Published in Crop Sci. 44:234-240 (2004).
© 2004 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
GENOMICS, MOLECULAR GENETICS & BIOTECHNOLOGY
Transgenic Cotton with Improved Resistance to Glyphosate Herbicide
O. L. May*,a,
A. S. Culpeppera,
R. E. Cernyb,
C. B. Cootsc,
C. B. Corkernd,
J. T. Cothrene,
K. A. Croonb,
K. L. Ferreirab,
J. L. Hartb,
R. M. Hayesf,
S. A. Huberb,
A. B. Martensb,
W. B. McCloskeyg,
M. E. Oppenhuizenb,
M. G. Pattersonh,
D. B. Reynoldsi,
Z. W. Shappleyj,
J. Subramanik,
T. K. Wittene,
A. C. Yorkl and
B. G. Mullinix, Jr.m
a Dep. of Crop & Soil Science, Univ. of Georgia, Coastal Plain Exp. Stn., P.O. Box 748, Tifton, GA 31793-0748
b Monsanto Company, 700 Chesterfield Parkway North, St. Louis, MO 63198
c Monsanto Company, 25920 Monsanto Road, Loxley, AL 36551
d Monsanto Company, P.O. Box 388, Stoneville, MS 38776
e Dep. of Soil and Crop Sciences, Texas A&M University, College Station, TX 77843-2474
f University of Tennessee, 605 Airways Boulevard, Jackson, TN 38301
g Univ. of Arizona, Forbes 303, Tucson, AZ 85718
h Auburn Univ., 108 Extension Hall, Auburn, AL 36849
i Dep. of Plant and Soil Sciences, Mississippi State Univ., Box 9555, Mississippi State, MS, 39762
j Monsanto Company, 1472 Pecan Ridge Drive, Collierville, TN 38017
k Univ. of Arizona, 37860 Smith-Enke Road, Maricopa, AZ 85239
l Dep. of Crop Science, North Carolina State Univ., Box 7620, Raleigh, NC 27695-7620
m Jr., Dep. Exp. Stat., Univ. Georgia, Coastal Plain Exp. Stn., P.O. Box 748, Tifton, GA 31793-0748
* Corresponding author (lmay{at}tifton.uga.edu).
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ABSTRACT
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Glyphosate [N-(phosphonomethyl)glycine] herbicide can be topically applied twice at rates as high as 0.84 kg a.e. (acid-equivalent) ha–1 to glyphosate-resistant cotton (Gossypium hirsutum L.) cultivars until the fourth true leaf stage, with the requirement of at least 10 d and two nodes of growth between applications. But, such cultivars are not reproductively resistant to glyphosate applied topically or imprecisely directed after the four-leaf stage because glyphosate can curtail pollen development and ovule fertilization, which potentially reduces yield. Extending glyphosate resistance past the four-leaf stage would provide growers with additional weed management options. Our objective was to test under field conditions glyphosate resistance of cotton germplasm transformed with gene constructs previously shown to impart extended glyphosate resistance in the greenhouse. Four or six transgenic cotton lines containing one of several constructs conferring extended glyphosate resistance, plus the current glyphosate-resistant control (‘Coker 312’-1445), were tested at nine U.S. locations in 2001. Within locations, treatment designs consisted of cross-classified arrangements of transgenic lines and glyphosate rates [0, 1.68, and 2.52 kg a.e. ha–1]. Treated plots received glyphosate over-the-top of cotton at four growth stages (3-, 6-, 10-, and 14-leaf crop stages). Compared with Coker 312-1445, extended glyphosate resistance was expressed as higher yields when glyphosate was applied topically at the four growth stages. Mature plant mapping confirmed extended glyphosate resistance of the new transgenic cotton through similar fruit distribution and weight with or without glyphosate treatment. The capability to apply glyphosate topically to cotton later in crop development will facilitate weed management and could reduce dependence on directed herbicides.
