David R. Gealy

Gene Flow Between Red Rice (Oryza sativa) and Herbicide-Resistant Rice (O. sativa): Implications for Weed Management1

Red rice has long been a troublesome, conspecific weed of cultivated rice. Rice varieties carrying certain herbicide-resistant traits acquired through genetic modification (herbicide-resistant varieties) now offer new options for red rice control. In concert with this innovation is the risk of gene flow, which can result in the transfer of that specific herbicide resistance to red rice and thus render this weed control measure ineffective. Gene flow in concept is simple, however, the parameters that determine the establishment of a new trait in a weed population are complex. Cross-pollination to make hybrid seed and the subsequent fate of those hybrid families in the general weed population are some of the biological factors that influence gene flow between red rice and cultivated rice. Natural outcrossing among rice plants is generally low. Most of the pollen dispersal studies published to date indicated that rice × rice outcrossing rates were less than 1.0%. Numerous reports summarized in this study suggest that outcrossing rates between rice and red rice can be highly variable but usually are similar to or lower than this level. However, once hybrids form, they may introgress into a red rice population within only a few generations. If hybrid seed families are to persist and establish herbicide-resistant red rice populations, they must successfully compete in the crop–weed complex. The ability to survive a herbicide applied to a herbicide-resistant rice variety would be a strong selective advantage for these hybrid families. Thus, the well-established principles of weed resistance management appear to be relevant for herbicide-resistant crop systems and should be used in combination with practices to minimize coincident flowering to mitigate the potential impact of gene flow from herbicide-resistant rice into red rice. For the rice–red rice crop–weed complex, there are both biological factors and agricultural practices that can work together to preserve these new weed control options. Nomenclature: Red rice, Oryza sativa L. #3 ORYSA; rice, Oryza sativa L. Additional index words: Genetically modified organism, glufosinate, glyphosate, herbicide tolerant, imazethapyr, introgression, outcrossing, pollen transfer, transgenic rice. Abbreviations: ALS, acetolactate synthase (EC 4.1.3.18); bar, bialophos resistant; EPA, Environmental Protection Agency; GMO, genetically modified organism; PCR, polymerase chain reaction; USDA, U.S. Department of Agriculture.

Food security and climate change: on the potential to adapt global crop production by active selection to rising atmospheric carbon dioxide

Agricultural production is under increasing pressure by global anthropogenic changes, including rising population, diversion of cereals to biofuels, increased protein demands and climatic extremes. Because of the immediate and dynamic nature of these changes, adaptation measures are urgently needed to ensure both the stability and continued increase of the global food supply. Although potential adaption options often consider regional or sectoral variations of existing risk management (e.g. earlier planting dates, choice of crop), there may be a global-centric strategy for increasing productivity. In spite of the recognition that atmospheric carbon dioxide (CO2) is an essential plant resource that has increased globally by approximately 25 per cent since 1959, efforts to increase the biological conversion of atmospheric CO2 to stimulate seed yield through crop selection is not generally recognized as an effective adaptation measure. In this review, we challenge that viewpoint through an assessment of existing studies on CO2 and intraspecific variability to illustrate the potential biological basis for differential plant response among crop lines and demonstrate that while technical hurdles remain, active selection and breeding for CO2 responsiveness among cereal varieties may provide one of the simplest and direct strategies for increasing global yields and maintaining food security with anthropogenic change.

Field evaluation of seed production, shattering, and dormancy in hybrid populations of transgenic rice (Oryza sativa) and the weed, red rice (Oryza sativa).

The genetic and agronomic consequences of transferring glufosinate (Liberty) herbicide resistance from transgenic rice (Oryza sativa L.) lines to the noxious weed red rice (Oryza sativa L.) were evaluated under field conditions. Replicated field trials in Louisiana (LA) and Arkansas (AR) were conducted in 1997 to evaluate ten vegetative and reproductive traits of eight F(2) populations produced from controlled crosses of two transgenic, glufosinate-resistant rice lines and four red rice biotypes. Plant vigor and plant density at both locations were similar among populations derived from either transgenic or non-transgenic parents. Significant differences in plant height and maturity were observed among LA populations produced from transgenic lines when compared to corresponding populations developed from non-transgenic material. However, values for these traits were not greater than those detected in the red rice biotypes. Seed dormancy and seed production were not significantly different at either location among transgenic and non-transgenic populations. Dominant Mendelian segregation of glufosinate resistance was detected in 40% of the populations evaluated. Results of this study indicated that those populations segregating for glufosinate resistance responded in a location-specific manner with respect to life history and fecundity traits.

