# New resistance against downy mildew and lettuce mosaic potyvirus in wild Lactuca spp

Brigitte B. Maisonneuve, Yannick Bellec, Sylvie Souche, Hervé Lot

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Brigitte B. Maisonneuve, Yannick Bellec, Sylvie Souche, Hervé Lot. New resistance against downy mildew and lettuce mosaic potyvirus in wild Lactuca spp. Eucarpia leafy vegetables 99, Jun 1999, Olomouc, Czech Republic. hal-02770149

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European Association for Research
on Plant Breeding

Eucarpia Leafy Vegetables '99

Proceedings of the Eucarpia Meeting
on Leafy Vegetables Genetics and Breeding

Olomouc, the Czech Republic
8-11 June, 1999

Aleš Lebeda & Eva Křístková

Editors

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Palacký University Olomouc
Czech Republic
1999
# New resistance against downy mildew and lettuce mosaic potyvirus in wild *Lactuca* spp.

B. Maisonneuve<fnref n="1" />,<fnref n="2" />, Y. Bellec<fnref n="2" />, S. Souche<fnref n="3" /> & H. Lot<fnref n="3" />

## Summary

New European isolates of *B. lactucae* and of LMV are able to overcome the disease resistance genes used in commercial varieties. Wild *Lactuca* spp. constitute a large pool for new genes. Several new resistance genes against European *B. lactucae* isolates were identified in wild *Lactuca* species and introduced in *L. sativa* lines. The resistance from two *L. virosa* accessions was efficient against every isolate. The results suggested at least 2 dominant genes in each accession controlled the resistance (2 independent in one accession, two tightly linked in the other one). A complete stable resistance was observed in artificial tests with absence of spores. Therefore, this resistance was assumed to be comparable to *Dm* gene. The spectra of resistance of lines derived from two *L. saligna* were similar with resistance from cv. Ninja. This mechanism seems more complex than *Dm* gene with apparition of late and sparse sporulation on many plants in some segregating populations. This late sporulation was also observed in some commercial varieties and other INRA resistant lines derived from *L. serriola*.

In a large collection of different *Lactuca* spp., one *L. virosa* (PIVT1398), was identified as resistant to different European LMV isolates overcoming *mo1¹* or *mo1²* genes. A study of *L. virosa* segregating F₂ population suggested one major dominant gene for this resistance. *Mo3* was proposed to designate this gene. Its expression in heterozygous plant was variable with different isolates. Its expression was consistent with a dominant gene against LMV-0, but incomplete dominant gene against LMV-13 (necrotic phenotype). The introgression of this gene in *L. sativa* is in progress despite several difficulties.

An accumulation in one genotype of several resistance genes against each disease could be a strategy to increase the durability of these new genes.

*Lactuca serriola*, *L. saligna*, *L. virosa*, LMV, *Bremia lactucae*, resistance genes, genetics of resistance, accumulation of genes

## Introduction

Lettuce is an important vegetable crop for fresh market. For consumer, the leaves must be exempt of disease symptoms and of pesticide residues. Among the most important diseases, downy mildew (Bremia lactucae Regel) and lettuce mosaic potyvirus (LMV) are constant risks for this crop. For both diseases, resistance genes have been introduced in cultivars since 30 to 40 years (1, 2, 11). Nevertheless, several isolates identified in Europe can overcome the *Dm* and *mo* resistance genes used in commercial varieties.

For *B. lactucae*, several genes efficient against NL16 isolate were overcome in the last few years, e.g. resistance from Samourai (RZ), Ninja (S&G) or Bacarès (Vilmorin) (presented in a communication at this meeting). Likewise, European virus isolates can

<footnote n="1">¹ INRA, Station de Génétique et d'Amélioration des Fruits et Légumes, Domaine St Maurice, BP94, 84143 Montfavet Cedex, France</footnote>

<footnote n="2">² INRA, Station de Génétique et d'Amélioration des Plantes, Route de St Cyr, 78026 Versailles Cedex, France</footnote>

<footnote n="3">³ INRA, Station de Pathologie Végétale, Domaine St Maurice, BP94, 84143 Montfavet Cedex, France</footnote>
overcome the LMV tolerance conferred by *mol*¹ and *mol*² (9). Therefore, we are looking in wild *Lactuca* spp. for new sources of resistance to both diseases.