Abbreviations: a.e., acid equivalent
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INTRODUCTION
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GROWERS have overwhelmingly adopted glyphosate-resistant (Roundup Ready, Monsanto Co., Chesterfield, MO) cotton cultivars since introduction in 1997. In 2002, about 72% of the U.S. hectarage was planted to cultivars containing the Monsanto glyphosate resistance gene alone, or combined with Monsanto's Bollgard gene (derived from Bacillus thuringiensis var. kurstaki) conferring protection from certain lepidopteran pest insects (Perlak et al., 1991; USDA-AMS, 2002). The popularity of glyphosate-resistant cotton cultivars reflects the broad-spectrum weed control possible with glyphosate, plus capability to farm cotton on more hectares compared with traditional weed management approaches (Culpepper and York, 1998; York, 1997).
The limitation of the current cotton glyphosate resistance technology is a short period of reproductive resistance to moderate glyphosate rates applied topically. Producers are instructed to apply glyphosate topically at rates as high as 0.84 kg a.e. ha–1 not later than the four-leaf growth stage, but with a limit of two applications while allowing at least 10 d and two nodes of growth between sprays. Thereafter, glyphosate must be carefully post-directed such that minimal leaf contact occurs; otherwise, yield loss can result (Kerby and Voth, 1998). Unlike nonglyphosate-resistant cotton that is killed by glyphosate, current glyphosate resistant cotton is vegetatively resistant to most glyphosate rates throughout crop development, but reduced pollen fertility and thus yield loss can result when the herbicide is applied topically or directed imprecisely beyond the recommended four-leaf stage limit (Jones and Snipes, 1999; Pline et al., 2002; Pline et al., 2003; Vargas et al., 1998). Yield losses from nonrecommended glyphosate treatments result from fruit abortion, incompletely filled fruit at fruiting sites critical for yield, or fruit set high on the plant that fail to open before once-over harvest or before a crop terminating freeze (Kalaher et al., 1997; Vargas et al., 1998). Fruit abortion and those not filled normally result from pollen sterility and reduced pollination resulting from altered androecium architecture following nonrecommended glyphosate treatments (Pline et al., 2002, 2003). Extending reproductive resistance to glyphosate later in crop development and to higher rates would provide growers with more options to utilize this effective herbicide.
Commercialization of the first slate of cotton cultivars featuring pest management traits has revealed the necessity of detailed screening of transformed germplasm to assess the merits of transgenically imparted traits, plus any undesired nontarget effects (Jenkins et al., 1997). Thereafter, a single donor parent must be chosen for trait introgression to initiate cultivar development, a critical event in the process of receiving required deregulated status from governmental regulatory agencies before commercialization of a new transgenic trait. The germplasm line Coker 312-1445, derived from transformation event no. 1445 in Coker 312, served as the initial donor parent of the glyphosate resistance trait to current glyphosate-resistant cultivars (Johnson, 1996; Nida et al., 1996). Coker 312-1445 is thus the positive control to which efforts to extend glyphosate resistance are compared.
Cotton containing transgenically enhanced pest management traits poses challenges to discern target and nontarget transformation effects because of cotton's indeterminate fruiting habit (Jenkins et al., 1990). Cotton produces fruit over an extended time, resulting in the ability to compensate for fruit loss at lower nodes through maturation of fruit at higher nodes and those more distal to the main stem, when environmental conditions permit compensation and harvest (Mann et al., 1997; Bednarz and Roberts, 2001). As such, assessing reproductive reaction to herbicides solely through measurement of yield, can lead to erroneous decisions on the merits of transformation entries or candidate cultivars because resistance to the herbicide and fruit set compensation can be confounded. Instead, monitoring fruit weight and distribution through mature plant mapping combined with measurement of yield is a comprehensive method of measuring reproductive reaction to application of herbicides.
Our objective was to test under field conditions glyphosate resistance of cotton germplasm transformed with gene constructs previously shown to impart extended glyphosate resistance in the greenhouse.
