Variation in tolerance mechanisms to fluazifop-P-butyl among selected zoysiagrass lines

Abstract

Breeding herbicide tolerance into new cultivars can improve safety and weed control in turfgrass systems. The sensitivity to fluazifop-P-butyl of 27 zoysiagrass (Zoysiaspp.) lines was screened under greenhouse conditions to identify potential tolerant germplasm for breeding programs. The herbicide rate that caused 50% biomass reduction (GR50) and the rate that caused 50% injury (ID50) were calculated to select the three most-tolerant and the five most-susceptible lines for studying the physiological mechanisms responsible for fluazifop-P-butyl tolerance. The differences in GR50and ID50between susceptible and tolerant lines ranged from 4-fold to more than 10-fold. Cytochrome P450–mediated metabolism was not detected in fluazifop-P-butyl–tolerant lines. Sequencing of theACCasegene confirmed that none of the seven previously reported mutations conferring resistance to acetyl-CoA carboxylase (ACCase)-inhibiting herbicides in other species were present in any of the tolerant or susceptible zoysiagrass lines studied. An Ala-2073-Thr substitution was identified in two tolerant lines, but this mutation did not completely explain the tolerant phenotype. No clear differences in absorption and translocation rates of14C-radiolabeled fluazifop-P-butyl were observed among most lines, with the exception of a susceptible line that exhibited greater translocation than two of the tolerant lines. Metabolite profiles did not differ between tolerant and susceptible lines. Our results suggest that the diversity in tolerance to fluazifop-P-butyl in zoysiagrass germplasm is most likely the result of a combination of different, minor, additive non–target site mechanisms such as translocation rate and compartmentation after absorption.

Transgressive segregation and maternal genetic effects of non–target site fluazifop-P-butyl tolerance in Zoysia spp.

“…The tolerant parental line 5337-2 exhibited less than 5% GR, while the susceptible parental lines 123, 5330-23, and 252 suffered 10%, 18%, and 75% GR, respectively (Figure 1). It is worth noting that Liu et al (2019a) determined that the tolerance to fluazifop-P-butyl in 5337-2 was not the result of any known resistance mutations in the ACCase gene and was likely due to non-target site mechanisms, although those researchers did not find any major differences in fluazifop-P-butyl metabolism between this line and susceptible lines. Although differences between the tolerant and susceptible lines were also detected for injury, those differences did not follow the same trend as for GR.…”

“…Three Zoysia spp. lines from the University of Florida zoysiagrass germplasm collection and previously characterized by Liu et al (2019a) as fluazifop-P-butyl tolerant (5337-2, 5459-10, and 5504-6) and three more-sensitive lines (123, 252, and 5330-23), here arbitrarily referred to as susceptible, were propagated from field plots and transferred to controlled conditions (28 ± 5 C, 75% to 85% relative humidity, and natural illumination providing a 12-to 14-h photoperiod) in 2017 and 2018. These parental lines were planted in 28 cm by 54 cm by 6 cm trays and maintained under greenhouse conditions with adequate fertilization, daily irrigation, and weekly mowing at 3.5 cm.…”

“…This rate was selected because it allows visual estimation of the differences in fluazifop-P-butyl injury without causing plant death. Also, it allows the quantification of growth reduction (Liu et al 2019a). For each line, a nontreated control was maintained for comparison.…”

“…However, this requires identifying ACCase tolerance variation in germplasm that can be used for breeding. A previous study screening 27 experimental lines of zoysiagrass (Liu et al 2019a) identified zoysiagrass genotypes exhibiting up to 10-fold difference in biomass reduction and greater than 4-fold difference in percent injury levels when treated with fluazifop-P-butyl. This wide variation of this desirable trait may facilitate the development of new AOPP-tolerant varieties.…”

“…Identifying and crossing lines with high tolerance would permit segregation of lines that tolerate label or higher than label rates that could then be generated and selected. Liu et al (2019a) suggested that multiple non-target site mechanisms may be responsible for the tolerance observed in zoysiagrass, but their research focused on physiological and biochemical factors, and the inheritance of the tolerance was not studied.…”