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a USDA-ARS Rice Research Unit, 1509 Aggie Dr., Beaumont, TX 77713
b LSU AgCenter, Agronomy Dep., Baton Rouge, LA 70803
c LSU AgCenter, 1373 Caffey Road, Rayne, LA 70578
d RiceTec, Inc., P.O. Box 1305, Alvin, TX 77512
e USDA-ARS-DBNNRC, P.O. Box 287, Stuttgart, AR 72160
f IRRI and Chinese Academy of Agricultural Sciences, Institute of Crop Sciences, Beijing, 100081, China. Mention of a trademark or proprietary product does not constitute a guarantee or warranty of the product by the U.S. Department of Agriculture and does not imply its approval to the exclusion of other products that also can be suitable
* Corresponding author (shannon.pinson{at}ars.usda.gov).
ABSTRACT
Three rice (Oryza sativa L.) germplasm lines designated as TIL:455 (Reg. No. GP-112, PI 642943), TIL:514 (Reg. No. GP-113, PI 642944), and TIL:642 (Reg. No. GP-114, PI 642945) contain a total of eight novel alleles with sheath blight (caused by Rhizoctonia solani Kühn) resistance backcrossed into an elite U.S. rice genetic background having good grain quality traits, early maturity, and glabrous leaves and hulls. These three germplasm lines were jointly released in 2007 by the USDA-ARS, the Louisiana State University Agricultural Center, and the International Rice Research Institute. The novel sheath blight resistance alleles contained in these lines originated from ‘TeQing’ (PI 536047), a high-yielding rice cultivar from China that is well known in the United States for its strong resistance to sheath blight disease. With the aim of identifying improved germplasm suitable for introducing novel sheath blight resistance alleles into tropical japonica breeding programs, a combination of phenotypic and molecular data was used to select three lines from among 262 random backcross introgression lines.
Abbreviations: IRRI, International Rice Research Institute • LSU AgCenter, Louisiana State University Agricultural Center • QTL, quantitative trait locus • RFLP, restriction fragment length polymorphism • SB, sheath blight • SBR, sheath blight resistance • SSR, simple sequence repeat • TIL, TeQing-into-Lemont introgression line
Three rice (Oryza sativa L.) germplasm lines designated TIL:455 (Reg. No. GP-112, PI 642943), TIL:514 (Reg. No. GP-113, PI 642944), and TIL:642 (Reg. No. GP-114, PI 642945) contain a total of eight novel sheath blight resistance (SBR) alleles backcrossed into an elite U.S. rice genetic background having good grain quality traits, early maturity, and glabrous leaves and hulls. These three germplasm lines were jointly released in 2007 by the USDA-ARS, the Louisiana State University Agricultural Center (LSU AgCenter, Baton Rouge and Crowley, LA), and the International Rice Research Institute (IRRI, Philippines). The novel SBR alleles contained in these lines originated from ‘TeQing’ (PI 536047), which has strong resistance to sheath blight disease, caused by the fungal pathogen Rhizoctonia solani Kühn. These three lines were selected from among 262 backcross introgression lines developed at IRRI. Selection for germplasm release was based on phenotypic and molecular data collected collaboratively by the USDA-ARS Rice Research Unit, Beaumont TX; the USDA-ARS Dale Bumpers National Rice Research Center, Stuttgart, AR; the LSU AgCenter, Crowley, LA; and RiceTec, Inc., Alvin, TX. The aim was to identify SBR lines that were most suitable for introducing novel SBR alleles into tropical japonica breeding programs, such as those in the United States.
Methods
The three germplasm lines were selected on the basis of both phenotypic and molecular data from among 262 backcross introgression lines first developed at IRRI, being derived from a cross of ‘Lemont’ (PI 475833) by TeQing followed by two to four backcross cycles using Lemont as the recurrent parent, and finishing with three to five generations of selfing. Phenotypic similarity to the recurrent parent was used to determine when to move each line from the backcrossing into the selfing phase. Selection for recurrent phenotype was exercised during the selfed generations. Lemont is an early-maturing semidwarf long-grain cultivar developed by the USDA-ARS in conjunction with the Texas Agricultural Experiment Station at the Texas A&M University Agricultural Research and Extension Center, Beaumont, TX (Bollich et al., 1985). TeQing (also known as Te Qing or Teqing) is a high-yielding rice cultivar from China that is well known in the United States for its high resistance to sheath blight disease (Li et al., 1995; Pinson et al., 2005).
The primary purpose for creating this large set of TeQing-into-Lemont introgression lines (TILs) was to serve as a gene-mapping population that would complement a set of Lemont/TeQing recombinant inbred lines that has been used to map major genes and quantitative trait loci (QTLs) for several economically important traits, including resistance to blast disease, caused by Magnaporthe grisea (anamorph Pyricularia grisea [= P. oryzae]) (Tabien et al., 2000, 2002); bacterial leaf blight disease, caused by Xanthomonas oryzae pv. oryzae (Li et al., 1999); and sheath blight (SB) disease (Li et al., 1995; Pinson et al., 2005). By isolating specific genomic regions from TeQing in a predominantly Lemont genetic background, these TILs were designed to allow researchers to verify the genomic location of putatively mapped QTLs and to measure their genetic effect (i.e., agronomic usefulness) after introgression into a genetic background adapted to the southern United States.
