Cultivar BRS 433FL B2RF: upland cotton with high-quality fiber, insect resistance and glyphosate tolerance for the Brazilian Savanna

Cultivar BRS 433FL B2RF is a high-quality fiber upland cotton with high lint yield potential and resistance to several diseases. The introgressed transgenic events Bollgard II® and Roundup Ready Flex® confer enhanced resistance to lepidopteran pests and tolerance to the herbicide glyphosate, respectively. Fiber length exceeds 32.5 mm and fiber strength 33 gf tex-1.


INTRODUCTION
The major objectives of cotton breeding programs are to improve yield, pest resistance and fiber quality of cultivars capable of meeting synthetic fiber competition and suited for the new spinning technology (Morello et al. 2010, Smith et al. 2011, Morello et al. 2012. Thus, an enhanced fiber quality is essential for the competitiveness of cotton fiber in the textile industry. Moreover, along with conventional agronomic traits (Suassuna et al. 2020), commercial biotechnological traits must also be improved in tropical environments to reinforce the pest and weed management and raise the competitiveness of Brazilian cotton (Suassuna et al. 2018).
Gossypium hirsutum L. latifolium Hutch, also called upland cotton, accounts for over 90% of the cotton fiber global production. Since the 1990s, the cotton production area in the Brazilian cerrado (savanna) has expanded, mainly as a result of the breeding efforts to develop regionally adapted, high-yielding cultivars as well as improved management practices (Morello et al. 2015, Silva Neto et al. 2016, Barroso et al. 2017, Suassuna et al. 2018, which have made the Brazilian cotton internationally competitive. High pest pressure and weed competition occur in cerrado region (Salgado et al. 2002, Bentivenha et al. 2016, reducing the crop profitability. Therefore, the Embrapa´s cotton breeding program focus on to develop high-yielding cotton cultivars with superior fiber quality and with transgenic insect resistance and herbicide tolerance, adapted to the Brazilian cerrado.

GENETIC ORIGIN AND DEVELOPMENT
Cultivar BRS 433FL B2RF was developed by backcrossing BRS 336 (recurrent parent), followed by pedigree selection for the incorporation of the events MON 15985 (B2) and MON 88913 (RF). Cultivar BRS 336 [(Chaco 520 x BRS Itaúba) x Delta Opal] has an outstanding fiber quality, high fiber yield, is well-adapted to the climate of the savanna and semiarid regions in central Brazil and resistant to bacterial blight (causal agent Xanthomonas citri subsp. malvacearum) and to cotton blue disease (causal agent Cotton Leafroll Dwarf Virus) (Morello et al. 2012). The donor, DP 164 B2RF {DP 565/3*[DP 565 x (Cocker 312 RF x DP 50 B2)]} is a high-yielding cotton cultivar, in spite of the medium fiber length (2.5% SL = 29.5 mm) and medium fiber strength (28.5 gf tex -1 ) and is susceptible to bacterial blight and cotton blue disease virus. Cultivar BRS 433FL B2RF was originated from a hybridization of BRS 336 and DP 164 B2RF, followed by three backcrosses (BRS 336/4*DP 164 B2RF). Initial cross was performed in 2011, and the crosses BC 1 , BC 2 and BC 3 from 2012 to 2013. Plants of BC 3 F 1 and BC 3 F 2 were selfed in controlled environment (nethouses). From BC 3 F 3 population were selected B2 and RF homozygous plants using a specific real-time PCR assay.
From these selected plants, 168 progeny rows (BC 3 F 3:4 ) were tested in the 2013/2014 growing season. Cultivar BRS 433FL B2RF was derived from a F 3:4 progeny row (CNPA GO 2014 33 B2RF) after two successive single plant selections in the BC 3 F 2 and BC 3 F 3 generations. Individual plants were selected for fiber quality, lint yield, disease resistance (cotton blue disease and bacterial blight) and plant architecture (height, lodging tolerance, length of branches etc.). Progeny CNPA GO 2014 33 B2RF (F 4 ) was grown in the second season of 2014 in a nethouse and evaluated in field tests at two locals as advanced breeding line (F 5

