Weimin Dai

Innate factors causing differences in gene flow frequency from transgenic rice to different weedy rice biotypes

BACKGROUND The compatibility and outcrossing rates between transgenic rice and weedy rice biotypes have been studied in some previous cases. However, few studies have addressed the reasons for these differences. The present study compared the compatibility and outcrossing rates between transgenic rice and selected weedy rice biotypes using manual and natural crossing experiments to elucidate the key innate factors causing the different outcrossing rates. RESULTS Hybrid seed sets from manual crossing between transgenic rice and weedy rice varied from 31.8 to 82.7%, which correlated directly with genetic compatibility. Moreover, the significant differences in the quantity of germinated donor pollens and pollen tubes entering the weedy rice ovule directly contributed to the different seed sets. The natural outcrossing rates varied from 0 to 6.66‰. The duration of flowering overlap was the key factor influencing natural outcrossing. Plant and panicle height also affected outcrossing success. CONCLUSION From this study, it is concluded that the likelihood of gene flow between transgenic rice and weedy rice biotypes is primarily determined by floral synchronisation and secondarily influenced by genetic compatibility and some morphological characteristics.

A comparative study of competitiveness between different genotypes of weedy rice (Oryza sativa) and cultivated rice

BACKGROUND Competition from weedy rice can cause serious yield losses to cultivated rice. However, key traits that facilitate competitiveness are still not well understood. To explore the mechanisms behind the strong growth and competitive ability, replacement series experiments were established with six genotypes of weedy rice from different regions and one cultivated rice cultivar. RESULTS (1) Weedy rice from southern China had the greatest impact on growth and yield of cultivated rice throughout the entire growing season. Weedy rice from the northeast was very competitive during the early vegetative stage while the competitive effects of eastern weedy rice were more detrimental at later crop-growth stages. (2) As the proportion of weedy rice increased, plant height, tillers, above-ground biomass, and yield of cultivated rice significantly declined; the crop always being at disadvantage regardless of proportion. (3) Weedy biotypes with greater diversity as estimated by their Shannon indexes were more detrimental to the growth and yield of cultivated rice. CONCLUSION Geographic origin (latitude) of weedy rice biotype, its mixture proportion under competition with the crop and its genetic diversity are determinant factors of the outcome of competition and the associated decline in the rice crop yield.

Cytoplasmic-genetic male sterility gene provides direct evidence for some hybrid rice recently evolving into weedy rice

Weedy rice infests paddy fields worldwide at an alarmingly increasing rate. There is substantial evidence indicating that many weedy rice forms originated from or are closely related to cultivated rice. There is suspicion that the outbreak of weedy rice in China may be related to widely grown hybrid rice due to its heterosis and the diversity of its progeny, but this notion remains unsupported by direct evidence. We screened weedy rice accessions by both genetic and molecular marker tests for the cytoplasmic male sterility (CMS) genes (Wild abortive, WA, and Boro type, BT) most widely used in the production of indica and japonica three-line hybrid rice as a diagnostic trait of direct parenthood. Sixteen weedy rice accessions of the 358 tested (4.5%) contained the CMS-WA gene; none contained the CMS-BT gene. These 16 accessions represent weedy rices recently evolved from maternal hybrid rice derivatives, given the primarily maternal inheritance of this trait. Our results provide key direct evidence that hybrid rice can be involved in the evolution of some weedy rice accessions, but is not a primary factor in the recent outbreak of weedy rice in China.

A model of the relationship between weedy rice seed-bank dynamics and rice-crop infestation and damage in Jiangsu Province, China.

BACKGROUND A heavy infestation of weedy rice leading to no harvested rice has never been predicted in China due to a lack of knowledge about the weedy rice seed bank. We studied the seed-bank dynamics of weedy rice for three consecutive years and analyzed the relationship between seed-bank density and population density in order to predict future weedy rice infestations of direct-seeded rice at six sites along the Yangtze River in Jiangsu Province, China. RESULTS The seed-bank density of weedy rice in all six sites displayed an increasing trend with seasonal fluctuations. Weedy rice seeds found in the 0-10 cm soil layer contributed most to seedling emergence. An exponential curve expressed the relationship between cultivated rice yield loss and adult weedy rice density. Based on data collected during the weedy rice life-cycle, a semi-empirical mathematic model was developed that fits well with the experimental data in a way that could be used to predict seed-bank dynamics. CONCLUSIONS By integrating the semi-empirical model and the exponential curve, weedy rice infestation levels and crop losses can be predicted based on the seed-bank dynamics so that a practical control can be adopted before rice planting.

