Jonathan Gressel

The paucity of plants evolving genetic resistance to herbicides: possible reasons and implications.

Abstract Resistances to antibiotics and pesticides except herbicides rapidly developed following their introduction. Despite repeated use of herbicides only a few cases of acquired genetic resistance have been reported. By extrapolation from analogous situations, it is suggested that this is due to a combination of low selection pressure of most herbicides, lower fitness of resistant weed strains in the absence of herbicide, the ability of herbicide thinned strands of susceptible weeds to produce relatively more seeds, as well as to the large soil reservoir of susceptible weed seeds. The few reported cases of resistance are to persistent, high selection pressure herbicides supporting our contentions.

Correlation between CuZn superoxide dismutase and glutathione reductase, and environmental and xenobiotic stress tolerance in maize inbreds

Abstract A hypothesis was tested that there would be cross-tolerance between various oxidant stresses, using drought-tolerant and intolerant maize inbreds. Membrane leakage was measured to determine oxidant damage without needing quantification of the enzymes and other factors involved in protection. Paraquat and acifluorfen tolerances paralleled drought and SO2 tolerances using this test. Thus, membrane leakage caused by oxidant generating herbicides can also be used to predict drought tolerance; a much harder character to ascertain. Drought and photooxidative herbicide tolerances were both significantly correlated with high levels of CuZn superoxide dismutase (EC 1.15.1.1.) and with glutathione reductase (EC 1.6.4.2) activities. High levels of just superoxide dismutase or just glutathione reductase did not correlate with any of the tolerances.

Inexpensive non-toxic flocculation of microalgae contradicts theories; overcoming a major hurdle to bulk algal production.

There are two major energy and cost constraints to bulk production of single cell microalgae for biofuels or feed: expensive culture systems with high capital costs and high energy requirements for mixing and gas exchange; and the cost of harvesting using high-speed continuous centrifugation for dewatering. This report deals with the latter; harvesting by flocculation where theory states that alkaline flocculants neutralize the repelling surface charge of algal cells, allowing them to coalesce into a floc. It had been assumed that with such electrostatic flocculation, the more cells to be flocculated, the more flocculant needed, in a linear stoichiometric fashion, rendering flocculation overly expensive. Counter to theory of electrostatic flocculation, we find that the amount of alkaline flocculant needed is a function of the logarithm of cell density, with dense cultures requiring an order of magnitude less base than dilute suspensions, with flocculation occurring at a lower pH. Various other theories abound that flocculation can be due to multi-valent cross-linking, or co-precipitation with phosphate or with magnesium and calcium, but are clearly not relevant with the flocculants we used. Monovalent bases that cannot cross-link or precipitate phosphate work with the same log-linear stoichiometry as the divalent bases, obviating those theories, leaving electrostatic flocculation as the only tenable theory of flocculation with the materials used. The cost of flocculation of dense cultures with this procedure should be below

Somatic hybridization of an atrazine resistant biotype of Solanum nigrum with Solanum tuberosum : Part 1: Clonal variation in morphology and in atrazine sensitivity.

1.00/T algae for mixed calcium:magnesium hydroxides.

Multienzyme oxygen radical detoxifying system correlated with paraquat resistance in Conyza bonariensis

SummaryPlants were regenerated from protoplast fusion experiments with haploid Solanum tuberosum L. and an atrazine resistant biotype of the normally susceptible S. nigrum L. Sixty clones which were unlike the parents were selected by types of hairs and leaf pigmentation of young shoots and characterized by: chromosome numbers, response to atrazine, branching, hairs of the calyx, shapes and pigmentation of leaves, and morphology of flowers. Twenty five clones showed vegetatively stable differences from the parental clones; enough combinations of mixed characters suggested the clear origin from fusants of at least nine clones. Observed diversities within and between protoplastderived clones are interpreted as expressions of variation during the development of the regenerants.

Cross tolerance to herbicidal and environmental oxidants of plant biotypes tolerant to paraquat, sulfur dioxide, and ozone

Abstract A biotype of Conyza bonariensis from an area with repeated paraquat treatments and a wild type from a pristine area were analyzed for resistance to paraquat and for enhanced levels of three enzymes thought necessary to detoxify oxygen radicals. There was no chlorophyll destruction in 24 hr in either biotype. The I 50 of the resistant biotype was a 100-fold higher than that of the sensitive biotype in measurements of CO 2 fixation. Superoxide dismutase levels in the total cell extracts were similar in both biotypes but there was a 60% higher level in the chloroplast stromal extract of the resistant type. There was also 150% increase in the ascorbate peroxidase level and 200% increase in glutathione reductase activity in the stromal extracts of the resistant biotype. These data indicate that resistance is probably due to the enhancement of this three-enzyme organellar system that detoxifies the oxygen entities formed by paraquat.

