Plant Cell Reports
Plant Cell Reports (1993) 12:312-315
9 Springer-Verlag1993
In vitro selection for Russian wheat aphid in wheat (Triticum aestivum) R . S . Zemetra 1, D.J. Sehotzko
1,
(Diuraphis noxia) resistance
C . M . Smith 2, and M. Lauver 1
1 Department of Plant, Soil, and EntomologicalSciences, Universityof Idaho, Moscow, Idaho 83843, USA z Department of Entomology,Kansas State University, Manhattan, Kansas 66506, USA Received September 1, 1992/Revised version received January 27, 1993 - Communicatedby G. C. Phillips
Abstract. The Russian wheat aphid (Diuraphis noxia) is a pest on wheat (Triticum aestivum) in many regions of the world. The aphid injects a phytotoxin when it feeds. Identification of somaclonal variants with phytotoxin resistance may shorten development time for resistance. Wheat calli from the susceptible cultivar 'Stephens' were exposed to an extract from the aphid. Five plants were regenerated from 100 treated ealli. Resistance to the aphid was observed in both the R z and R 3 generations. One of the six R3 populations had improved resistance for leaf curling and leaf folding, while another had improved response for chlorosis damage. These results indicate that the use of aphid extract on wheat callus offers an alternate method for development of resistance to the Russian wheat aphid.
Introduction The Russian wheat aphid, Diuraphis noxia (Kurdjumov), is a serious pest of wheat in the United States and other regions of the world. The Russian wheat aphid (RWA) was first identified in the United States in 1986 (Webster et al. 1987) and has since spread through the wheat production areas of the western United States. Losses due to the aphid in the United States from 1988 to 1990 have been estimated at $352 million (Massey 1991). Damage by D. noxia occurs both from direct feeding and from the effect of a phytotoxin it injects during feeding. Detection and control of RWA by biological control agents and contact insecticides is difficult because the phytotoxin prevents the leaves in proximity to feeding aphids from unfurling (Valiulis 1 9 8 6 ) . Currently, systemic insecticides are the only method for control of the
Correspondence to." R. S. Zemetra
aphid in commercial fields (Pike 1988; Anonymous 1989). Plant resistance to the Russian wheat aphid has recently been found in unimproved wheat germplasm, primarily from central Asia (du Toit 1987, 1988; Nkongolo et al. 1989; Quick et al. 1991; Zemetra et al. 1990; Smith et al. 1991; Harvey and Martin 1990; Souza et al. 1991). The disadvantage with these sources of resistance is that a series of backcrosses are necessary to obtain the RWA resistance in an adapted genetic background. The use of somaclonal variation to recover resistance in an adapted cultivar could reduce the time necessary to recover RWA resistance (Larkin et al. 1984) Somaclonal variation in wheat has been utilized to recover plants with improved freezing tolerance (Lazar et al. 1988). Variation was observed for this trait in both the R2 and R3 generations. Using in vitro selection, Pauly et al. (1987) selected for bacterial blight (Pseudomonas syringae pv. syringae) resistance in wheat. A toxin derived from the bacteria was used to screen the wheat callus. Variability for resistance was found in the plants regenerated from this callus for response to the bacteria. A crude extract using whole RWA was found to induce phytotoxin-like symptoms in wheat leaves (Kruger and Hewitt 1984) and cause damage to wheat callus (Schroeder-Teeter 1991) indicating a crude RWA extract might be used to screen for RWA resistance in vitro. The objectives of this study were to determine if somaclonal variants could be identified in the callus stage for RWA resistance and if the plants regenerated from this callus would express resistance to the aphid over several generations.
