Heritability of Field Resistance to Potato Leafrou Virus in Cultivated Potato

Heritability of field resistance to potato leafroU virus in cultivated potato

C. R. BROWN', D. CORSINI^ J. PAVEK^ and P. E. THOMAS' ' USDA/ARS, 24106 North Bunn Road, Prosser, WA, 99350, USA; ^USDA/ARS, PO Box AA, Aberdeen, ID, 83210, USA IVlth 1 figure and I table Received February 3. 1997jAccepted July 15, 1997 Communicated by W. E. Weber

Abstract resistance to late blight (Ross 1958, 1966). Resistance to infec- Potato progenies in a line x tester mating design and the clonal parents tion appears to have been introduced to many pedigrees were screened for field resistance to potato leafroll virus (PLRV) to through use of the variety 'Aquila' in crossing programmes determine the heritability of this trait. Twelve advanced potato clones (Davidson 1980). Combining ability for resistance to infection or varieties were crossed as pistillate parents to two pollen testers. in STT cultivars and advanced breeding clones has been The seedling progenies and clonal parents were exposed to aphid- explored by Baerecke (1955, 1958) Brown (1979, 1984), and transmitted potato leafroll virus for two growing seasons. Cumulative Brown et al. (1984a,b) and Brandolini and coworkers (1992). infection by potato leafroll virus was determined by post-season sero- Resistance to titre build-up has been characterized in cultivated logical indexing of foliage grown from sprouted tubers after 2 years of STT materials and attributed to a mode of inheritance involving exposure. Narrow-sense heritability was estimated from regression of mid-parent on progeny as h^ = 0.72. This estimate indicates a high level two complementary genes (Barker and Solomon 1990; Barker of useabie genetic variance for PLRV resistance in advanced breeding et al. 1994). Resistance to infection by aphid transfer and resist- materials. Although variation in resistance to PLRV appears to be a ance to titre build-up after graft challenge were found to be quantitative trait in susceptible and moderately resistant clones, per- distinct traits that were not necessarily correlated when assessed formance of the most resistant parents suggests that genes with major as secondary infection (e.g. indexed in foliage grown from the effects may be present. These results are similar to the conclusions of daughter tubers of inoculated plants) in advanced breeding other researchers who found one or two genes controlling the pheno- clones (Solomon-Blackburn and Barker 1993). Resistance to types of extreme resistance, resistance to infection, or suppression of titre build-up in the section Etuberosa was first described by virus titre. Jones (1979) for S. brevidens Phil, and in somatic hybrids with cultivated potato by Austin et al. (1985). Complex hybrids Key words: Solanum tuberosum — ELISA — oligogenic inherit- incorporating S. etuberosum Lindl. of this series were produced ance — PLRV — progeny test and tested for PLRV resistance by Chavez et al. (1988). Resist- ance in 5. etuberosum functions by means of suppression of Potato leafroll virus (PLRV), an aphid-vectored persistent titre build-up rather than resistance to infection. Aphid- and luteovirus, is one of the most destructive pathogens afflicting graft-inoculated plants were serologically non-reactive against potato (Hooker 1981). It is vectored primarily by the green PLRV, and appeared to be free of infection, but were shown to peach aphid (Myzus persicae Sulz.) and shares with this insect a be infected in all cases by back-testing through grafting to susceptible hosts. Extreme resistance to PLRV has been world-wide distribution spanning temperate to tropical climatic described in dipioid breeding materials several sexual cycles zones. Although tuber-transmitted, it can be eradicated from removed from wild species ancestry (Swiezynski et al. 1989). seedstock relatively easily with thermotherapy and meristem Another case of extreme resistance to PLRV, controlled by a excision (Hooker 1981). The maintenance and use of virus-free single dominant gene, was reported by Brown and Thomas seedstock, and control of green peach aphid with insecticides (1994) in the wild dipioid species S. chacoense Bitt. In both are the primary means of restricting presence and spread of the studies resistance was effective when challenged by aphid- and virus. graft-inoculation, and attempted graft transmission from chal- Genetic resistance to PLRV has long been of interest to lenged plants to suitable back-test hosts failed to transmit virus. breeders. Certain varieties or breeding clones express field resistance (also called 'resistance to infection' or 'partial resistance'), Pathogen-derived coat protein mediated resistance to PLRV and show a lower incidence of infection compared with clones in transgenic potato has been described by Barker et al. (1992), that are more susceptible, but no variety has been described as Kawchuk et al. (1991), Wilk et al. (1991), Presting et al. (1995) immune. Field resistance scores are reproducible among differ- and Brown et al. (1995). In all cases resistance was determined ent experiments, but the absolute incidence of infection rises to be titre reduction as opposed to resistance to infection. Trans- in both resistant and susceptible clones as inoculum pressure genic plants that produced nuclear transcripts of sense and increases (Baerecke 1955, 1958, Hammann et al. 1968, David- antisense versions of the coat protein gene of PLRV became son 1980; Brown 1984; Bagnall and Tai 1986). infected with the virus, but at significantly lower titre levels Resistance to infection in long-day adapted Solanum tub- than untransformed controls. erosum ssp. tuberosum L. (STT) and S. tuberosum ssp. andigena The purposes of this study were to analyse the heritability of varieties appears to have been derived from introgression of S. resistance to PLRV infection in a population consisting of a demissum Lindl. as a fortuitous by-product of breeding for group of 12 breeding clones and varieties and to identify parents