Several genes from *L. serriola* have been introduced into some cultivars (e.g. several *Dm* genes; (2)) because the two species can be easily crossed. The use of *L. saligna* and *L. virosa* has been more restricted; however, some resistance genes from *L. saligna* have been introduced into *L. sativa* (8, 10). A possible method for overcoming the crossability barrier with *L. saligna* and *L. virosa* was achieved by *in vitro* rescue of immature hybrid embryos (5).

Investigations of new sources of resistance to *B. lactucae* in *L. serriola*, *L. saligna* and *L. virosa* have been in progress at INRA for several years. Some resistance efficient to NL16 have been introgressed into *L. sativa* and we are controlling their efficiency against new European isolates.

For LMV, we have recently screened a large collection of *Lactuca* with virulent isolates. One accession of *L. virosa* appeared interesting, consequently a program to study this resistance and introgress it into *L. sativa* has been investigated.

## Material and methods

### Plant material

Two morphologically different accessions of *L. saligna* (CR16 and CR17) received from I. Crute (NVRS, Wellesbourne) and two accessions of *L. virosa* collected by INRA (LS238 and LS241) were used for *B. lactucae* resistance studies. The interspecific crosses with *L. saligna* and the $BC_1$ [*(*L. sativa* x *L. virosa*)* x *L. sativa*] were made by *in vitro* culture of immature embryo (5). The progenies obtained at INRA from 3 *L. serriola* resistant to NL16 were also tested with new European isolates. Lines derived from these interspecific crosses by backcrosses with several *L. sativa* (e.g. cv. Girelle, cv. Ravel) or crosses between interspecific progenies; and then selfpollination were generated. Evaluation of this resistance and its inheritance study were made on this material issued from four to seven cross generations before selfing; the genetic background could be considered as *L. sativa* one.

In contrast, the research on LMV resistance was achieved in wild species. In preliminary experiments, 116 *L. sativa* genotypes were screened for resistance to virulent LMV-E isolates. Because no interesting resistance was found, then 115 wild *Lactuca* accessions were screened. The most interesting accession, *L. virosa* (PIVT1398) received from CPRO (Wageningen, NL), was extensively studied for its LMV resistance. The study was made on *L. virosa* material: $F_1$ between susceptible *L. virosa* (PIVT280) and resistant PIVT1398 and derived $F_2$ and $F_3$ families.

### Tests for resistance to *B. lactucae*

European isolates has been used in this study, NL16 and isolates overcoming NL16 resistant varieties. The new isolates SAR1, SAR3 and I3 attacking respectively Ninja, Samourai and Bacarès, were supplied by Novartis Seeds (France); 49/83, overcoming previously Mariska resistance (6), was supplied by A. Lebeda. Lettuces were tested at seedling stage in closed plastic boxes with soil substratum. After sowing, the lettuce seeds were incubated at 6 °C for 24 h to 48 h to overcome seed dormancy; and then the boxes were transferred to a growth chamber for 3 weeks (16/12 °C day/night tempera-
tures, 16 h per day). Isolates were multiplied on susceptible cultivar seedlings cultivat-
ted on filter paper moistened with a nutrient solution. Seven day-old seedlings were
inoculated by spraying a spore suspension (2 x 10⁵ spores/ml). Seven and 14 days after
inoculation, presence or absence of asexual sporulation were scored.

