Developing Multiple Pest Resistant Wheat Germplasm

Principal Investigator(s): Allan K. Fritz, Department of Agronomy
and UNIT(S): 785.532.7245, akf@k-state.edu
John P. Fellers, Department of Plant Pathology
785.532.2367, jfellers@k-state.edu
INTRODUCTION

The USDA-ARS Plant Science Unit, Manhattan, KS has a specific goal to develop multiple pest resistant elite germplasm adapted to the Southern Great Plains. By developing elite material with pest resistance, and releasing those materials to wheat programs throughout the Southern Great Plains, greater emphasis can be placed on agronomic traits, milling and baking quality and performance by each respective state or private breeding program. New combinations of resistance genes, as well as new sources of resistance, can be identified and backcrossed into elite germplasm and varieties grown in the Southern Great Plains. These lines in turn can be used as superior sources of resistant germplasm in varietal crosses without reducing emphasis on quality or yield. The simultaneous development of molecular markers linked to track resistance genes greatly facilitates the ability to develop hard winter wheat germplasm and cultivars having multiple resistance genes. Development and use of molecular markers is particularly important as breeding programs move toward the use of minor rust resistance genes.

Two significant pathogens that attack wheat are leaf rust and Wheat Streak Mosaic Virus. They are responsible for millions of bushels of lost production. Wheat varieties that have been released in the past contain only a few specific leaf rust resistance genes that prevent infection and little to no resistance to WSMV. When resistant varieties are grown over large acreages, a significant selection pressure is placed on the pathogen. Leaf rust tends to overcome resistance to single genes rapidly leading to a shortened usefulness of that particular variety. It is not know whether WSMV adapts quickly due to the lack of resistance.

Breeding programs are now integrating genes that provide a ‘slow’ rusting type of resistance that will provide longer protection. There are two important loci that provide slow rusting resistance to both leaf rust and stripe (yellow) rust, Lr34/Yr18 and Lr46/Yr29. In the last two years these genes have been crossed into adapted germplasm and are being evaluated for resistance. With WSMV, there are two sources of resistance. A translocated chromosome arm of Thinopyum intermedium, present in KS93WGRC27, provides good resistance in field conditions. CO960293 is a Colorado line that contains temperature sensitive virus resistance. The goal of this work is two fold. First is to continue to find better molecular markers for Lr46/Yr29 so that the genes can be integrated more efficiently into adapted germplasm. The second is to screen a collection of CYMMIT synthetic hexaploids for new sources of resistance to WSMV.

OBJECTIVE

The longterm objective of this project is to develop multiple pest resistant germplasm in established elite breeding lines adapted to the Southern Great Plains. This germplasm can then be utilized by breeding programs to improve disease and insect pest resistance in their own respective programs. This program and the germplasm produced is unique because it contains new resistance genes in proven elite lines

RATIONALE AND SIGNIFICANCE

Each year in the Southern Great Plains yields are reduced an estimated 20-25% by both plant diseases and insect damage. Under average cropping conditions, that amounts to an estimated $500 million loss. The results of this loss can be measured not only in reduced on-farm yields, but also as lower milling and baking quality of the harvested crop. Yield loss is usually expressed as smaller kernels and lighter grain with poor intrinsic protein quality. By improving the overall disease and insect resistance backgrounds of wheat grown we can improve the overall quality of the crop.

Leaf rust and stripe (yellow) rust are considered as globally and regionally important diseases in wheat where new pathotypes constantly challenge the efforts of plant breeders. Resistance based on race-specific, major genes has been short-lived in the Southern Great Plains. Using three decades of expertise that CIMMYT has gained in genetic characterization and germplasm improvement activities, as well as, expertise available in the Agronomy, Plant Pathology and USDA-ARS groups at KSU we propose to make further advances to understand the nature of broad-spectrum disease resistance. We also propose to fine map the chromosomal regions that are known to confer broad spectrum resistance to these rust diseases with the goal of isolating the genes involved. We have made substantial progress toward these goals in the case of the durable resistance genes Lr46/Yr29. We have simultaneously been developing germplasm having Lr46/Yr29 in hard winter wheat backgrounds with the goal of releasing germplasm and markers for Lr46/Yr29 to breeders in the region. This gene is currently present in only in unadapted spring wheat germplasm.

Several lines of evidence indicate that the Lr46/Yr29 locus is probably functionally different than the previously identified resistance genes that function in gene-for-gene interactions.

1) The gene provides only partial resistance and the effects are greatest at the adult-plant stage.

2) The resistance gene effects seem to be non-specific, that is, no rust pathotypes have been identified that are completely virulent on lines carrying these genes.

3) The gene does not confer an obvious hypersensitive reaction to the pathogen, as do most genes involved in gene-for-gene interactions.

4) The gene appears to affect the physiology of the plant to a small extent even when the pathogen is not prevalent. This can be observed as a leaf-tip necrosis in some environments.

5) The gene confers resistance to at least two different rust diseases. Lr46 is either identical or tightly linked to Yr29, which has a similar adult-plant resistance phenotype.

Warm winters over the last few years have allowed insect vectors to over winter and lead to larger areas of infection of WSMV. Wheat curl mite (Aceria tosichella) transmits the virus and placed a high infection pressure on currently grown varieties. In 2006, it was estimated that WSMV was responsible for 18.1 % crop loss in west central Kansas (Appel et al. 2006). Included in that figure was a 38% crop loss in susceptible cultivars. As mentioned above, wild wheat relatives are good sources for disease resistance. CYMMIT has made available over 500 synthetic hexaploid lines made from numerous Ae tauschii lines. These have not been screened for WSMV resistance. It may be possible to identify new sources of resistance from these CYMMIT lines.

Progress Report
2nd Quarter FY08 (October 1, 2007 – December 31, 2007)

Accomplishments since writing of the grant:

The map region for Lr46 has been narrowed to 0.8 cM on chromosome 1B. In particular, marker 3680 is only 0.2 cm from Lr46 which is very accurate for using in marker assisted selection. On another front, two wheat BAC clones have been partially sequenced and gene candidates are being mapped. There is a P450 gene that has been mapped in a large recombinant population and found to be between Lr46 and 3680. This has allowed us to orient the BAC contigs and start chromosome walking to the gene. Also from the BACs, we have found a new microsatelite marker that maps in the same position as the P450 gene. The P450 gene was used to probe the BAC library and new clones have been fingerprinted. A BAC clone was isolated and a NADH-like gene was used as a marker and it was found to co-segregate with Lr46. Again, the BAC library was probed and a new BAC was selected for sequencing. The BAC was sequenced and we are now mapping with new probes.

Other genomic sources are being used. Aegilops tauschii has had its genome fingerprinted. In other words, the genome was cut up into smaller pieces, cloned, and these clones were oriented in order. We have found that the genes we are using for markers are also on the D genome. We have found a long continuous region in the D genome that we can use to try to find the gene. BACs from this region are being probed with the current markers to find the co-linearity between the B and the D genome. In other words. we are seeing if the gene order along the chromosome is the same on 1D as it is on 1B.

On another front, we have started making populations for two new leaf rust resistance genes, Lr52 and an Lr gene from an Iranian land race. We have constructed a preliminary linkage map with SSR markers. A graduate student is now looking at cDNA clones and resistance gene-like sequences to see if they are mapping in the region. F3 lines are being evaluated for resistance.