Tubulin-isotype analysis of two grass species-resistant to dinitroaniline herbicides

Dinitroaniline Herbicide Resistance and Mechanisms in Weeds

Dinitroanilines are microtubule inhibitors, targeting tubulin proteins in plants and protists. Dinitroaniline herbicides, such as trifluralin, pendimethalin and oryzalin, have been used as pre-emergence herbicides for weed control for decades. With widespread resistance to post-emergence herbicides in weeds, the use of pre-emergence herbicides such as dinitroanilines has increased, in part, due to relatively slow evolution of resistance in weeds to these herbicides. Target-site resistance (TSR) to dinitroaniline herbicides due to point mutations in α-tubulin genes has been confirmed in a few weedy plant species (e.g., Eleusine indica, Setaria viridis, and recently in Lolium rigidum). Of particular interest is the resistance mutation Arg-243-Met identified from dinitroaniline-resistant L. rigidum that causes helical growth when plants are homozygous for the mutation. The recessive nature of the TSR, plus possible fitness cost for some resistance mutations, likely slows resistance evolution. Furthermore, non-target-site resistance (NTSR) to dinitroanilines has been rarely reported and only confirmed in Lolium rigidum due to enhanced herbicide metabolism (metabolic resistance). A cytochrome P450 gene (CYP81A10) has been recently identified in L. rigidum that confers resistance to trifluralin. Moreover, TSR and NTSR have been shown to co-exist in the same weedy species, population, and plant. The implication of knowledge and information on TSR and NTSR in management of dinitroaniline resistance is discussed.

The Mitochondrial Genome of Eleusine indica and Characterization of Gene Content within Poaceae

Plant mitochondrial (mt) genome assembly provides baseline data on size, structure, and gene content, but resolving the sequence of these large and complex organelle genomes remains challenging due to fragmentation, frequent recombination, and transfers of DNA from neighboring plastids. The mt genome for Eleusine indica (Poaceae: goosegrass) is comprehensibly analyzed here, providing key reference data for an economically significant invasive species that is also the maternal parent of the allotetraploid crop Finger millet (Eleusine coracana). The assembled E. indica genome contains 33 protein coding genes, 6 rRNA subunits, 24 tRNA, 8 large repetitive regions 15 kb of transposable elements across a total of 520,691 bp. Evidence of RNA editing and loss of rpl2, rpl5, rps14, rps11, sdh4, and sdh3 genes is evaluated in the context of an updated survey of mt genomic gene content across the grasses through an analysis of publicly available data. Hypothesized patterns of Poaceae mt gene loss are examined in a phylogenetic context to clarify timing, showing that rpl2 was transferred to the nucleus from the mitochondrion prior to the origin of the PACMAD clade.

Molecular bases for sensitivity to tubulin-binding herbicides in green foxtail

We investigated the molecular bases for resistance to several classes of herbicides that bind tubulins in green foxtail (Setaria viridis L. Beauv.). We identified two alpha- and two beta-tubulin genes in green foxtail. Sequence comparison between resistant and sensitive plants revealed two mutations, a leucine-to-phenylalanine change at position 136 and a threonine-to-isoleucine change at position 239, in the gene encoding alpha2-tubulin. Association of mutation at position 239 with herbicide resistance was demonstrated using near-isogenic lines derived from interspecific pairings between green foxtail and foxtail millet (Setaria italica L. Beauv.), and herbicide sensitivity bioassays combined with allele-specific PCR-mediated genotyping. Association of mutation at position 136 with herbicide resistance was demonstrated using herbicide sensitivity bioassays combined with allele-specific PCR-mediated genotyping. Both mutations were associated with recessive cross resistance to dinitroanilines and benzoic acids, no change in sensitivity to benzamides, and hypersensitivity to carbamates. Using three-dimensional modeling, we found that the two mutations are adjacent and located into a region involved in tubulin dimer-dimer contact. Comparison of three-dimensional alpha-tubulin models for organisms with contrasted sensitivity to tubulin-binding herbicides enabled us to propose that residue 253 and the vicinity of the side chain of residue 251 are critical determinants for the differences in herbicide sensitivity observed between organisms, and that positions 16, 24, 136, 239, 252, and 268 are involved in modulating sensitivity to these herbicides.

CYTOSKELETAL PERSPECTIVES ON ROOT GROWTH AND MORPHOGENESIS

Growth and development of all plant cells and organs relies on a fully functional cytoskeleton comprised principally of microtubules and microfilaments. These two polymeric macromolecules, because of their location within the cell, confer structure upon, and convey information to, the peripheral regions of the cytoplasm where much of cellular growth is controlled and the formation of cellular identity takes place. Other ancillary molecules, such as motor proteins, are also important in assisting the cytoskeleton to participate in this front-line work of cellular development. Roots provide not only a ready source of cells for fundamental analyses of the cytoskeleton, but the formative zone at their apices also provides a locale whereby experimental studies can be made of how the cytoskeleton permits cells to communicate between themselves and to cooperate with growth-regulating information supplied from the apoplasm.