313 Materials and methods.
Results and Discussion
In vitro culture and regenerationprocedure. The cnitivar 'Stephens',
After treatment with the RWA extract the caUi appeared necrotic, with the calli changing to a brown to dark brown color. After two treatments with extract all the callus treated appeared necrotic. Upon transfer to shoot medium, only 5 out of 100 calli developed shoots after 4 to 8 weeks. Roots were successfidly regenerated on all 5 plantlets and all 5 plants (A, B, C, D, and E) matured to produce seed. Seed production varied among the R t plants with three plants (A, B and C) appearing partially sterile. Three of the four R2 populations (A, D, and E) showed segregation for RWA resistance in the greenhouse (Table 1). Population E showed segregation for leaf rolling for both more susceptible and more resistant plants. Population C showed little segregation for resistance with a greater mean chlorosis rating than the check Stephens indicating that R1 plant C was most likely an escape from the callus screening. The segregation pattern of the A, D, and E populations indicates that the R1 plants were heterozygous for the tissue culture induced modification. The R2 plants from these three populations (A, D, and E) that were the best for resistance were allowed to mature and were completely fertile.
a soft white winter wheat cultivar susceptible to the Russian wheat aphid, was selected as the donor for explant material. Eleven to 12 d old immature embryos were used to initiate callus. The callus initiation medium was similar to Sears and Deckard (1982) with the following modifications; 100 mg/L-asparagine, 0A mg/L thiamineHcl, 100 mg/L inositol and 2 mg/L 2,4-dichlorophenoxyacetic acid (2,4-D). After 1 month in the dark at 18~ the newly formed caUuswas transferred to a growth medium that was identical to the initiation medium except for lowering the 2,4-13 level to 1 m g / L The callus remained on growth media for 4 weeks and during this time was treated with 80 ttl of RWA extract at 2 week intervals. The extract was applied directly to the callus using a Pasteur pipet. One hundred calli were treated with extract. Two weeks after the second extract treatment, callus was moved to shoot regeneration medium containing half-strength macro salts, 0.5 mg/L 2,4-D and 0.5 mg/L kinetin (Nabors et al. 1983). Calli that developed shoots were then moved to a medium containing half-strength macro salts without 2,4-D or ldnetin. After roots had developed the plants were transferred to soil and grown to maturity.
Russian wheat aphid extract preparation. The RWA extract was prepared by homogenizing 0.5 g of freeze-dried aphids in 15 ml of 0.35 M NaCI using a Wheaton Instruments overhead stirrer. Ice was used to maintain homogenization temperature at 1-2~ The extract was centrifuged at 18,000 g for 5 rain. The superaatant was forced through a sterile 0.2/tin filter. Fresh extract was prepared for each treatment.
Whole plant evaluation. Greenhouse evaluation of R 2 and R3 plants for RWA resistance followed the procedure described by Smith et al. (1991). The R1, R2, and R3 generation notation follows that of Yurkova et al. (1982) where the regenerated plants are the R 1 generation and progeny from R 1 plants are the R2 generation. Five aphids were applied to each seedling at the two leaf stage and the plants were evaluated for damage after 3 weeks of aphid feeding. Symptoms measured were leaf rolling (ceding parallel to the leaf mid-vein), leaf folding (fold perpendicular to the leaf midvein), and chlorosis. For each damage symptom, the plants were rated on a 0 to 3 scale where 0 = no damage to 3 = all leaves showing damage with plants dead or dying (Zemetra et al. 1990). Plants with the best resistance were selected from the R2 populations that showed segregation for resistance and were allowed to mature and set seed. Both a Susceptible check cultivar (Stephens) and a resistant check (Border oats Avena sativa L.) were included in greenhouse evaluation of the R z populations. The number of plants evaluated per R 1 plant varied from 15 to 30 depending on R1 seed production. The regenerate plant B did not produce adequate seed with the proper vigor to be included in evaluations of the R2 populations. The R 2 plants with the lowest combined leaf rolling, leaf folding, and leaf chlorosis scores were saved and allowed to produce seed in the greenhouse. Progeny of two plants from the three R2 selected populations were then screened for RWA resistance. The six R3 populations plus the susceptible check Stephens were evaluated using a randomized complete block design with 5 replications. Each replication contained 12 plants of one entryper tray. Data was analyzed using SAS-ANOVA CATMOD (SAS, Institute 1985). The contrasts within CATMOD were to compare the frequency of score distributions of the individual R3 populations to the frequency of score distributions of the standard susceptible check Stephens.
Table 1. Mean and range scoresa of leaf rolling (LR), leaf folding (LF), and chiorosis (C) for the R 2 populations of A, D, and E, Stephens (Ste.) wheat and Border oats based on greenhouse evaluation for Russian wheat aphid resistance.
pop.
plants tested
LR mean range
LF mean range
C mean range
A
15
1.5
1- 2
0.2
0- 1
2.1
2- 3
C
1
1.8
1- 3
0A
0-1
2.6
2-3
D
30
1.9
1- 2
0.5
0-1
2.1
1-3
E
30
1.8
0-3
0.3
0-1
2.0
1-3
Ste.