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Table 1: Per cent healthy as determined by ELISA of clonal parents 'Lemhi Russet' 'Chipeta' and mean progeny of both pollen Clone/Variety Parental Clones progenies progenies sources. Means not sharing the same letter are significantly difler- 87Tr2210-r 42.5 def 6.3 def 6.3 def ent at the P < 0.05 level using Dun- 87Tr2246-l' 20.0 fgh 2.5 def 5.0 def can's Multiple Range Test AWN85540-r 78.4 ab 29.3 abcde 10.0 cdef A80559-2^ 6.3 g 2.5 def Of A8469-5' 28.8 defg 12.5 cdef 1.2 ef A85519-6' 23.8 defg 23.8 abcdef 18.8bcdef A85530-10' 50.5 ode 15.0 cdef 13.8 cdef 'Achirana'-* 73.7 abc 37.5 abc 13.8 cdef 'Serrana'' 46.3 def 11.2 cdef 7.5 def R241-16' 77.5 ab 29.8 abed 48.9 a R247-1' 53.8 bed 36.2 abc 26.2 abcdef 'Abnaki"*" 95.0 a 45.0 ab 26.3 abcdef 'Chipeta"-' = 1.3 h na na 'Lemhi Russet''- Oh na na

na = not applicable. 'Pollen parents. Origin of clonelcultivar: ^USDA/ARS, Aberdeen, ID, USA; 'INTA, Balcarce, Argentine; "Dept of Plant Breeding, Cornell University, Ithaca, NY, USA; 'Colorado State University, Center, CO, USA; 'University of Maine, Presque Isle, ME, USA; 'USDA/ARS, Prosser, WA, USA.