Tests for LMV resistance

European isolates were used in this study, the common isolate (LMV-0) and some
isolates identified on some tolerant varieties: LMV-E, LMV-1, LMV-13 (3). The ino-
culum was an extract from 2-3 week-old infected lettuce plants (cv. Trocadero for LMV-
0 or cv. Mantilia for other isolates). One gram of infected leaf tissue was grinded in 4
ml buffer containing 100 mg/ml of activated charcoal and carborundum. Young plants
were mechanically inoculated once or twice (day 0 and 2). Plants were maintained in
a growth chamber (22 °C /12 to16 °C day/night temperatures, 16 h per day) for LMV-
0 and LMV-E. Experiments using the other strains (LMV-1 and LMV-13) were con-
ducted at Montfavet with plants in an insect-proof greenhouse. Symptom observation
as well as DAS-ELISA (Double Antibody Sandwich-ELISA) was used to evaluate the
plant resistance. One gram of leaf tissue was collected from different young leaves,
grinded with a roller press in 4 ml of PBS-Tween-PVP buffer and tested by DAS-ELISA
using polyclonal antibodies.

**Results and discussion**

New sources of resistance to *B. lactucae*

Seven sources of resistance to *B. lactucae*, previously identified against NL16 and 49/83, were tested against new European isolates in comparison with new resistance of commercial varieties (Table 1). Four out of these 7 lines were resistant to every isolate (two *L. serriola* and the both *L. virosa*). As expected, TTE, genitor of one of the resistance used by Vilmorin, was overcome by I3. Isolate SAR3, identified on Ninja, attacked the both *L. saligna*.

Observation of plants with sparse and late sporulation was made in several lines
and varieties in our test conditions; this late sporulation was scored especially in in-
teraction I3/R18, but also with different isolates on R38 and R36 resistance factors as
well as on our lines derived from *L. saligna*, and sometimes on TTE and FHD.

In segregating population with resistance from CR16 or CR17, this phenotype with
late and weak sporulation was screened 14 days after inoculation (dpi), and then incre-
ased the percentage of susceptible plants. For example, in F₂ (SaBV x Girelle) the ratio
resistant to susceptible plants at 7 dpi (120 to 33 with NL16 and 110 to 42 with 49/83)
was consistent with one dominant gene (χ² = 0.96 and 0.56 respectively). But at 14 dpi,
some sporulation was scored on more than 50% of plants (51 res to 92 susc with NL16
and 66 res to 83 susc with 49/83). In experiment with weaker inoculation (2 x 10³ sp/
ml), the ratio at 14 dpi fitted with 3 : 1 in some segregating populations. These results
suggested that the mechanism of resistance in these both *L. saligna* CR16 and CR17
as observed previously in other *L. saligna* (4) was different from the *Dm* gene mecha-
nism. A difference of susceptibility between SaBV and SaAB observed with SAR2,
isolate supplied by Novartis (results not shown), suggested that the both resistant fac-
tors must be different.
<table><caption>**Table 1.** Interaction of NL16 resistant genotypes with European isolates of *B. lactucae*</caption><thead><tr><th>Variety/line</th><th>R factor <sup>a</sup><br>or source<br>of resistance</th><th>NL16</th><th>Isolates of <i>B. lactucae</i><br>49/83</th><th>SAR1</th><th>SAR3</th><th>I3</th></tr></thead><tbody><tr><td colspan="7"><strong>Commercial varieties <sup>b</sup></strong></td></tr><tr><td>Mariska</td><td>R18</td><td>R</td><td>+</td><td>+</td><td>R</td><td>R *</td></tr><tr><td>Samourai</td><td></td><td>R</td><td>+</td><td>+</td><td>R</td><td>*</td></tr><tr><td>Argelès</td><td>R38</td><td>R *</td><td>nd</td><td>R *</td><td>R</td><td>+</td></tr><tr><td>Bacarès</td><td></td><td>R</td><td>R</td><td>R *</td><td>R</td><td>+</td></tr><tr><td>Ninja</td><td>R36</td><td>R</td><td>R</td><td>R</td><td>+</td><td>R *</td></tr><tr><td>Angie</td><td>Nd</td><td>R</td><td>R</td><td>R</td><td>+</td><td>R</td></tr><tr><td colspan="7"><strong>INRA lines</strong></td></tr><tr><td colspan="7"><strong>Resistance from <i>L. serriola</i></strong></td></tr><tr><td>F<sub>7</sub> YYD</td><td>LS102 (R17)</td><td>R</td><td>R</td><td>R</td><td>R</td><td>R</td></tr><tr><td>F<sub>7</sub> FHD</td><td>LS162</td><td>R</td><td>R</td><td>R</td><td>R</td><td>R</td></tr><tr><td>F<sub>7</sub> TTE</td><td>Nd</td><td>R</td><td>R</td><td>R</td><td>R</td><td>+</td></tr><tr><td colspan="7"><strong>Resistance from <i>L. saligna</i></strong></td></tr><tr><td>F<sub>6</sub> SaAB</td><td>CR16</td><td>73 R</td><td>73 R</td><td>27 R</td><td>20 +</td><td>20 R</td></tr><tr><td>F<sub>4</sub> SaBV</td><td>CR17</td><td>68 R</td><td>72 R</td><td>27 R</td><td>20 +</td><td>20 R</td></tr><tr><td colspan="7"><strong>Resistance from <i>L. virosa</i></strong></td></tr><tr><td>F<sub>6</sub> ViAE</td><td>LS241</td><td>197 R</td><td>340 R</td><td>349 R</td><td>410 R</td><td>143 R</td></tr><tr><td>F<sub>4</sub> ViCQ</td><td>LS238</td><td>250 R</td><td>357 R</td><td>383 R</td><td>413 R</td><td>219 R</td></tr></tbody></table>