10
1.9
1- 2
0.3
0-1
2.3
2-3
oats
10
0,0
0
0.0
0
1.0
1
a Scoring for LR, LF, and C was on a 0 - 3 rating with 0 being resistant and 3 being susceptible
Based on Chi square analysis of the frequency distribution of the six R 3 populations, only populations originating from plant A population showed a significant difference from the check cultivar Stephens (Table 2). Population A-2-2 was different for leaf rolling and leaf folding while population A-2-1 was
314 different than Stephens for chlorosis. Variation was still present in all six populations for response to the aphid but the range of susceptible to resistant was less than that observed in the R2 generation screening. The variation in the response of R 2 and Rs populations both for range and change in degree of variation is similar to that observed by Lazar et al. (1988) for somaclonal variation for freezing tolerance. The plants with the lowest combined leaf rolling, leaf folding, and ehlorosis score in the R3 populations were more resistant than those of the R2 populations. The level of resistance of the superior Ra plants was equivalent to wheat accessions identified in germplasm as carrying RWA resistance (Zemetra et al. 1990; Smith et al. 1991; Souza et al. 1991).
Table 2. Chi square analysis of the wheat R3 population frequency distributions compared to the check cultivar Stephens for leaf rolling (LR), leaf folding (LF) and chlorosis ((2) after feeding by the Russian wheat aphid.
LR
LF
Prob.
~
C
Population
X~
Prob.
~
Prob.
A-2-1
0.05
0.82
0.07
0.79
5.48
0.02
A-2-2
4.87
0.02
3.78
0.05
1.69
0.19
D-l-1
0.04
0.83
0.08
0.78
0.24
0.62
D-l-2
0.26
0.60
0.02
0.89
0.03
0.85
E-l-1
0.72
0.39
0.23
0.63
0.00
0.97
E-l-5
0.13
0.71
0.02
0.89
0.37
0.54
This is the f'n'st report of development of insect resistance in wheat using an insect extract for in vitro selection to identify potentially resistant somaclonal variants. The presence of a susceptible escape was not surprising because the level of phytotoxin may have been variable in the whole aphid extract used for screening or, due to the form of application, some cells within the callus may not have been exposed to the phytotoxin. Refinement of this extract or isolation of the RWA phytotoxin would greatly improve the callus selection technique. Field testing of the elite R4 populations is needed to determine if the level of resistance developed is adequate under field conditions since response to the phytotoxin may be only part of the damage caused by the aphid feeding on wheat. It still may be necessary to control the aphid population number to keep damage below an economic threshold. The decrease in the amount of leaf folding and rolling
observed in plants of population A-2-2 would facilitate aphid population control by allowing the use of biological and contact insecticide control methods for the Russian wheat aphid (Zemetra et al. 1990). A moderate level of resistance would also decrease the potential for the development of new biotypes of D. noxia that could occur with germplasm containing aphid immunity (Zemetra et al. 1990). The resistance developed in this research could also be combined with that identified inthe wheat germplasm collections to develop a horizontal resistance to the aphid, again decreasing the likelihood of biotype development. Further work is needed on the genetics of the tissue culture derived resistance to determine the origin of the resistance variation in the populations. Whether segregation follows a Mendelian pattern or not would determine if the segregation was due to heterozygosity for the resistance or heterogeneity due to a multicellular origin for the regenerated plantlet. Additional research is also needed on the germplasm resistance before the development of horizontal resistance to the Russian wheat aphid utilizing both tissue culture and germplasm derived resistance can Occur.
Acknowledgements.
Contribution No. 92747 of the Idaho Agricultural Experiment Station. Research funded by the Idaho Agricultural Experiment Station, IMAGE, and USDA/CSRS grant No. 89-34205-4296.