Dinitroaniline herbicide-resistant transgenic tobacco plants generated by co-overexpression of a mutant alpha-tubulin and a beta-tubulin

Dinitroaniline herbicides are used for the selective control of weeds in arable crops. Dinitroaniline herbicide resistance in the invasive weed goosegrass was previously shown to stem from a spontaneous mutation in an alpha-tubulin gene. We transformed and regenerated tobacco plants with an alpha/beta-tubulin double gene construct containing the mutant alpha-tubulin gene and showed that expression of this construct confers a stably inherited dinitroaniline-resistant phenotype in tobacco. In all transformed lines, the transgene alpha- and beta-tubulins increased the cytoplasmic pool of tubulin approximately 1.5-fold while repressing endogenous alpha- and beta-tubulin synthesis by up to 45% in some tissues. Transgene alpha- and beta-tubulin were overexpressed in every plant tissue analyzed and comprised approximately 66% of the total tubulin in these tissues. Immunolocalization studies revealed that transgene alpha- and beta-tubulins were incorporated into all four microtubule arrays, indicating that they are functional. The majority of the alpha/beta-tubulin pools are encoded by the transgenes, which implies that the mutant alpha-tubulin and the beta-tubulin can perform the majority, if not all, of the roles of microtubules in both juvenile and adult tobacco plants.

Double mutation in eleusine indica alpha-tubulin increases the resistance of transgenic maize calli to dinitroaniline and phosphorothioamidate herbicides

The repeated use of dinitroaniline herbicides on the cotton and soybean fields of the southern United States has resulted in the appearance of resistant biotypes of one of the world's worst weeds, Eleusine indica. Two biotypes have been characterized, a highly resistant (R) biotype and an intermediate resistant (I) biotype. In both cases the resistance has been attributed to a mutation in alpha-tubulin, a component of the alpha/beta tubulin dimer that is the major constituent of microtubules. We show here that the I-biotype mutation, like the R-biotype mutation shown in earlier work, can confer dinitroaniline resistance on transgenic maize calli. The level of resistance obtained is the same as that for E. indica I- or R-biotype seedlings. The combined I- and R-biotype mutations increase the herbicide tolerance of transgenic maize calli by a value close to the summation of the maximum herbicide tolerances of calli harbouring the single mutations. These data, taken together with the position of the two different mutations within the atomic structure of the alpha/beta tubulin dimer, imply that each mutation is likely to exert its effect by a different mechanism. These mechanisms may involve increasing the stability of microtubules against the depolymerizing effects of the herbicide or changing the conformation of the alpha/beta dimer so that herbicide binding is less effective, or a combination of both possibilities.

Dinitroaniline herbicide resistance and the microtubule cytoskeleton

Dinitroaniline herbicides have been used for pre-emergence weed control for the past 25 years in cotton, soybean, wheat and oilseed crops. Considering their long persistence and extensive use, resistance to dinitroanilines is fairly rare. However, the most widespread dinitroaniline-resistant weeds, the highly resistant (R) and the intermediate (I) biotypes of the invasive goosegrass Eleusine indica, are now infesting more than 1000 cotton fields in the southern states of the USA. The molecular basis of this resistance has been identified, and found to be a point mutation in a major microtubule cytoskeletal protein, alpha-tubulin. These studies have served both to explain the establishment of resistance and to reveal fundamental properties of tubulin gene expression and microtubule structure.

Herbicide resistance caused by spontaneous mutation of the cytoskeletal protein tubulin

The dinitroaniline herbicides (such as trifluralin and oryzalin) have been developed for the selective control of weeds in arable crops. However, prolonged use of these chemicals has resulted in the selection of resistant biotypes of goosegrass, a major weed. These herbicides bind to the plant tubulin protein but not to mammalian tubulin. Here we show that the major alpha-tubulin gene of the resistant biotype has three base changes within the coding sequence. These base changes swap cytosine and thymine, most likely as the result of the spontaneous deamination of methylated cytosine. One of these base changes causes an amino-acid change in the protein: normal threonine at position 239 is changed to isoleucine. This position is close to the site of interaction between tubulin dimers in the microtubule protofilament. We show that the mutated gene is the cause of the herbicide resistance by using it to transform maize and confer resistance to dinitroaniline herbicides. Our results provide a molecular explanation for the resistance of goosegrass to dinitroanaline herbicides, a phenomenon that has arisen, and been selected for, as a result of repeated exposure to this class of herbicide.