The first case of glyphosate-resistant weeds was discovered in Lolium rigidum from
Australia in 1996 (Powles et al., 1998). So far, over 30 weed species have been
reported as having evolved resistance to glyphosate globally (Heap, 2015). An important factor that has increased the evolution of glyphosate-resistant weeds has been transgenic glyphosate-resistant crops such as maize, soybean and cotton (Powles & Yu, 2010). In these crops, extensive applications of glyphosate instead of other means of weed control have increased selection intensity for glyphosate resistance in weeds (Powles & Yu, 2010).
Mechanisms of glyphosate resistance in weeds have been shown to be both target site and non-target site. In target site based resistance, either a modification of the site of action (EPSPS enzyme) or enzyme over-expression/gene amplification endows resistance to the weed species (Powles & Yu, 2010; Sammons & Gaines, 2014). The first case of target site based resistance (enzyme modification) for glyphosate was reported in Eleusine indica (Baerson et al., 2002; Lee & Ngim, 2000). In this case, a
modification resulted from a single nucleotide change (a serine substitution at Pro-106) to cause resistance (Baerson et al., 2002; Lee & Ngim, 2000).
In another resistant biotype of Eleusine indica, another nucleotide substitution (Pro-106
27 Subsequently, a target site based resistance mechanism to glyphosate has also been reported for other weed species such as Lolium rigidum from three different parts of the
world: Australia (Bostamam et al., 2012; Wakelin & Preston, 2006b), South Africa (Yu et al., 2007) and USA (Simarmata & Penner, 2008), and another species, Lolium multiflorum from USA (Jasieniuk et al., 2008) and Chile (Perez-Jones et al., 2007).
Nucleic acid substitution at Pro-106 endows a modest degree of glyphosate resistance to weed species (Powles & Yu, 2010).
A double amino acid substitution in the EPSPS gene was recently reported in Eluesine indica (Yu et al., 2015). In this double amino acid substitution which is known as TIPS,
modifications resulted from two nucleotide changes at positions Thr-102-Ile plus Pro- 106-Ser (TIPS) (Yu et al., 2015). The TIPS has been described in the first generation of
Roundup Ready maize (Sammons & Gaines, 2014). However, this is the first example of the TIPS conferring glyphosate resistance to a weed species. In contrast to the single mutation Pro-106, the TIPS confers an extremely high level of glyphosate resistance (over 180-fold based on R/S LD50 ratio) (Yu et al., 2015).
Recently, another mechanism of resistance was reported which conferred glyphosate resistance to Amaranthus palmeri (Gaines et al., 2010). Studies of the mechanism of
glyphosate resistance in one population of Amaranthus palmeri showed similar
herbicide absorption and translocation between resistant and susceptible biotypes (Culpepper et al., 2006). Furthermore, EPSPS from both resistant and susceptible
biotypes was inhibited equally by glyphosate (Gaines et al., 2010). However, the
resistant biotype was 6- to 8-fold more resistant to glyphosate compared to the susceptible biotype (Culpepper et al., 2006). Molecular investigations showed that
resistant biotype of Amaranthus palmeri had a higher genomic copy number of the
EPSPS gene which was positively correlated with EPSPS expression (Gaines et al.,
2010). Therefore, the authors concluded that gene amplification (over-expression) was the mechanism of glyphosate resistance for this biotype (Gaines et al., 2010). This over-
expression has also been documented for glyphosate-resistance within Amaranthus tuberculatus, Lolium multiflorum, Kochia scoparia and Amaranthus spinosus
(Sammons & Gaines, 2014).
Restricted herbicide translocation is another mechanism of glyphosate resistance (non- target site) in glyphosate-resistant weed biotypes. In fact, non-target site based glyphosate resistance is more common among weeds species than target site
28
mechanisms (Powles & Yu, 2010). In a study of the cause of restricted glyphosate translocation using 31P nuclear magnetic resonance (NMR), glyphosate was found to enter the cytoplasm of both glyphosate-resistant and susceptible Conyza canadensis at
the same rate, but a large amount of glyphosate was sequestered in vacuoles shortly after spraying with glyphosate while this rapid sequestration of glyphosate was not observed in susceptible plants (Ge et al., 2010). Also, there was a positive correlation
between the levels of resistance to glyphosate and the extent of vacuole sequestration of glyphosate in resistant Lolium multiflorum from Brazil and Chile, and Lolium rigidum
from Australian and Italy (Ge et al., 2012). It has been suggested that the transporters
associated with vacuolar membranes like ATP-binding cassette (ABC) transporters might have roles in the vacuolar sequestration of glyphosate (Yuan et al., 2007).
Glyphosate non-target site based resistance has been reported in Lolium rigidum (Adu-
Yeboah et al., 2014; Wakelin et al., 2004), Conyza canadensis (Feng et al., 2004), Lolium multiflorum (Michitte et al., 2007) and Sorghum halepense (Riar et al., 2011b).
However, it was reported that some weed species such as Lolium rigidum accumulated
both target site and non-target site mechanism of glyphosate resistance and as a result, the studied populations of Lolium rigidum was more resistant to glyphosate compared to
those Lolium rigidum populations in which resistance was only conferred by one
mechanism of resistance (Bostamam et al., 2012; Yu et al., 2007).
A less known mechanism of resistance has also been observed for Ambrosia trifida in
which the mature treated leaves of the glyphosate-resistant biotype show rapid necrosis 12 hours after glyphosate application (Sammons & Gaines, 2014). Further studies showed that glyphosate translocation was substantially restricted possibly due to this rapid leaf necrosis (Sammons & Gaines, 2014). The molecular and physiological basis of this mechanism has not been completely elucidated.
The inheritance of glyphosate-resistance has been investigated in a number of biotypes. Depending on the type of glyphosate resistance mechanism, varied modes of inheritance have been documented. It was reported that a target site mechanism of resistance was inherited as a single nuclear gene with partial dominance in an Australian Lolium rigidum population (Preston et al., 2009b) and a population of Eleusine indica from
Malaysia (Ng et al., 2004). The mode of inheritance of glyphosate resistance in one
population of Lolium rigidum with restricted herbicide translocation was reported to be
29 was also reported for Conyza canadensis (Zelaya et al., 2004) and Lolium multiflorum
(Vargas et al., 2007). Wakelin and Preston (2006a) reported that glyphosate resistance
in four populations of Lolium rigidum was inherited as a single dominant gene. Okada
and Jasieniuk (2014) reported that the glyphosate resistance in Conyza bonariensis was
controlled by two different genes. In Amaranthus palmeri with the EPSPS gene
amplification mechanism of resistance, the inheritance involves multiple nuclear genes (Mohseni-Moghadam et al., 2013).