References Anonymous (1989) Great Plains Agricultural Council, Publication No 129, Ft. Collins, CO, USA du Toit F (1987) Cereal Res Commun 15:175-179 du Toit F (1988) Cereal Res Commun 16:105-106 Harvey TL, Martin TJ (1990) Cereal Res Commun 18:127-129 Kruger GI-IJ, Hewitt PH (1984) In: Waiters MC (ed) Progress in Russian wheat aphid (Diuraphis noxia Mordw.) research in the Republic of South Africa. Technical Communication 191, Department of Agriculture, Republic of South Africa Larldn PF, Ryan SA, Brettel RIS, Seowcroft WR (1984) Theor Appl Genet 67:443-445 Lazar MD, Chen THH, Gusta LV, Kartha KK (1988) Theor Appi Genet 75:480-484 Massey W (1991) Great Plains Agricultural Council Pub 139 Nabors MW, Heyser JW, Dykes TA, Demott KJ (1983) Planta 157:385-391 Nkongolo KK, Quick JS, Meyer WL, Peairs FB (1989) Cereal Res Commun 17:227-232 Pauly MH, Shane WW, Gengenbach BG (1988) Crop Sci 27:340344 Pike KS, Suomi D (1988) Coop Ext Washington State Unix.Ext Bull 1486 Quick JS, Nkongolo KK, Meyer WL, Peairs FB, Weaver B (1991) Crop Sci 31:50-53 SAS Institute (1985) Version 5 ed SAS Institute Inc Cary, North Carolina Schroeder-Teeter S (1991) MS Thesis, Univ of Idaho, Moscow, Idaho, USA
315 Sears RG, Deckard EL (1982) Crop Sci 22:546-550 Smith CM, Schotzko D, Zemetra RS, Souza EJ, Schroeder-Teeter S (1991) J Econ Entomol 84:328-332 Souza E, Smith CM, Sehotzko DJ, Zemetra RS (1991) Euphytica 57:221-225 Valiulis D (1986) Agrichem Age 30:10-11
Webster JA, Stark KJ, Burton BI (1987) J Econ Entomol 80:944949 Yurkuva GN, Levenko BA, Novoshilov (1982) Biochem Physiol Pflanz 177:337-344 Zemetra RS, Schotzko D, Smith CM, Souza EJ (1990) Cereal Res Commun 18:223-227
Plant Cell Reports (1993) 12:312-315
9 Springer-Verlag1993
In vitro selection for Russian wheat aphid in wheat (Triticum aestivum) R . S . Zemetra 1, D.J. Sehotzko
1,
(Diuraphis noxia) resistance
C . M . Smith 2, and M. Lauver 1
1 Department of Plant, Soil, and EntomologicalSciences, Universityof Idaho, Moscow, Idaho 83843, USA z Department of Entomology,Kansas State University, Manhattan, Kansas 66506, USA Received September 1, 1992/Revised version received January 27, 1993 - Communicatedby G. C. Phillips
Abstract. The Russian wheat aphid (Diuraphis noxia) is a pest on wheat (Triticum aestivum) in many regions of the world. The aphid injects a phytotoxin when it feeds. Identification of somaclonal variants with phytotoxin resistance may shorten development time for resistance. Wheat calli from the susceptible cultivar 'Stephens' were exposed to an extract from the aphid. Five plants were regenerated from 100 treated ealli. Resistance to the aphid was observed in both the R z and R 3 generations. One of the six R3 populations had improved resistance for leaf curling and leaf folding, while another had improved response for chlorosis damage. These results indicate that the use of aphid extract on wheat callus offers an alternate method for development of resistance to the Russian wheat aphid.