Allele Types of Rc Gene of Weedy Rice from Jiangsu Province, China

Weedy rice (Oryza sativa f. spontanea), the predominant type of which has a red pericarp, seriously inhibits growth and yield of direct-seeded rice in Jiangsu Province, China. In this study, we randomly selected 10 weedy rice accessions from 10 plots in Jiangsu, and then sequenced the full lengths of their Rc genes (approximately 6.4 kb). In addition, we collected 166 different full-length Rc genes in the Oryza genus from the literature and from GenBank. A collinearity sequence analysis showed that the 10 weedy rice accessions from Jiangsu all had the same wild-type allele of the Rc gene. Single nucleotide polymorphisms indicated that the nucleotide polymorphisms (Π = 0.19) and the proportion of segregation sites (θw = 0.28) of the Rc genes in the 10 weedy rice accessions from Jiangsu were higher than those in 56 weedy rice accessions from USA (Π = 0.09 and θw = 0.07). Haplotype and phylogenetic analyses showed that the Rc genes of weedy rice accessions from Jiangsu were not revertants of the rc gene found in Asian cultivated rice (O. sativa) varieties with white pericarp. In addition, Rc gene sequences of the rice varieties Lvdao from Lianyungang, Jiangsu and Tangdao from Anhui were more similar to those of cultivated rice than to the weedy rice from Jiangsu. These findings support the continued quarantine of weedy rice and clarify the evolutionary mechanism of the red pericarp found in the weedy rice of Jiangsu.

Early flowering and rapid grain filling determine early maturity and escape from harvesting in weedy rice

BACKGROUND Early maturity is an important trait that is essential to the survival of weedy rice. To explore the mechanism of early maturity in weedy rice, the reproductive development of a large sample of weedy rice accessions and cultivars was compared in a common garden study. A selected sample of both weedy and cultivated rice was sown at different dates in two years to study in more detail their flowering and grain-filling patterns. RESULTS The weedy rice from three major cropping regions matured 7-8 days earlier than their associated cultivars. Representative weedy rice accessions planted on conventional sowing dates flowered 3-26 days earlier than cultivars; delayed sowing caused divergence in the flowering regimes in weedy rice. However, regardless of the sowing date, weedy rice filled its grain 7-21 days faster than cultivars in both study years. Vegetative and reproductive traits of weedy and cultivated rice have different patterns of variation with delayed planting. CONCLUSION Early maturity is an essential factor determining the persistence of weedy rice by contributing to the escape of its seed from being harvested with the rice crop. Both early flowering and shorter grain-filling stages determine early maturity, and flowering is more plastic than grain filling.

Feral rice from introgression of weedy rice genes into transgenic herbicide-resistant hybrid-rice progeny

Pollen-mediated transgenic flow of herbicide resistance occurs bidirectionally between transgenic cultivated rice and weedy rice. The potential risk of weedy traits introgressing into hybrid rice has been underestimated and is poorly understood. In this study, two glufosinate-resistant transgenic rice varieties, hybrid rice (F1), and their succeeding generations (F2-F4) were planted for 3 years in field plots free of weedy rice adjacent to experimental weedy-rice fields. Weedy-rice-like (feral) plants that were both glufosinate-resistant and had red-pericarp seed were initially found only among the F3 generations of the two glufosinate-resistant transgenic hybrid cultivars. The composite fitness (an index based on eight productivity and weediness traits) of the feral progeny was significantly higher than that of the glufosinate-resistant transgenic hybrid (the original female parent of the feral progeny) under monoculture common garden conditions. The hybrid rice progeny segregated into individuals of variable height and extended flowering. The hybrid rice F2 generations had higher outcrossing rates by pollen reception (0.96-1.65%) than their progenitors (0.07-0.98%). The results show that herbicide-resistant weedy rice can rapidly arise by pollen-mediated gene flow from weedy to transgenic hybrid rice, and their segregating pollen-receptive progeny pose a greater agro-ecological risk than transgenic varieties. The safety assessment and management regulations for transgenic hybrid rice should take into account the risk of bidirectional gene flow.

Transgenic restorer rice line T1c-19 with stacked cry1C*/bar genes has low weediness potential without selection pressure

Abstract Stacked (insect and herbicide resistant) transgenic rice T1c-19 with cry1C*/bar genes, its receptor rice Minghui 63 (herein MH63) and a local two-line hybrid indica rice Fengliangyou Xiang 1 (used as a control) were compared for agronomic performance under field conditions without the relevant selection pressures. Agronomic traits (plant height, tiller number, and aboveground dry biomass), reproductive ability (pollen viability, panicle length, and filled grain number of main panicles, seed set, and grain yield), and weediness characteristics (seed shattering, seed overwintering ability, and volunteer seedling recruitment) were used to assess the potential weediness without selection pressure of stacked transgene rice T1c-19. In wet direct-seeded and transplanted rice fields, T1c-19 and its receptor MH63 performed similarly regarding vegetative growth and reproductive ability, but both of them were significantly inferior to the control. T1c-19 did not display weed characteristics; it had weak overwintering ability, low seed shattering and failed to establish volunteers. Exogenous insect and herbicide resistance genes did not confer competitive advantage to transgenic rice T1c-19 grown in the field without the relevant selection pressures.