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MATERIALS AND METHODS
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Transgenic cotton germplasm lines, hereafter referred to as lines, putatively expressing extended resistance to glyphosate compared with the current glyphosate-resistance technology control, Coker 312-1445, were evaluated in field trials at nine U.S. locations in 2001 (Table 1). The transgenic lines tested for extended glyphosate resistance each descended from a different R0 transformed plant. Lines 1 through 4 were derived from transformed ‘Coker 130’ and were evaluated in field trials in the R4 generation, while lines 5 and 6 originated from transformed Coker 312 and were evaluated in the R3 generation. Each line resulted from transformation with one of several gene constructs conferring extended glyphosate resistance under greenhouse conditions. Transformation of Coker 130 and Coker 312 was accomplished with an Agrobacterium vector, but details of the transformation protocol and composition of the constructs remain proprietary. Monsanto conducted Southern DNA analyses of lines 1 through 6 to confirm that each contained a single copy of a construct imparting glyphosate resistance later in crop development. Coker 312-1445 had been previously assayed by Monsanto and found to contain one copy of the current construct conferring glyphosate resistance, but this construct was not present in the new transformed lines (Nida et al., 1996). Seed of the lines and Coker 312-1445 were sent to cooperators in packets containing 130 seeds each for planting with small-plot, cone-type seeders, to reduce stand variation within and among locations.
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Table 1. Locations, entries, and treatment and experimental designs employed in nine 2001 field trials of transgenic cotton expressing extended glyphosate-resistance.
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The treatment design at all locations consisted of three glyphosate rates [0, 1.68, and 2.52 kg a.e. ha–1] and four or six lines plus the Coker 312-1445 control (hereafter entries), but varied in arrangement among locations (Table 1). Glyphosate rates and entries were main plot and sub-plot factors, respectively, for split-plot treatment designs, with main plots arranged in randomized complete blocks with four replicates, while the same factors arranged in strip-plot treatment designs employed randomized complete block experimental designs with four replicates. The experimental unit was a combination of glyphosate rate and entry and consisted of two rows that varied in length and row spacing across locations, with row spacing reflecting prevalent local production systems (Table 2). The soil types at each trial site were representative of those on which cotton was locally produced (Table 3). Glyphosate was applied topically to cotton and sequentially to the same plots at the 3-, 6-, 10-, and 14-leaf crop stages with a backpack or tractor mounted applicator fitted with flat fan nozzles and calibrated to deliver 93.5 L ha–1 of carrier volume. Before each glyphosate application, plant nodes were counted on five plants per plot to document average plant growth stage.
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Table 2. Plot size and data enumerated from nine 2001 field trials of transgenic cotton tested for extended glyphosate resistance.
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Table 3. Soil types on which 2001 field trials of transgenic cotton expressing extended glyphosate resistance were conducted at nine U.S. locations.
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Remaining inputs for fertilization and pest management were based on local Cooperative Extension Service recommendations for cotton production, except that pest insects and volunteer weeds were aggressively controlled so as not to confound reaction to glyphosate with fruit loss from insects or weed competition. Trials were irrigated or nonirrigated depending on prevalent local practices.
Before machine harvest, distribution of fruit number and weight through box plant mapping of 3 m of one row of each plot was determined at all locations except Lubbock, TX. Box plant mapping involves counting fruit and hand harvest of seed-cotton from open fruit at crop maturity followed by agglomeration by fruiting site into that portion of the box representing a main stem node x sympodial fruiting site combination. We used box mapping procedures of Jenkins et al. (1990), except that the cotyledonary node was considered to be node zero and the few fruit set at position three or greater were collected together and considered to be fruit borne at the 3rd fruiting position on sympodial branches. Cotton matured on monopodial branches was enumerated by weight of seed-cotton and number of fruit from all plants within the 3 m section of row, except for occasional plants with aborted terminals that precludes box mapping. Fruit from plants with aborted terminals within the 3 m row section box mapped was recorded by weight and number. Weight of seed-cotton from fruit borne on plants with aborted terminals, plus that from box mapping, was added to the machine harvested yields to arrive at plot seed-cotton yield. At Lubbock, TX, mature plant mapping from 10 plants from 3 m of row determined presence or absence of fruit by fruiting site on sympodial branches and total fruit on monopodial branches, but this process did not determine fruit weight. Following box or plant mapping, 25 fruit were hand picked from each plot for determination of lint fraction calculated as ginned lint weight as a percentage of seed cotton weight. Plots were then harvested with spindle pickers at all locations except Lubbock, TX, where plots were harvested with a stripper apparatus. The stripped yields at Lubbock reflecting seed-cotton and other plant parts removed by the stripper harvester were considered to be seed-cotton yield for purposes of combined ANOVA across locations.