Rice and red rice interference. II. Rice response to population densities of three red rice (Oryza sativa) ecotypes

Abstract Red rice, which grows taller and produces more tillers than domestic rice and shatters most of its seeds early, is a major weed in many rice-growing areas of the world. Field experiments were conducted at Stuttgart, AR in 1997 and 1998 to evaluate the growth response of the Kaybonnet (KBNT) rice cultivar to various population densities of three red rice ecotypes. The ecotypes tested were Louisiana3 (LA3), Stuttgart strawhull (Stgstraw), and Katy red rice (KatyRR). Compared with KBNT alone, LA3, the tallest of the three red rice ecotypes, reduced tiller density of KBNT 51%, aboveground biomass at 91 d after emergence (DAE) 35%, and yield 80%. Stgstraw, a medium-height red rice, reduced KBNT tiller density 49%, aboveground biomass 26%, and yield 61%. KatyRR, the shortest red rice, reduced KBNT tiller density 30%, aboveground biomass 16%, and yield 21%. Tiller density of rice was reduced by 20 to 48% when red rice density increased from 25 to 51 plants m−2. Rice biomass at 91 DAE was reduced by 9 and 44% when red rice densities were 16 and 51 plants m−2. Rice yield was reduced by 60 and 70% at red rice densities of 25 and 51 plants m−2, respectively. These results demonstrate that low populations of red rice can greatly reduce rice growth and yield and that short-statured red rice types may affect rice growth less than taller ecotypes. Nomenclature: Red rice, Oryza sativa L. ORYSA, ‘KatyRR’, ‘LA3’, ‘Stgstraw’; rice, Oryza sativa L., ‘Kaybonnet’.

Growth Response of Rice (Oryza sativa) and Red Rice (O. sativa) in a Replacement Series Study1

A replacement series study was conducted in a greenhouse in 1998 and 1999 to evaluate the interference interactions among two rice cultivars and two red rice ecotypes. Plants were established in proportions of 3:0, 2:1, 1:2, and 0:3 (rice–red rice) plants/pot. Relative yield of Kaybonnet based on the shoot dry weight was lower than that of KatyRR or LA3, whereas PI 312777 was comparable to that of KatyRR and LA3. These results indicate that Kaybonnet was less competitive than PI 312777 when contrasted with KatyRR and LA3 red rice ecotypes. Kaybonnet (commercial rice cultivar) was dominated by both KatyRR (suspected rice × red rice cross) and LA3 (tall red rice ecotype) in tiller production, whereas PI 312777 (weed-suppressive cultivar) was comparable to either KatyRR or LA3. Both KatyRR and LA3 considerably reduced the leaf area of Kaybonnet. In contrast, PI 312777 reduced the growth of KatyRR, and its leaf area was comparable to that of LA3. The data suggest that high tillering capacity, as demonstrated by PI 312777, should be considered when breeding for rice cultivars that are competitive against weeds. This agronomic characteristic of rice may improve the success of reduced herbicide rate application programs. Nomenclature: Red rice, Oryza sativa L. #3 ORYSA ‘KatyRR’, ‘LA3’; rice, Oryza sativa L. ‘Kaybonnet’, ‘PI 312777’. Additional index words: Leaf area, red rice growth, relative yield, rice growth, strawhull. Abbreviations: DAE, days after emergence; RY, relative yield; RYT, relative yield total.

Factors Affecting the Outcrossing Rate between Clearfield™ Rice and Red Rice (Oryza sativa)