Introduction The Russian wheat aphid, Diuraphis noxia (Kurdjumov), is a serious pest of wheat in the United States and other regions of the world. The Russian wheat aphid (RWA) was first identified in the United States in 1986 (Webster et al. 1987) and has since spread through the wheat production areas of the western United States. Losses due to the aphid in the United States from 1988 to 1990 have been estimated at $352 million (Massey 1991). Damage by D. noxia occurs both from direct feeding and from the effect of a phytotoxin it injects during feeding. Detection and control of RWA by biological control agents and contact insecticides is difficult because the phytotoxin prevents the leaves in proximity to feeding aphids from unfurling (Valiulis 1 9 8 6 ) . Currently, systemic insecticides are the only method for control of the
Correspondence to." R. S. Zemetra
aphid in commercial fields (Pike 1988; Anonymous 1989). Plant resistance to the Russian wheat aphid has recently been found in unimproved wheat germplasm, primarily from central Asia (du Toit 1987, 1988; Nkongolo et al. 1989; Quick et al. 1991; Zemetra et al. 1990; Smith et al. 1991; Harvey and Martin 1990; Souza et al. 1991). The disadvantage with these sources of resistance is that a series of backcrosses are necessary to obtain the RWA resistance in an adapted genetic background. The use of somaclonal variation to recover resistance in an adapted cultivar could reduce the time necessary to recover RWA resistance (Larkin et al. 1984) Somaclonal variation in wheat has been utilized to recover plants with improved freezing tolerance (Lazar et al. 1988). Variation was observed for this trait in both the R2 and R3 generations. Using in vitro selection, Pauly et al. (1987) selected for bacterial blight (Pseudomonas syringae pv. syringae) resistance in wheat. A toxin derived from the bacteria was used to screen the wheat callus. Variability for resistance was found in the plants regenerated from this callus for response to the bacteria. A crude extract using whole RWA was found to induce phytotoxin-like symptoms in wheat leaves (Kruger and Hewitt 1984) and cause damage to wheat callus (Schroeder-Teeter 1991) indicating a crude RWA extract might be used to screen for RWA resistance in vitro. The objectives of this study were to determine if somaclonal variants could be identified in the callus stage for RWA resistance and if the plants regenerated from this callus would express resistance to the aphid over several generations.
313 Materials and methods.
Results and Discussion
In vitro culture and regenerationprocedure. The cnitivar 'Stephens',
After treatment with the RWA extract the caUi appeared necrotic, with the calli changing to a brown to dark brown color. After two treatments with extract all the callus treated appeared necrotic. Upon transfer to shoot medium, only 5 out of 100 calli developed shoots after 4 to 8 weeks. Roots were successfidly regenerated on all 5 plantlets and all 5 plants (A, B, C, D, and E) matured to produce seed. Seed production varied among the R t plants with three plants (A, B and C) appearing partially sterile. Three of the four R2 populations (A, D, and E) showed segregation for RWA resistance in the greenhouse (Table 1). Population E showed segregation for leaf rolling for both more susceptible and more resistant plants. Population C showed little segregation for resistance with a greater mean chlorosis rating than the check Stephens indicating that R1 plant C was most likely an escape from the callus screening. The segregation pattern of the A, D, and E populations indicates that the R1 plants were heterozygous for the tissue culture induced modification. The R2 plants from these three populations (A, D, and E) that were the best for resistance were allowed to mature and were completely fertile.
a soft white winter wheat cultivar susceptible to the Russian wheat aphid, was selected as the donor for explant material. Eleven to 12 d old immature embryos were used to initiate callus. The callus initiation medium was similar to Sears and Deckard (1982) with the following modifications; 100 mg/L-asparagine, 0A mg/L thiamineHcl, 100 mg/L inositol and 2 mg/L 2,4-dichlorophenoxyacetic acid (2,4-D). After 1 month in the dark at 18~ the newly formed caUuswas transferred to a growth medium that was identical to the initiation medium except for lowering the 2,4-13 level to 1 m g / L The callus remained on growth media for 4 weeks and during this time was treated with 80 ttl of RWA extract at 2 week intervals. The extract was applied directly to the callus using a Pasteur pipet. One hundred calli were treated with extract. Two weeks after the second extract treatment, callus was moved to shoot regeneration medium containing half-strength macro salts, 0.5 mg/L 2,4-D and 0.5 mg/L kinetin (Nabors et al. 1983). Calli that developed shoots were then moved to a medium containing half-strength macro salts without 2,4-D or ldnetin. After roots had developed the plants were transferred to soil and grown to maturity.
Russian wheat aphid extract preparation. The RWA extract was prepared by homogenizing 0.5 g of freeze-dried aphids in 15 ml of 0.35 M NaCI using a Wheaton Instruments overhead stirrer. Ice was used to maintain homogenization temperature at 1-2~ The extract was centrifuged at 18,000 g for 5 rain. The superaatant was forced through a sterile 0.2/tin filter. Fresh extract was prepared for each treatment.
