temperature. In contrast, plants of population J (glyphosate resistant) were significantly more resistant than population SP to glyphosate when treated under warm conditions, but at the cooler temperature, population J was as susceptible as population SP. Thus the mechanism of resistance in population J appeared to be suppressed under cool conditions, suggesting that application of glyphosate during winter might improve control of glyphosate-resistant perennial ryegrass infestations.

Cross-resistance to auxinic herbicides in dicamba-resistant Chenopodium album

ABSTRACT The responses of two dicamba-resistant Chenopodium album (fathen) populations (L and M) were compared with the responses of two dicamba-susceptible populations (A and P) to the auxinic herbicides mecoprop, clopyralid, 2,4‐D and aminopyralid in a preliminary experiment. The dicamba-resistant fathen was cross-resistant to the pyridine carboxylic acid herbicides clopyralid and aminopyralid, but not the phenoxy acid herbicides, 2,4-D and mecoprop. The level of cross-resistance to aminopyralid and picloram (another pyridine carboxylic acid herbicide) was investigated in two separate dose-response experiments. The results of the first dose-response experiment showed populations L and M were 12 and 19 times more resistant to aminopyralid, respectively, than susceptible populations (A and P). The dicamba-resistant fathen populations were also shown to be resistant to picloram although the levels of resistance ranged from three- to 17-fold. These levels of cross-resistance for both herbicides were confirmed in the second dose-response experiment. The results of this study help with planning control strategies for this resistance problem.

Perspectives on non-target site mechanisms of herbicide resistance in weedy plant species using evolutionary physiology

Research into evolutionary physiology involves principles from both the disciplines of evolution and physiology. However investigations into development of resistance to herbicides have often not given much consideration of evolutionary physiology. This paper suggests that a better understanding of herbicide resistance in weeds should be possible by investigating the evolutionary physiology involved.