Abstract The commercialization of imazethapyr-resistant (Clearfield™, CL) rice in the southern United States has raised serious concerns about gene flow to red rice, producing imazethapyr-resistant red rice populations. Our objectives were to determine the impact of planting date, CL cultivars, and red rice biotypes on outcrossing rate; and to investigate the relative contribution of flowering time of CL rice and red rice biotypes, together with air temperature and relative humidity (RH), on outcrossing rate. Field experiments were conducted at Stuttgart, Rohwer, and Kibler, AR, from 2005 to 2007, at three or four planting times from mid-April to late May. ‘CL161’ (inbred cultivar) and ‘CLXL8’ (hybrid) rice were planted in nine-row plots, with red rice planted in the middle row. Twelve red rice biotypes were used. The flowering of red rice and CL rice, air temperature, and RH were recorded. Red rice seeds were collected at maturity. To estimate outcrossing rate, resistance to imazethapyr was evaluated in subsequent years and confirmed using rice microsatellite markers. CLXL8 rice flowered 2 to 4 d earlier than CL161 rice, and flowering was completed within 1 wk in all plantings. The flowering duration of most red rice biotypes ranged from 4 to 17 d. Flowering synchrony of red rice biotypes and CL rice ranged from 0 to 100% at different plantings. In general, CLXL8 had greater flowering overlap and higher outcrossing rate with red rice than did CL161 rice. The outcrossing rate of red rice biotypes ranged from 0 to 0.21% and 0 to 1.26% with CL161 and CLXL8 rice, respectively. The outcrossing rate differed within each planting date (P < 0.05). Outcrossing was generally lower in mid-May and late May than in mid-April and late April planting times. Flowering synchrony and outcrossing rate were not correlated (r2 < 0.01). Outcrossing with CL161 was primarily influenced by red rice biotype. A minimum air temperature of > 24 C in the evening also favors outcrossing with CL161. With CLXL8 rice, outcrossing was most affected by RH. When RH was < 54%, outcrossing was less (0.12%) than when RH was ≥ 54% (0.38%). With CLXL8 rice, a minimum RH of ≥ 54%, from mid-morning to noon, increased outcrossing with red rice. To fully understand the interaction effects of these factors on outcrossing with red rice, controlled experiments are needed. Nomenclature: Imazethapyr; Red rice, Oryza sativa L.; Rice, Oryza sativa L. ORYSA.

Red rice (Oryza sativa) emergence characteristics and influence on rice yield at different planting dates.

Abstract Cultivated rice yield losses due to red rice infestation vary by cultivar, red rice density, and duration of interference. The competition effects of red rice could be influenced further by emergence characteristics, red rice biotype, and planting time of cultivated rice. We aimed to characterize the emergence of red rice biotypes at different planting dates and evaluate the effect of red rice biotype, rice cultivar, and planting date on cultivated rice yield loss. Field experiments were conducted at the Southeast Research and Extension Center, Rohwer, AR, and at the Arkansas Rice Research and Extension Center, Stuttgart, AR, in the summer of 2005 and 2006. The experimental design was a split-split plot with three or four replications. Planting time, ClearfieldTM (CL) rice cultivar, and red rice biotype were the main plot, subplot, and sub-subplot factors, respectively. There were three planting times from mid-April to mid-May at 2-wk intervals. CL rice cultivars, CL161 and hybrid CLXL8, and 12 red rice biotypes were planted. The emergence rate and coefficient of uniformity of germination differed among some red rice biotypes within a planting time. Planting date affected the emergence characteristics of red rice biotypes. The mean emergence rate of red rice was 0.043 d−1 in the mid-April planting and 0.058 d−1 in the late April planting. For the mid-April planting, 50% of red rice biotypes emerged in 20 ± 2 d compared with 15 ± 2 d for CL rice cultivars. Yield losses due to red rice biotypes generally increased in later planting dates, up to 49%. Yield losses due to interference from red rice biotypes ranged from 14 to 45% and 6 to 35% in CL161 and CLXL8, respectively. Cultivated rice became less competitive with red rice in later plantings, resulting in higher yield losses. Nomenclature: Red rice, Oryza sativa L. ORYSA; rice, Oryza sativa L. ‘CL161’, ‘CLXL8’

Maximum Outcrossing Rate and Genetic Compatibility between Red Rice (Oryza sativa) Biotypes and Clearfield™ Rice