Plant mapping results in fruit number and seed-cotton weight as functions of main stem node and sympodial fruiting site (Jenkins et al., 1990), but these data are usually unbalanced because of an incomplete cross-classified arrangement of fruiting sites and experimental units. Rather than conduct ANOVA of unbalanced fruiting site data to compare response of fruit distribution and weight among entries to glyphosate, we devised the following statistics to describe fruiting behavior, number and weight of sympodial fruit; monopodial fruit; first position fruit; second position fruit; third position fruit; bottom five main stem node, first position fruit; next five main stem node, first position fruit; and average node of first position sympodial fruit.
Glyphosate rates, entries (lines + Coker 312-1445), and the rate x entry interaction were considered fixed effects, while location and all interactions containing location were considered random effects in combined ANOVA over locations. The statistical analysis was complicated because within locations, cooperators chose various treatment designs to accommodate the cross-classified arrangement of entries and glyphosate rates, and not all entries and locations were cross-classified among locations because Monsanto deemed lines 5 and 6 to be too late maturing for production in North Carolina (Table 1). To compute ANOVA across locations, we ignored sources of variation for rep x glyphosate rate (Error A) of split-plot designs and rep x glyphosate rate and rep x entry (Errors A and B, respectively) for strip-plot treatment designs by analyzing data as a factorial arrangement of entries and glyphosate rates with a randomized complete block experimental design (Cochran and Cox, 1957, p. 305–306). We recognize that doing so causes the resulting residual error variance to differ and have more degrees of freedom relative to that from a split-plot or strip-plot treatment design, but the F tests for entry, glyphosate rate, and their interaction in the mixed model involved location x entry, location x rate, and location x rate x entry, respectively, in the denominator and thus were not affected by collapsing the error structure of split- or strip-plot designs into a single residual term.
To accommodate incomplete cross-classification of locations and entries, we conducted ANOVA of two data sets. One data set was comprised of nine locations with four lines plus Coker 312-1445, hereafter referred to as the nine locations x five entries data set, and ANOVAs of a second data set containing eight locations and six lines plus Coker 312-1445, denoted as the eight locations x seven entries data set (Table 1). Within the eight locations x seven entries data set, fruit weight statistics were computed from seven locations because the plant mapping procedure at Lubbock counts fruit by fruiting site, but not weight (Table 2). ANOVAs of lint fraction, seed-cotton yield, and fruit distribution statistics were conducted with SAS Procedure Mixed (SAS Institute, 2001). We relied on the capability of Procedure Mixed with the Satterthwaite approach for calculating degrees of freedom to construct the correct F tests for sources of variation in the mixed effects models and the PDIFF option of LSMEANS to calculate the standard error of differences among fixed effects means, when significant (P < 0.05) F tests for these sources of variation were encountered. Significant (P < 0.05) glyphosate rate x entry interactions were characterized by linear and quadratic polynomial analyses across glyphosate rates within each entry, using SAS PROC IML to calculate the coefficients for linear and quadratic effects of rate because levels of rate were unequally spaced. Results of the polynomial analyses within entries were summarized through the corresponding LSD (0.05) for comparing rate means within entries when the trend analysis revealed significant (P < 0.05) linear or quadratic rate effects.
