Germination and seed persistence of Amaranthus retroflexus and Amaranthus viridis: Two emerging weeds in Australian cotton and other summer crops.

Redroot pigweed (Amaranthus retroflexus L.) and slender amaranth (Amaranthus viridis L.) are becoming problematic weeds in summer crops, including cotton in Australia. A series of laboratory and field experiments were performed to examine the germination ecology, and seed persistence of two populations of A. retroflexus and A. viridis collected from the Goondiwindi and Gatton regions of Australia. Both populations of A. retroflexus and A. viridis behaved similarly to different environmental conditions. Initial dormancy was observed in fresh seeds of both species; however, germination reached maximum after an after-ripening period of two months at room temperature. Light was not a mandatory prerequisite for germination of both species as they could germinate under complete darkness. Although both species showed very low germination at the alternating day/night temperature of 15/5 C, these species germinated more than 40% between ranges of 25/15 C to 35/25 C. Maximum germination of A. retroflexus (93%) and A. viridis (86%) was observed at 35/25 C and 30/20, respectively. Germination of A. retroflexus and A. viridis was completely inhibited at osmotic potentials of -1.0 and -0.6 MPa, respectively. No germination was observed in both species at the sodium chloride concentration of 200 mM. A. retroflexus seedling emergence (87%) was maximum from the seeds buried at 1 cm while the maximum germination of A. viridis (72%) was observed at the soil surface. No seedling emergence was observed from a burial depth of 8 cm for both species. In both species, seed persistence increased with increasing burial depth. At 24 months after seed placement, seed depletion ranged from 75% (10 cm depth) to 94% (soil surface) for A. retroflexus, and ranged from 79% to 94% for A. viridis, respectively. Information gained from this study will contribute to an integrated control programs for A. retroflexus and A. viridis.

Herbicide resistance evolution can be tamed by diversity in irrigated Australian cotton: a multi-species, multi-herbicide modelling approach.

BACKGROUND: Because herbicide resistance evolves in very large populations over periods of many years, modelling is an important tool for investigating the dynamics of the problem. The Diversity model tracks the simultaneous evolution of resistance to multiple herbicides, using multiple genetic pathways, in several weed species at once. Tracking multiple species and simultaneous resistances is an important development in resistance modelling. We used the Diversity model to test weed management strategies for new cropping cotton varieties with multiple herbicide tolerances ('triple-stacked' varieties), in an Australian context. RESULTS: The diversity required for long-term control of three key weeds in Australian cotton goes beyond using three herbicides, especially where there is already a substantial background of existing resistance to one or more of these herbicides. Assuming some glyphosate resistance is already present, simulations showed that glyphosate-resistant summer grass populations reach 20 000 seeds m-2 within 12 years using the triple-stack herbicides (glyphosate, glufosinate and dicamba) and a minimum of other tactics. Adding three pre-emergent modes of action plus cultivation to the system effectively controls glyphosate-resistant grasses for over 30 years. In conditions where resistance genes are as frequent as 1 in 100, however, highly fecund weeds such as Conyza bonariensis are hard to control beyond 15 years even with very highly diverse management. CONCLUSIONS: Stacked herbicide tolerances in new crop varieties offers potential for increased herbicide diversity, but existing glyphosate-resistant weed populations need substantial extra management beyond what a glyphosate/glufosinate/dicamba resistance stack provides. More diverse systems can provide robust management over 30 years in the absence of very high levels of background resistance to other herbicides. © 2018 Society of Chemical Industry.

Complete chloroplast genome of glyphosate resistant Sonchus oleraceus L. from Australia, with notes on the small single copy (SSC) region orientation.

Sonchus oleraceus, common sowthistle, is an asteraceous weed in Australian agricultural systems and has recently developed resistance to glyphosate. We present the complete chloroplast sequence of S. oleracueus reconstructed from Illumina whole genome shotgun sequencing. This is the first complete chloroplast genome available for the genus Sonchus. The complete chloroplast sequence is 151,808 bp long. A Bayesian phylogeny of the chloroplast coding regions of the tribe Cichorieae (Asteraceae) is presented. The S. oleraceus chloroplast genome is deposited at GenBank under accession number MG878405.

Gene expression in response to glyphosate treatment in fleabane (Conyza bonariensis) - glyphosate death response and candidate resistance genes.

BACKGROUND: This study takes a whole-transcriptome approach to assess gene expression changes in response to glyphosate treatment in glyphosate-resistant fleabane. We assessed gene expression changes in both susceptible and resistant lines so that the glyphosate death response could be quantified, and constitutively expressed candidate resistance genes identified. There are three copies of the glyphosate target site (5-enolpyruvylshikimate-3-phosphate; EPSPS) gene in Conyza and because Conyza bonariensis is allohexaploid, there is a baseline nine copies of the gene in any individual. RESULTS: Many genes were differentially expressed in response to glyphosate treatment. Known resistance mutations are present in EPSPS2 but they are present in a glyphosate-susceptible line as well as resistant lines and therefore not sufficient to confer resistance. EPSPS1 is expressed four times more than EPSPS2, further reducing the overall contribution of these mutations. CONCLUSION: We demonstrate that glyphosate resistance in C. bonariensis is not the result of EPSPS mutations or overexpression, but due to a non-target-site mechanism. A large number of genes are affected by glyphosate treatment. We present a list of candidate non-target-site-resistance (NTSR) genes in fleabane for future studies into these mechanisms. © 2017 Society of Chemical Industry.

Complete chloroplast genome sequences of six lines of Echinochloa colona (L.) link.

Barnyard grass (Echinochloa colona, (L.) Link) is the wild relative of barnyard millet (E. frumentacea (Roxb.) Link). This species, widely distributed globally, is an agricultural weed and has developed resistance to several herbicides including glyphosate. This paper presents the complete chloroplast sequences of two haplotypes (139,718 bp & 139,719 bp) sequenced from six lines of E. colona from Australia. The E. colona chloroplast sequence is very similar to that of E. frumentacea (163-169bp =0.12% differences across the genome). The gene content, arrangement, and the inverted repeat structure is the same as in the other species of Echinochloa sequenced to date.

Complete chloroplast genome sequences of two species of Chloris grass, Chloris truncata Sw. and Chloris virgata R.Br.

Chloris truncata (windmill grass) and Chloris virgata (feathertop Rhodes grass) are both weedy grass species that have developed resistance to glyphosate in Australia. This paper describes the complete chloroplast genomes of these two species generated by high throughput shotgun sequencing. The chloroplast genome of C. truncata is 135,584 bp and C. virgata is 134,561 bp; both have a GC content of 38%. The gene content and order followed the conserved pattern observed across the subfamily Chloridoideae.

Complete chloroplast genome of glyphosate resistant Conyza bonariensis (L.) Cronquist from Australia.

Conyza bonariensis, flaxleaf fleabane, is a serious weed in Australian agricultural systems, particularly the north-east cropping system. We present the complete chloroplast sequence of C. bonariensis reconstructed from Illumina whole genome shotgun sequencing. This is the first complete chloroplast genome available for genus Conyza. The complete chloroplast sequence is 153,014 bp long, and has the same gene content and structure as other members of the tribe Astereae. A Bayesian phylogeny of the chloroplast coding regions of 18 representatives of Astereae is presented. The C. bonariensis chloroplast genome is deposited at GenBank under accession number MF276802.

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.