Abstract The transfer of the imazethapyr-resistant gene from Clearfield™ (CL) rice to red rice is an ecological risk. Flowering synchronization and genetic compatibility between cultivated rice and red rice could influence gene transfer. We examined the (1) variability in maximum outcrossing rate between 12 red rice biotypes and ‘CL161’ rice during their peak flowering overlap in the field and (2) genetic compatibility of red rice biotypes with CL161 rice. Experiments were conducted at Stuttgart, AR, and Fayetteville, AR, from 2005 to 2007. To evaluate the flowering synchrony of red rice and CL161 rice as well as its impact on outcrossing rate, field experiments were conducted at four planting times from early April to late May. The red rice biotypes were planted in the middle row of nine-row CL161 plots and flowering was monitored. Outcrosses were evaluated in subsequent years by herbicide response and simple-sequence-repeat marker assays. To determine compatibility, manual crosses were performed between 12 red rice biotypes and CL161 rice in the greenhouse. The flowering duration of all red rice types ranged from 5 to 16 d after the onset of flowering in contrast to 6 d in CL161 rice. Ten of the twelve types of red rice had ≥ 70% overlap in flowering time with CL161 rice in at least one planting date. The maximum field outcrossing rate between red rice biotypes and CL161 ranged from 0.03 to 0.25%. The field outcrossing rate between red rice biotypes differed (P < 0.01), but flowering synchronization was not directly related to outcrossing rate. Manual crosses resulted in seed sets of 49 to 94%. The majority of red rice biotypes had similar compatibility with CL161 rice. Thus, other factors must contribute to hybridization rates in the field. Follow-up experiments should investigate other plant factors and environmental influence on hybridization rate. Nomenclature: Imazethapyr; red rice, Oryza sativa L. ORYSA; rice Oryza sativa ‘CL161’.

Gene flow from weedy red rice (Oryza sativa L.) to cultivated rice and fitness of hybrids

BACKGROUND Gene transfer from weeds to crops could produce weedy individuals that might impact upon the evolutionary dynamics of weedy populations, the persistence of escaped genes in agroecosystems and approaches to weed management and containment of transgenic crops. The present aim was to quantify the gene flowrate from weedy red rice to cultivated rice, and evaluate the morphology, phenology and fecundity of resulting hybrids. Field experiments were conducted at Stuttgart and Rohwer, Arkansas, USA. Twelve red rice accessions and an imazethapyr-resistant rice (Imi-R; Clearfield) were used. RESULTS Hybrids between Imi-R rice x red rice were 138-150 cm tall and flowered 1-5 days later than the rice parent, regardless of the red rice parent. Hybrids produced 20-50% more seed than the rice parent, but had equivalent seed production to the majority of red rice parents. Seeds of all hybrids were red, pubescent and dehisced at maturity. For the majority of hybrids, seed germination was higher than that of the red rice parent. The gene flowrate from red rice to rice was 0.01-0.2% and differed by red rice biotype. The hybrids had higher fecundity and potential competitive ability than the rice parent, and in some cases also the red rice parent. CONCLUSIONS Red rice plants are vectors of gene flow back to cultivated rice and other weedy populations. The progeny of red rice hybrids from cultivated rice mother plants have higher chances of persistence than those from red rice mother plants. Gene flow mitigation strategies should consider this scenario.

The Impact of Herbicide-Resistant Rice (Oryza sativa L.) Technology on Phenotypic Diversity and Population Structure of US Weedy Rice

Use of herbicide-resistant rice cultivars in the United States has led to the emergence of herbicide-resistant weedy rice formed predominantly by hybridization of cultivars with historical weeds characterized by black hulls and awns. The use of herbicide-resistant (HR) Clearfield rice (Oryza sativa) to control weedy rice has increased in the past 12 years to constitute about 60% of rice acreage in Arkansas, where most U.S. rice is grown. To assess the impact of HR cultivated rice on the herbicide resistance and population structure of weedy rice, weedy samples were collected from commercial fields with a history of Clearfield rice. Panicles from each weedy type were harvested and tested for resistance to imazethapyr. The majority of plants sampled had at least 20% resistant offspring. These resistant weeds were 97 to 199 cm tall and initiated flowering from 78 to 128 d, generally later than recorded for accessions collected prior to the widespread use of Clearfield rice (i.e. historical accessions). Whereas the majority (70%) of historical accessions had straw-colored hulls, only 30% of contemporary HR weedy rice had straw-colored hulls. Analysis of genotyping-by-sequencing data showed that HR weeds were not genetically structured according to hull color, whereas historical weedy rice was separated into straw-hull and black-hull populations. A significant portion of the local rice crop genome was introgressed into HR weedy rice, which was rare in historical weedy accessions. Admixture analyses showed that HR weeds tend to possess crop haplotypes in the portion of chromosome 2 containing the ACETOLACTATE SYNTHASE gene, which confers herbicide resistance to Clearfield rice. Thus, U.S. HR weedy rice is a distinct population relative to historical weedy rice and shows modifications in morphology and phenology that are relevant to weed management.