Biology and glyphosate resistance in Chloris truncata (windmill grass) and Chloris virgata (feathertop Rhodes grass) in southern Australia

Abstract

Chloris truncata and C. virgata, which are major weeds in cotton and grain crops in the sub-tropical region of Australia, have recently emerged as potential weeds of the future in southern Australia. Glyphosate, an inhibitor of 5-enolpyruvylshikimate-3- phosphate synthase (EPSPS), is the most widely used non-selective post-emergence herbicide globally. As a result of over-reliance on glyphosate combined with dominance of reduced tillage systems for weed control, glyphosate-resistant populations of C. truncata have already been reported in Australia. C. virgata is also considered hard to kill with glyphosate, but resistance has not been reported so far in the literature. Studies on growth, development and seed biology of C. truncata and C. virgata were conducted to better understand the biology of these emerging weed species. Under field conditions, C. truncata and C. virgata required 748-786 degree-days (Cd) and 1200 Cd respectively to progress from emergence to mature seed production. Freshly produced seeds of C. virgata were dormant for about 2 months, whereas 16-40% of seeds of C. truncata germinated within a week after maturation. Seed dormancy of C. virgata was released by the pre-treatment with 564 mM NaClO for 30 minutes. Exposure to light significantly increased germination of C. truncata seed from 0-2% in the dark to 77-84% in the light, and of C. virgata seed from 2-35% in the dark to 72- 85% in the light. Seeds of these two species could germinate over a wide temperature range (10-40oC), with maximum germination at 20-25oC for C. truncata and 15-25oC for C. virgata. The predicted base temperature for germination was 9.2-11.2oC for C. truncata and much lower 2.1-3.0oC for C. virgata. Seedling emergence of C. virgata (76% for seeds present on soil surface) was significantly reduced by burial at 1 (57%), 2 (49%) and 5 cm (9%), whereas seedling emergence of C. truncata was completely inhibited by burial of seed even at a shallow depth (0.5 cm). Under field conditions, both C. truncata and C. virgata seeds persisted in the soil for at least 11 months and seasons with below-average spring-summer rainfall increased seed persistence. Detailed studies were undertaken to identify glyphosate-resistant populations and to understand the mechanism of glyphosate resistance in C. truncata and C. virgata. Glyphosate resistance (GR) was confirmed in five populations of C. truncata and four populations of C. virgata. GR plants were 2.4 to 8.7-fold (C. truncata) and 2 to 9.7-fold (C. virgata) more resistant and accumulated less shikimate after glyphosate treatment than susceptible (S) plants. The differences in shikimate accumulation indicated that glyphosate did reach the target site but inhibited the EPSPS enzyme of each population differently. Glyphosate absorption and translocation did not differ between GR and S plants of either C. truncata or C. virgata. Two target-site EPSPS mutations (Pro-106-Leu and Pro-106-Ser) were likely to be the primary mechanism of glyphosate resistance in C. virgata but no previously known target-site mutations were identified in C. truncata. The C. virgata population with Pro-106-Leu substitution was 2.9 to 4.9-fold more resistant than those with Pro-106-Ser substitution. The primary mechanism of resistance to glyphosate in C. truncata was a combination of target-site EPSPS mutation (Glu-91-Ala) and amplification of the EPSPS gene. There were 16 to 48-fold more copies of the EPSPS gene in GR plants compared to S plants, with the number of EPSPS copies found to be variable both between and within populations.