A strategy to provide long‐term control of weedy rice while mitigating herbicide resistance transgene flow, and its potential use for other crops with related weeds

Abstract We tested a hypothesis that biotypes of various species that evolved resistance to one oxidant stress might have cross tolerance to oxidant stresses from other sources. A paraquat-resistant Conyza bonariensis biotype was tolerant to SO 2 , and to atrazine, acifluorfen, and two experimental pyrimidinedione derivatives, herbicides that generate or induce the generation of oxidants in the light. Its resistance was between 1.5 and 10 times greater than that of the sensitive biotype. Two strains of Lolium perenne , one paraquat tolerant and one SO 2 tolerant, were compared for their tolerances to paraquat. Both were at least 10 times more tolerant to paraquat than the sensitive strain. The paraquat- and SO 2 -tolerant Lolium strains both had 70% more superoxide dismutase and 20% more glutathione reductase than the sensitive strain. One of four ozone-tolerant Nicotiana tabacum cultivars was 16 times more tolerant to paraquat than the ozone-sensitive cultivar. The ozone-tolerant cultivar had 60% more of the three superoxide dismutase isoenzymes present than the sensitive biotype. These results suggest that tolerance in these cases is related to a general ability to more rapidly detoxify the generated oxygen species, as we have previously shown with C. bonariensis .

Low pesticide rates may hasten the evolution of resistance by increasing mutation frequencies

Transgenic herbicide-resistant rice is needed to control weeds that have evolved herbicide resistance, as well as for the weedy (feral, red) rice problem, which has been exacerbated by shifting to direct seeding throughout the world-firstly in Europe and the Americas, and now in Asia, as well as in parts of Africa. Transplanting had been the major method of weedy rice control. Experience with imidazolinone-resistant rice shows that gene flow to weedy rice is rapid, negating the utility of the technology. Transgenic technologies are available that can contain herbicide resistance within the crop (cleistogamy, male sterility, targeting to chloroplast genome, etc.), but such technologies are leaky. Mitigation technologies tandemly couple (genetically link) the gene of choice (herbicide resistance) with mitigation genes that are neutral or good for the crop, but render hybrids with weedy rice and their offspring unfit to compete. Mitigation genes confer traits such as non-shattering, dwarfism, no secondary dormancy and herbicide sensitivity. It is proposed to use glyphosate and glufosinate resistances separately as genes of choice, and glufosinate, glyphosate and bentazone susceptibilities as mitigating genes, with a six-season rotation where each stage kills transgenic crop volunteers and transgenic crop x weed hybrids from the previous season.

Morphogenesis in Trichoderma: Photoinduction and RNA

At very low pesticide rates, a certain low proportion of pests may receive a sublethal dose, are highly stressed by the pesticide and yet survive. Stress is a general enhancer of mutation rates. Thus, the survivors are likely to have more than normal mutations, which might include mutations leading to pesticide resistance, both for multifactorial (polygenic, gene amplification, sequential allelic mutations) and for major gene resistance. Management strategies should consider how to eliminate the subpopulation of pests with the high mutation rates, but the best strategy is probably to avoid too low application rates of pesticides from the outset.

A revolving dose strategy to delay the evolution of both quantitative vs major monogene resistances to pesticides and drugs

Abstract 1. 1. Trichoderma viride cultured in petri dishes will sporulate if given a short illumination after reaching a certain level of maturation. 2. 2. Sporulation can be suppressed by fluorouracil (FU) and azaguanine given for short periods before or after photoinduction without affecting growth. 3. 3. The FU suppression of sporulation can be overcome by adding uracil, but uracil will not competitively prevent long-term FU inhibition of growth. 4. 4. The RNA of Trichoderma was fractionated into the usual species as well as an ultraviolet absorbing peak in the ribosomal region that can be resolved electrophoretically and on methylated albumin columns but does not appear in the analytical ultracentrifuge. 5. 5. FU and azaguanine are incorporated into all types of RNA (with varying specificities). An additional sRNA is eluted from methylated albumin columns between the 4 S and 5 S RNA. 6. 6. Pulse labeling with FU yielded RNA with the same elution profile as uracil labeled RNA of untreated fungus, suggesting FU incorporation into messenger RNA. 7. 7. Although FU is rapidly incorporated into RNA, a considerable time elapses before there is an inhibition of total precursor uptake as well as the rate of the uptake into nucleic acids and proteins. It is surmised from the results that photoinduction of sporulation requires RNA-mediated action. The precise mechanism of FU action on the RNA is a subject for further study.