Hossein Ghanizadeh

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.

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.

Spectrophotometric technique for measuring herbicide deposition from wiper applicators

ABSTRACT Weed wipers have been used to apply herbicides selectively to tall weeds in pastures for several decades, but few reliable techniques exist to quantify the relative output of wiper applicators under varying operating conditions. A new technique is described in which clopyralid is applied by wiper applicators to plastic sleeves on uniform, upright, flexible posts with herbicide deposition quantified using a spectrophotometer. The technique was used to compare the relative output of three commercially available wipers: the Rotowiper, Eliminator and Weedswiper. At a constant speed of 5 km/h, large variability was found in the quantities of herbicide deposited by the three wiper applicators. When applied at a range of speeds, there was a small negative correlation for herbicide output as speed increased, although variability in output resulted in this trend being significant for the Weedswiper, but not for the other wiper applicators. The Weedswiper applied less herbicide than the other two wipers, although the relative output from the Eliminator compared with the Rotowiper differed between the two experiments.

Restricted glyphosate translocation in Lolium multiflorum is controlled by a single incomplete dominant nuclear gene

ABSTRACT The mode of inheritance for herbicide resistance was investigated in a population of Lolium multiflorum (Italian ryegrass) with restricted glyphosate translocation mechanism of resistance. The degree of dominance for glyphosate resistance in Italian ryegrass was evaluated in the progenies of F1 families which were created by reciprocal-crossing between resistant (R) and susceptible (S) parental individuals. The results showed an intermediate level of glyphosate resistance for F1 families compared to that of the parental types. The phenotypic resistance segregation was also investigated using backcross families which were created by pair-crossing the F1 individuals to the susceptible population, and the observed segregation values fitted a one-gene model very well. Thus it appears that glyphosate resistance in the Italian ryegrass obtained from a New Zealand vineyard with restricted herbicide translocation is controlled by a single incompletely dominant nuclear gene.