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RESULTS AND DISCUSSION
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Combined ANOVA over locations revealed entry x glyphosate rate interactions (P < 0.05) for seed-cotton yields in the nine locations x five entries and eight locations x seven entries data sets (Table 4). The entry x glyphosate rate interactions were characterized by higher seed-cotton yields of the new lines compared with Coker 312-1445 when treated with glyphosate at either rate (Table 5). Averaged over locations of the nine locations x five entries (data not shown) and the eight locations x seven entries data sets, yields of the lines were the same with or without glyphosate treatment (Table 5). Yields of Coker 312-1445 were not different from the lines when not treated with glyphosate in either data set. Plus, regressions of yield on rates within entries revealed linear declines for Coker 312-1445, as it suffered nearly 50% yield losses on topical treatment with glyphosate on four occurrences at the 3-, 6-, 10-, and 14-leaf stages. Similar yield losses for current glyphosate-resistant cotton from nonrecommended topical glyphosate applications after the four-leaf stage have been reported (Baughman et al., 1999). We also found glyphosate x entry interactions (P < 0.05) for lint fraction in both data sets, caused by 1445 increasing in lint fraction across glyphosate rates, while lint fractions of the new lines were not affected (data not shown). These findings for lint fraction and seed-cotton yields indicate that the new transgenic cotton allows glyphosate to be applied later in crop development and at higher rates compared with current glyphosate-resistant cotton.
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Table 4. Results from the ANOVA of seed-cotton yields for new transgenic lines expressing extended glyphosate-resistance compared with the current glyphosate-resistant technology control Coker 312-1445.
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Table 5. Glyphosate rate x entry seed-cotton yields averaged over eight locations, excluding Lewiston, NC, from 2001 field trials of six new transgenic lines and Coker 312-1445 when glyphosate was applied sequentially over-the-top at the 3-, 6-, 10-, and 14-leaf stages.
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Resistance to herbicides and/or insects based solely on yield can result in erroneous decisions concerning target effects of such transgenes and the general merits of transformation events. Cotton's nature to fruit over an extended period and even initiate a late crop when weather allows can confer yield compensation from fruit loss earlier in crop development, especially in the southern U.S. cotton belt with its long growing season (Mann et al., 1997; Bednarz and Roberts, 2001). In the northern cotton belt, climatic restrictions on maturation of fruit borne at nodes high on the plant and distal sympodial positions might allow yield to measure reaction to glyphosate. Therefore, fruit number and weight statistics from box and plant mapping are helpful when combined with yield to discern cotton's resistance to glyphosate.
There exist several measures of cotton's fruiting behavior capable of discriminating between yields reflecting normal fruit production and yields representing compensation for fruit loss at lower nodes. Compared with Coker 312-1445 at either rate of glyphosate, the lines produced higher weight and number of sympodial fruit (Table 6) and displayed no evidence that sympodial fruit production was shifted to monopodial branches because weight and number of monopodial fruit did not vary among lines or glyphosate rates (data not shown). Additional evidence that the lines produced fruit in a normal manner when glyphosate was topically applied four times was lack of variation in first position fruit weight or number among rates (Table 7) and no increase in either second or third position fruit weight or number (Tables 8, 9). In contrast, Coker 312-1445 never recovered from the four-glyphosate applications as it matured less-yield at first and second position fruiting sites.
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Table 6. Glyphosate rate x entry sympodial fruit weight and number averaged over locations from 2001 field trials of six transgenic lines and Coker 312-1445 when glyphosate was applied sequentially over-the-top at the 3-, 6-, 10-, and 14-leaf stages.
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Table 7. Glyphosate rate x entry means for weight and number of first position sympodial fruit averaged over locations from 2001 field trials of six transgenic lines and Coker 312-1445 when glyphosate was applied sequentially over-the-top at the 3-, 6-, 10-, and 14-leaf stages.
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Table 8. Glyphosate rate x entry means for weight and number of second position sympodial fruit averaged over locations from 2001 field trials of six transgenic lines and Coker 312-1445 when glyphosate was applied sequentially over-the-top at the 3-, 6-, 10-, and 14-leaf stages.