Whole plant evaluation. Greenhouse evaluation of R 2 and R3 plants for RWA resistance followed the procedure described by Smith et al. (1991). The R1, R2, and R3 generation notation follows that of Yurkova et al. (1982) where the regenerated plants are the R 1 generation and progeny from R 1 plants are the R2 generation. Five aphids were applied to each seedling at the two leaf stage and the plants were evaluated for damage after 3 weeks of aphid feeding. Symptoms measured were leaf rolling (ceding parallel to the leaf mid-vein), leaf folding (fold perpendicular to the leaf midvein), and chlorosis. For each damage symptom, the plants were rated on a 0 to 3 scale where 0 = no damage to 3 = all leaves showing damage with plants dead or dying (Zemetra et al. 1990). Plants with the best resistance were selected from the R2 populations that showed segregation for resistance and were allowed to mature and set seed. Both a Susceptible check cultivar (Stephens) and a resistant check (Border oats Avena sativa L.) were included in greenhouse evaluation of the R z populations. The number of plants evaluated per R 1 plant varied from 15 to 30 depending on R1 seed production. The regenerate plant B did not produce adequate seed with the proper vigor to be included in evaluations of the R2 populations. The R 2 plants with the lowest combined leaf rolling, leaf folding, and leaf chlorosis scores were saved and allowed to produce seed in the greenhouse. Progeny of two plants from the three R2 selected populations were then screened for RWA resistance. The six R3 populations plus the susceptible check Stephens were evaluated using a randomized complete block design with 5 replications. Each replication contained 12 plants of one entryper tray. Data was analyzed using SAS-ANOVA CATMOD (SAS, Institute 1985). The contrasts within CATMOD were to compare the frequency of score distributions of the individual R3 populations to the frequency of score distributions of the standard susceptible check Stephens.
Table 1. Mean and range scoresa of leaf rolling (LR), leaf folding (LF), and chiorosis (C) for the R 2 populations of A, D, and E, Stephens (Ste.) wheat and Border oats based on greenhouse evaluation for Russian wheat aphid resistance.
pop.
plants tested
LR mean range
LF mean range
C mean range
A
15
1.5
1- 2
0.2
0- 1
2.1
2- 3
C
1
1.8
1- 3
0A
0-1
2.6
2-3
D
30
1.9
1- 2
0.5
0-1
2.1
1-3
E
30
1.8
0-3
0.3
0-1
2.0
1-3
Ste.
10
1.9
1- 2
0.3
0-1
2.3
2-3
oats
10
0,0
0
0.0
0
1.0
1
a Scoring for LR, LF, and C was on a 0 - 3 rating with 0 being resistant and 3 being susceptible
Based on Chi square analysis of the frequency distribution of the six R 3 populations, only populations originating from plant A population showed a significant difference from the check cultivar Stephens (Table 2). Population A-2-2 was different for leaf rolling and leaf folding while population A-2-1 was
314 different than Stephens for chlorosis. Variation was still present in all six populations for response to the aphid but the range of susceptible to resistant was less than that observed in the R2 generation screening. The variation in the response of R 2 and Rs populations both for range and change in degree of variation is similar to that observed by Lazar et al. (1988) for somaclonal variation for freezing tolerance. The plants with the lowest combined leaf rolling, leaf folding, and ehlorosis score in the R3 populations were more resistant than those of the R2 populations. The level of resistance of the superior Ra plants was equivalent to wheat accessions identified in germplasm as carrying RWA resistance (Zemetra et al. 1990; Smith et al. 1991; Souza et al. 1991).
Table 2. Chi square analysis of the wheat R3 population frequency distributions compared to the check cultivar Stephens for leaf rolling (LR), leaf folding (LF) and chlorosis ((2) after feeding by the Russian wheat aphid.
LR
LF
Prob.
~
C
Population
X~
Prob.
~
Prob.