After U.S. quarantine in 2001 and seed increase in 2002, the 262 TILs from IRRI were evaluated in replicated inoculated trials for SBR at the LSU AgCenter Rice Research Station in Crowley, LA, in both 2003 (three replications) and 2004 (four replications). Inoculated field plots were two rows wide by 2.4 m in length. TeQing-into-Lemont introgression lines that appeared resistant or moderately resistant in 2003 were also evaluated at two additional field sites in 2004 (Beaumont and Alvin, TX) and then evaluated again in Beaumont, TX, in 2005 for a total of five year-location environments. These SB evaluations were conducted according to the SB screening nursery specifications in standard use by the pathologists at these locations and included three replications of three-row plots at Beaumont and two replications of two-row plots at Alvin. To enhance stand establishment in the SB plots, seed was treated with 0.6 mL of Vitavax-3F (Uniroyal Chemical Company, Inc., Middleburg, CT), 0.4 mL of Icon 6.2FS (Rhone-Poulenc Ag Co., Research Triangle Park, NC), and 0.02 g of Gibgro 10% Powder (Nufarm Americas, Inc., Burr Ridge, IL) per 100 g of rice seed. The plots were inoculated and rated using standard methods and a disease rating scale of 0 to 9, where each unit of the scale approximates the proportion of aboveground plant length showing disease symptoms (Marchetti and Bollich, 1991). Lemont and ‘Cocodrie’ (PI 606331) were included as susceptible controls, TeQing served as a resistant control, and ‘Saber’ (PI 633624), a U.S. rice variety with TeQing in its parentage (McClung et al., 2004), was used as a moderately resistant control. Increased resistance of the TILs as compared to the recurrent parent, Lemont, was considered indicative of their containing effective TeQing SBR alleles. Dunnet's test was used to compare the SB ratings of the TILs with those for Lemont. As is common among disease rating systems where susceptible plots often appear falsely resistant because of inoculation difficulties, individual replications that were rated two units higher (more resistant) than the other two or three ratings per that same year-location were considered "escapes" and were omitted from the data considered by the Dunnet's test. None of the selected TILs were significantly more resistant than Lemont in all five year-location studies. Disease ratings varied by year-location, with higher incidence in Crowley in 2003 than other studies, and lower disease incidence in Alvin in 2004 than elsewhere (Table 1 ). Not even TeQing was significantly more resistant than Lemont in Alvin, TX, due to low disease incidence, having only two replications, and high variance of response between the replications. Data on plant height, heading time, seed shape, apparent amylose content and alkali spreading value of the grain, glabrous/pubescent leaves, and plant architecture were collected from noninoculated plots grown in 2003, 2004, and 2005 in Beaumont, TX, and planted using untreated seed.
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Molecular marker data indicate that the improved SBR exhibited by Saber results in part from the introgression of TeQing alleles at QTLs on chromosomes 2, 3, and 12 (Table 2, and unpublished data on the rest of the 111 SSR loci observed). Saber also contains the three SBR QTLs is reportedly originating from Lemont and located on chromosomes 8 and 11 (Table 2).
TIL:642, which possesses a single SBR QTL from TeQing located near the centromere on chromosome 12, displayed SBR levels equal to or better than that of Saber. This region of chromosome 12 also contains a major gene for blast resistance in TeQing (Tabien et al., 2002), and accordingly, TIL:642 exhibited resistance to IC-17 and IB-49 races of Pyricularia oryzae (previously P. grisea) in studies using greenhouse inoculations. TIL:642 contains Lemont alleles at 103 out of 111 loci characterized, indicating that it is genetically similar to the recurrent parent. In addition to the QTL-containing region on chromosome 12, small introgressions were also found on chromosomes 1, 3, 8, and 9.
TIL:455 contains four SBR QTLs from TeQing combined with the three QTLs originating from Lemont. While the QTL on chromosome 3 is already available to U.S. rice breeders in Saber, TIL:455 also provides three novel SBR QTLs, on chromosomes 4, 9, and 10, now contained in an early-maturing rice line with grains that are long and clear (not chalky). TIL:455 had slightly increased resistance compared with Saber in four of the five year-locations (Table 1). The phenotype of TIL:455 in uninoculated plots was very similar to that of Lemont, distinguished only by its slightly taller height and sometimes by a slightly later heading time (Table 3 ). At a genotypic level, TIL:455 contained Lemont alleles at 102 of the 111 loci characterized. In addition to the QTL-containing regions already noted, several additional TeQing alleles were also identified on chromosomes 3 and 10.
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Availability
Seed will be maintained and distributed by the USDA-ARS National Small Grains Collection, P.O. Box 307, Aberdeen, ID. Recipients of the seed are asked to make appropriate recognition if this germplasm contributes to research or development of a new germplasm, cultivar, or genetic stock.
Acknowledgments
The Rockefeller Foundation is acknowledged for funding the initial backcrossing and molecular analyses that led to the development of the 262 introgression lines at IRRI.
Footnotes
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.
Received for publication November 2, 2007.
References
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