with the highest general combining ability for resistance to allowed to emerge and grow for 4 weeks after emerging. Plants were PLRV. then assayed for PLRV by a two-step ELISA procedure (Kaniewski and Thomas 1988). A composite tissue sample of lOOmg derived in equal parts from apical, middle, and basal leaves, was triturated in Materials and Methods ELISA buffer, and tested at a final dilution of 1:3. Absorbances were Twelve clones of potato, Solanum tuberosum L., were selected as female measured at 2h after addition of substrate. Absorbances less than 0.1 and two as male (pollen) parents. The origins ofthe clones are indicated were classed as virus-free (i.e. healthy) and those above as infected (i.e. in Table 1. The parents used here were selected to include a range of infected). Plot means were expressed as percentage of healthy plants. resistance values known from previous studies (Corsini et al. 1994). The The data were analysed using the SAS General Linear Model (SAS choice of mating design was necessitated by the common occurrence of Institute, Cary, NC) for a factorial experiment. Heritability was deter- male sterility in the group of clones. Each female was crossed to both mined by regression of the mid-parent value on the mean performance pollen parents, generating a set of 24 progenies using standard hybrid- of the progeny. A linear regression equation was estimated and the ization techniques (Plaisted 1980). One-hundred and sixty sexual seed- regression coefficient, b, served as an estimate of h^ (Falconer 1989). ling progeny from each female were evaluated, half of these were derived Means of clonal parents and progenies were tested for differences using from mating with 'Lemhi Russet' and other half from mating with Duncan's Multiple Range Test (Steel and Torrie 1980). 'Chipeta.' Fourteen clonal parents, 12 females, and two males, were included. The true seed of the seedling progeny was sown in flats, transplanted to multicelled flats and subsequently transplanted into the Results field. The clonal parents were planted as cut seed pieces on the same The analysis of variance (ANOVA) of per cent healthy revealed date. The seedlings arising from the hybrid progenies and the 14 clonal that in the progenies tested only females and in the clonal test parents involved were planted in a randomized complete block design consisting of four blocks. The plots consisted of 20 hills planted at parental clones were a significant source of variation. The plot 61 cm spacing in rows 86 cm apart. Thus, each progeny plot consisted of mid-parent values on progeny performance and the estimated of 20 different genotypes while the parental plots each consisted of 20 regression line are shown in Fig. 1. The slope of this line, hills of a single clonal genotype. Plots were established on May 15, b = 0.72, is an estimate of h^ (Levings and Dudley 1963, Fal- 1994 and May 20, 1995 at the Washington State University, Irrigated coner 1989). Agriculture Research and Extension Centre, Roza Unit, WA, USA. The fohage was removed mechanically using a beater 120 days after planting in both years. Green peach aphids {Myzus persicae Sulz.) from Discussion colony MP-1 were reared on Physalis floridana L. infected with PLRV The crossing of potential parents with common testers is an isolate 12243 at the USDA/ARS Entomology Research Laboratory, efficient way to test parental value. The two parents with the Yakima, WA, USA. Viruliferous aphids were transferred to healthy highest progeny means were 'Abnaki' at 34.4% healthy and Datura tatula L. and populations were allowed to multiply for 4 weeks NYR 241.16 with 39.3% healthy averaged over both pollen on this newly infected host. Cut pieces of the virus-infected Datura testers. The variety 'Abnaki' emerged from a research effort in tatula leaves bearing x 10 aphids each were placed on each plant twice the north-eastern US to develop varieties resistant specifically during each growing season. Applications were made on July 7 and 21 to PLRV (Akeley et al. 1971). The breeding clone NYR241.16 in 1994 and July 10 and 24 in 1995. No insecticides with aphicidal properties were applied. A single tuber was harvested from each plant was selected from cycle 5 of a Neotuberosum population at the end of the first season. Tubers were stored at 4°C for 8 months developed at Cornell University, and represents an example and then planted whole to establish the second exposure season. The of PLRV resistance derived from S. tuberosum ssp. andigena 20 tubers harvested from each plot were combined and planted as a (Mufioz 1980). This clone appears as a parent in the pedigrees plot the second year. Hence, the incidence of PLRV in the plot at the ofa number of virus-resistant breeding clones selected in Idaho end of the experiment reflected cumulative infection over two seasons (Corsini et al. 1994). The high percentage of progeny resistant of exposure. At the end ofthe second season, one tuber per plant was to PLRV from these clones after two cumulative seasons of harvested and maintained at 4°C for 6 months. Single eyes were excised exposure invites more detailed analysis of the genetic factors from each tuber. These were planted in 1:1 soil:peat mix in flats and involved. Heritability of field resistance to PLRV in cultivated potato 587