A-2-1
0.05
0.82
0.07
0.79
5.48
0.02
A-2-2
4.87
0.02
3.78
0.05
1.69
0.19
D-l-1
0.04
0.83
0.08
0.78
0.24
0.62
D-l-2
0.26
0.60
0.02
0.89
0.03
0.85
E-l-1
0.72
0.39
0.23
0.63
0.00
0.97
E-l-5
0.13
0.71
0.02
0.89
0.37
0.54
This is the f'n'st report of development of insect resistance in wheat using an insect extract for in vitro selection to identify potentially resistant somaclonal variants. The presence of a susceptible escape was not surprising because the level of phytotoxin may have been variable in the whole aphid extract used for screening or, due to the form of application, some cells within the callus may not have been exposed to the phytotoxin. Refinement of this extract or isolation of the RWA phytotoxin would greatly improve the callus selection technique. Field testing of the elite R4 populations is needed to determine if the level of resistance developed is adequate under field conditions since response to the phytotoxin may be only part of the damage caused by the aphid feeding on wheat. It still may be necessary to control the aphid population number to keep damage below an economic threshold. The decrease in the amount of leaf folding and rolling
observed in plants of population A-2-2 would facilitate aphid population control by allowing the use of biological and contact insecticide control methods for the Russian wheat aphid (Zemetra et al. 1990). A moderate level of resistance would also decrease the potential for the development of new biotypes of D. noxia that could occur with germplasm containing aphid immunity (Zemetra et al. 1990). The resistance developed in this research could also be combined with that identified inthe wheat germplasm collections to develop a horizontal resistance to the aphid, again decreasing the likelihood of biotype development. Further work is needed on the genetics of the tissue culture derived resistance to determine the origin of the resistance variation in the populations. Whether segregation follows a Mendelian pattern or not would determine if the segregation was due to heterozygosity for the resistance or heterogeneity due to a multicellular origin for the regenerated plantlet. Additional research is also needed on the germplasm resistance before the development of horizontal resistance to the Russian wheat aphid utilizing both tissue culture and germplasm derived resistance can Occur.
Acknowledgements.
Contribution No. 92747 of the Idaho Agricultural Experiment Station. Research funded by the Idaho Agricultural Experiment Station, IMAGE, and USDA/CSRS grant No. 89-34205-4296.
References Anonymous (1989) Great Plains Agricultural Council, Publication No 129, Ft. Collins, CO, USA du Toit F (1987) Cereal Res Commun 15:175-179 du Toit F (1988) Cereal Res Commun 16:105-106 Harvey TL, Martin TJ (1990) Cereal Res Commun 18:127-129 Kruger GI-IJ, Hewitt PH (1984) In: Waiters MC (ed) Progress in Russian wheat aphid (Diuraphis noxia Mordw.) research in the Republic of South Africa. Technical Communication 191, Department of Agriculture, Republic of South Africa Larldn PF, Ryan SA, Brettel RIS, Seowcroft WR (1984) Theor Appl Genet 67:443-445 Lazar MD, Chen THH, Gusta LV, Kartha KK (1988) Theor Appi Genet 75:480-484 Massey W (1991) Great Plains Agricultural Council Pub 139 Nabors MW, Heyser JW, Dykes TA, Demott KJ (1983) Planta 157:385-391 Nkongolo KK, Quick JS, Meyer WL, Peairs FB (1989) Cereal Res Commun 17:227-232 Pauly MH, Shane WW, Gengenbach BG (1988) Crop Sci 27:340344 Pike KS, Suomi D (1988) Coop Ext Washington State Unix.Ext Bull 1486 Quick JS, Nkongolo KK, Meyer WL, Peairs FB, Weaver B (1991) Crop Sci 31:50-53 SAS Institute (1985) Version 5 ed SAS Institute Inc Cary, North Carolina Schroeder-Teeter S (1991) MS Thesis, Univ of Idaho, Moscow, Idaho, USA
315 Sears RG, Deckard EL (1982) Crop Sci 22:546-550 Smith CM, Schotzko D, Zemetra RS, Souza EJ, Schroeder-Teeter S (1991) J Econ Entomol 84:328-332 Souza E, Smith CM, Sehotzko DJ, Zemetra RS (1991) Euphytica 57:221-225 Valiulis D (1986) Agrichem Age 30:10-11
Webster JA, Stark KJ, Burton BI (1987) J Econ Entomol 80:944949 Yurkuva GN, Levenko BA, Novoshilov (1982) Biochem Physiol Pflanz 177:337-344 Zemetra RS, Schotzko D, Smith CM, Souza EJ (1990) Cereal Res Commun 18:223-227