PERFORMANCE CHARACTERISTICS
Cultivar BRS 433FL B2RF has similar height and maturity as cv. BRS 336 when grown in São Desidério (lat 12° 05' 33" S, long 45° 48' 08" W, alt 776 m asl), BA. "BRS 433FL B2RF" grows to a medium/tall plant height (118 -135 cm at maturity) when treated with 50 -75 g ai of a plant growth regulator. "BRS 433FL B2RF" has trichomes on main stem and on the leaves. The leaves are normal-shaped and have glands and nectaries. Bracts have more than 12 lobes. Generally, the first fruiting branch is on the fifth node, and the plant has a pyramidal morphology. Flowers petals, anthers and pollen are cream-colored. Green fruits when completely developed are elliptical and generally have five and occasionally four locules. Open bolls resist shattering and are suitable for mechanical harvesting, but are not stormproof. The lint is white and the seeds have a white fuzz as well.
First flowers of the cultivar appear 55 to 60 days after emergence (DAE), and the first open boll appear at about 110 to 120 DAE (at 776 m asl). (Table 1). Under the evaluated environmental conditions and application of harvest-aid treatments, harvesting is possible between 165 and 180 DAE.
"BRS 433FL B2RF" is resistant to cotton blue disease, caused by Cotton leafroll dwarf virus-CLRDV and bacterial blight, caused by the bacterium Xanthomonas citri subsp. malvacearum. The cultivar is moderately susceptible to ramularia leaf spot, caused by the fungi Ramulariopsis pseudoglycines, and susceptible to ramulosis, caused by the fungi Colletotrichum gossypii var. cephalosporioides (Table 1). Foliar fungicide sprays are required in order to reduce losses due to ramularia leaf spot. Cotton blue disease and bacterial blight resistance were assessed in two independent field trials without control of the virus vector (Aphis gossypii). All evaluated plants showed no symptoms of bacterial blight. Likewise, no typical symptoms of cotton blue disease were recorded on evaluated plots. Additionally genotyping tests were performed with the molecular markers linked to resistance genes to cotton blue disease and bacterial blight, DC20027 and CIR 246, respectively . Ramularia leaf spot was assessed on a 1 -5 severity score scale based on the leaf lesion area, ranging from 1 ( resistant) to 5 (extremely susceptible) in two field trials without chemical control. Cotton ramulosis resistance was evaluated on artificially inoculated plants on controlled environment, using a methodology proposed by Cia et al. (2002).    (2015); d Disease severity (grades from 1 = resistant to 5 = highly susceptible) -data of two trials without fungicide application. Resistance to bacterial blight was confirmed by using SNP markers ); e Relative disease index, where 1.0 is resistant and 0 is susceptible (Cia et al. 2002) -data of plants artificially inoculated with Colletotrichum gossypii var. cephalosporioides; f Cotton cultivar IMA 1318 was used as a resistant/ partial resistant check.
In the growing seasons 2016/2017 and 2017/2018, cv. BRS 433FL B2RF was compared with cv. DP 1536 B2RF with the same biotechnology traits and improved fiber quality. In 2016/2017, fiber length of cv. BRS 433FL B2RF was significantly longer (33.2 mm) than of cv. DP 1536 B2RF (30.6 mm). Both cultivars had a high fiber strength (Table 3), without significant differences for this trait. In this season, the overall yield means were higher than in the previous ones. Averaged across seven trials, cottonseed yield and lint yield of cv. BRS 433FL B2RF and DP 1536 B2RF were not significantly different (Table 3), although DP 1536 had a higher lint percentage. In the 2017/2018 growing season, fiber length was significantly higher in BRS 433FL B2RF (32.4 mm) than in DP 1536 B2RF (30.8 mm), while fiber strength was statistically the same again. There were no significant differences in cottonseed yield; however, due to the difference in lint percentage, the lint yield of cv. DP 1536 B2RF was higher (Tables 3 and 5). With a relatively low lint percentage, the cottonseed yield potential of cv. BRS 433FL B2RF is very high, enough to ensure high fiber productivity (Tables 3 and 5). On the other hand, high volume instrument (HVI) measurements consistently indicated superior physical properties for an upland cotton cultivar, e.g., mean micronaire reading of 4.4; fiber length (UHML) 32.8 mm and mean fiber strength 33.9 gf tex -1 . This fiber quality is comparable to the obsolete low-yielding long staple cultivar BRS Acácia (Acala type) and the recurrent parent BRS 336 (Table 1).
The accepted base for upland cotton is a mean fiber length of 27.8 mm (Smith et al. 2011). Fiber length has long been considered one of the primary determinants of cotton quality. Staple length influences yarn strength and yarn uniformity. Individual fibers are converted to yarn by a series of techniques of twisting the fiber strands. Longer fibers can be twisted around each other more than shorter fibers. This layering reinforces the yarn, making it more resistant to breakage. Thus, yarn made from long is stronger than yarn made from short fibers. Fiber strength also affects the types of cotton products that can be made from yarn. Therefore, cv. BRS 433FL B2RF may be useful in the cotton industry, for the production of high-quality yarns and fabrics and would achieve high fiber prices. With regard to breeding, cv. BRS 433FL B2RF can be used in cotton breeding programs as a donor parent with high-quality fiber.

SEED MAINTENANCE AND DISTRIBUTION
Cultivar BRS 433FL B2RF was registered by the Ministry of Agriculture, Livestock and Food Supply (no 36253). Foundation seed is produced by Embrapa and licenced seed companies are responsible for the certified seed production, together with Embrapa.