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Table 9. Glyphosate rate x entry means for weight and number of third position sympodial fruit averaged over locations from 2001 field trials of six transgenic lines and Coker 312-1445 when glyphosate was applied sequentially over-the-top at the 3-, 6-, 10-, and 14-leaf stages.
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Cotton yield potential is highly dependent on harvest of fruit borne on first position sympodial sites (Kerby and Buxton, 1981; Jenkins et al., 1990; Bednarz and Roberts, 2001). When glyphosate is applied at the 6-, 10-, and 14-leaf stages, the androecia of flowers at first position, main-stem nodes in about the first half of the fruiting zone have become primordial structures or are in development. Consequently, pollen fertility in these flowers is sensitive to glyphosate when applied according to the regime in our studies (Pline et al., 2002, 2003). Hence, one measure of extended glyphosate resistance is the capability to produce yield at lower main stem node, first position, fruiting sites. An ANOVA of the initial first-position fruiting site across eight locations (excluding Lewiston, NC, but including Lubbock, TX) found that in the absence of glyphosate, the new entries and Coker 312-1445 began to fruit at about the same main stem node (mean = node 5; range = 0.6 nodes, SE = 0.5), so we compared fruit number and weights summed from the first five first positions at main stem nodes five to nine and also for the next five nodes, 10 to 14. Compared with Coker 312-1445 at either glyphosate rate, the lines produce more cotton at the first five, first position, fruiting sites (Table 10). None of the lines lost yield at the bottom five, first position fruiting sites as glyphosate was applied, while Coker 312-1445 experienced a linear decline in seed-cotton yield and fruit number over rates. Results were similar at the next five, first-position fruiting sites (Table 11). These data also did not demonstrate that the new lines produced more yield at higher 1st position nodes reflecting compensation for fruit loss at lower nodes. In summary, these statistics crafted from the fruit number and weight data corroborate the conclusions drawn from the yield data that the new transgenic cotton extends resistance to glyphosate.
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Table 10. Glyphosate rate x entry means for weight and number of first position sympodial fruit set at main stem nodes 5-9 averaged over locations from 2001 field trials of six transgenic lines and Coker 312-1445 when glyphosate was applied sequentially over-the-top at the 3-, 6-, 10-, and 14-leaf stages.
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Table 11. Glyphosate rate x entry means for weight and number of first position, sympodial fruit set at main stem nodes 10 through 14 averaged over locations from 2001 field trials of six transgenic lines and Coker 312-1445 when glyphosate was applied sequentially over-the-top at the 3-, 6-, 10-, and 14-leaf stages.
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The nine locations at which the lines expressing extended glyphosate resistance were tested span most of the U.S. upland cotton belt from Arizona to North Carolina. These locations represent diverse environments, encompassing variation in soil type, season length, heat unit accumulation, picker or stripper harvest methods, and rain-fed or irrigated production. Thus, the reaction of the lines to glyphosate over these environments defines a reasonably broad inference space to infer the success or not in extending cotton's resistance to glyphosate. These experiments confirm extended glyphosate resistance of the new transgenic cotton to glyphosate later in crop development and at higher rates compared with current glyphosate-resistant cotton. On the basis of yield and fruit distribution, the new lines were resistant to glyphosate when applied according to the protocol in these studies, and thus define a set of candidates from which to choose the donor of the extended glyphosate resistance trait (tradename Roundup Ready Flex). Commercialization of this technology is not envisioned until 2006 or later, but soon thereafter should further enhance the convenience of cotton production. Although the role of extended glyphosate-resistant cotton technology in weed management continues to be refined, the capability to apply glyphosate topically between the 4- and 14-leaf stages and at higher rates compared with current glyphosate-resistant cotton could reduce dependence on directed herbicides historically used on much of the cotton hectarage, while providing growers additional weed control options.
Received for publication April 29, 2003.
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TOP
ABSTRACT
INTRODUCTION