50 Progeny(%) = -0.46 -hO .72 X [Midparent(%)] " 45 r== 0.47 ** 40 • 35 o

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'Serrana' (Huarte et al. 1986) was released by the national Baerecke, M.-L., 1955: Untersuchungen zur Blattrollresistenz. Proc. potato programme of Argentina and described as possessing 2nd Conf. Pot. Vir. Dis., Lisse-Wageningen, The Netherlands, 1954, resistance to PLRV. It was identified as the best general com- 111—119. biner for PLRV resistance in a previous study of parental breed- , 1958: BlattroUresistenzzUchtung. In: H. Kappert, and W. Rudorf (Hrsg.), Handbuch der Pflanzenzuchtung, 2. Aufl., Bd. Ill, 97—106. ing value (Brandolini et al. 1992). It ranked eighth out of 12 in Parey, Berlin. our study and would not be considered a parent with good Bagnall, R. H., and G. C. C. Tai, 1986: Potato leafroll virus: Evaluation combining ability. The cumulative infection of two seasons of of resistance in potato cultivars. Plant Dis. 70, 621—623. exposure described here may be a more severe test of resistance Barker, H., and R. N. Solomon, 1990: Evidence of simple genetic than that carried out in the study reported by Brandolini et al. control in potato of ability to restrict potato leafroll virus con- (1992). centration in leaves. Theor. Appl. Genet. 80, 188—192. Barker and Solomon (1990) and Barker et al. (1994) showed , B. Reavy, A. Kumar, K. D. Webster, and M. A. Mayo, 1992: that the segregation patterns of low and high PLRV titres Restricted virus multiplication in potatoes transformed with the coat derived from crosses involving PLRV-resistant parents were protein gene of potato leafroll luteovirus: Similarities with a type of consistent with control by two complementary genes. Resist- host gene-mediated resistance. Ann. Appl. Biol. 120, 55—64. , R. M. Solomon-Blackburn, J. W. Me Nichol, and J. E. Bradshaw, ance to PLRV in advanced clones and varieties of S. tuherosum 1994: Resistance to potato leafroll virus multiplication is under major ssp. tuherosum and ssp. andigena has been thought to be a gene control. Theor. Appl. Genet. 88, 754—758. polygenically controlled trait (Baerecke 1955, 1958, Davidson Brandohni, A., P. D. S. Cahgari, and H. A. Mendoza, 1992: Combining 1980, Brown 1979, 1984). The high heritability estimated from resistance to potato leafroll virus (PLRV) with immunity to potato this mating design and the outstanding performance of the viruses X and Y (PVX and PVY). Euphytica 61, 37—42. sexual progeny of certain clonal parents invite reexamination Brown, C. R., 1979: General combining ability for PLRV resistance in of that assumption. The high percentage of healthy individuals Andigena germplasm. Am. Potato J. 56, 456. in progenies of the top-performing parents are consistent with , 1984: Genetic studies and breeding of resistance to potato viruses: a major phenotypic effect controlled by few genes. Several ofthe PLRV, PVY, and PVX. Present and Future Strategies for Potato parent progeny comparisons deviated from the general trend Breeding and Improvement. Rpt. 26 Plan. Conf., Int. Potato Ctr., described here. This was most notable for AWN85540-1 and Lima, Peru, 17—44. A85530-1 where the relatively high percentage of the parent , and P. E. Thomas, 1994: Resistance to potato leafroll virus was not matched by correspondingly high progeny perform- derived from Solanum chacoense: Characterization and inheritance. Euphytica 74, 51—57. ance. This of course argues for caution in the interpretation of , E. N. Fernandez-Northcote, U. Jayasinghe, and L. Salazar, this data. It is possible that mechanisms and genetic control of 1984a: Breeding virus-resistant potato cultivars for developing coun- resistance may differ among the clonal parents tested. A more tries. CIP Circ. 12, 1^. detailed study of inheritance of resistance in these clones and , , , and , 1984b: Mejoramiento de resistencia a the mapping of loci controlling resistance appears, however, to PLRV y PVY en papa. Memorias: XII Reunion de la Asociacion be warranted. Latinoamericana de la Papa, Paipa-Boyoca, 295—307. , O. P. Smith, V. D. Damsteegt, C.-P. Yang, L. Fox, and P. E. Thomas, 1995: Suppression of PLRV titer in transgenic 'Russet References Burbank' and 'Ranger Russet'. Am. Pot. J. 72, 590—597. Akeley, R. V., H. J. Murphy, and R. C. Cetas, 1971: 'Abnaki': a new Chavez, R., C. R. Brown, and M. Iwanaga, 1988: Transfer of resistance high-yielding potato variety resistant to Verticilhum wilt and leafroU. to PLRV titer buildup from Solanum tuberosum to a tuber-bearing Am. Potato J. 48, 230—233. Solanum gene pool. Theor. Appl. Genet. 76, 129—135. Austin, S., M. A. Baer, and J. P. Helgeson, 1985: Transfer of resistance Corsini, D. L., J. J. Pavek, M. Martin, and C. R. Brown, 1994: Potato to potato leafroll virus from Solanum brevidens into Solanum tub- germplasm with combined resistance to leafroll virus and viruses X erosum by somatic fusion. Plant Sci. 39, 75—82. and Y. Am. Potato J. 71, 377—385. 588 BROWN, CORSINI, PAVEK and THOMAS

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