<table><caption>**Table 2.** Segregation of resistance in populations derived from crosses between resistant *L. virosa* and susceptible cultivars inoculated with 4 isolates of *B. lactucae*</caption><thead><tr><th rowspan="3">Isolates</th><th colspan="2">ViBK6-2-2<br>(resistance from LS241)</th><th colspan="2">ViCQ2-4-4<br>(resistance from LS238)</th><th colspan="2">ViCP5-1-1<br>(resistance from LS238)</th></tr><tr><th colspan="2">No of<br>seedlings</th><th colspan="2">No of<br>seedlings</th><th colspan="2">No of<br>seedlings</th></tr><tr><th>Res : susc</th><th>χ<sup>2</sup><br>3 : 1</th><th>Res : susc</th><th>χ<sup>2</sup><br>15 : 1</th><th>Res : susc</th><th>χ<sup>2</sup><br>3 : 1</th></tr></thead><tbody><tr><td>NL16</td><td>293 : 64</td><td>9.5 **</td><td>308 : 18</td><td>0.3</td><td>332 : 97</td><td>1.3</td></tr><tr><td>49/83</td><td>522 : 137</td><td>6.2 *</td><td>554 : 48</td><td>3.0</td><td>436 : 128</td><td>1.6</td></tr><tr><td>SAR1</td><td>524 : 118</td><td>15.0 **</td><td>296 : 25</td><td>1.3</td><td>345 : 93</td><td>3.3</td></tr><tr><td>SAR3</td><td>468 : 115</td><td>8.7 **</td><td>500 : 43</td><td>2.6</td><td>473 : 142</td><td>1.2</td></tr></tbody></table>

<footnote><sup>a</sup> R factor proposed by IBEB (presented at this meeting);</footnote>

<footnote><sup>b</sup> test on 10 to 50 plants per variety; interactions were scored as R for no sporulation, score R * for a sparse and late sporulation on only few plants of resistant variety; score * for mixed reaction (5R, 7*, 3+); score + for profuse sporulation on all plants.</footnote>

<footnote>* Significantly different from hypothesis (P<0.05)</footnote>

<footnote>** Significantly different from hypothesis (P<0.01)</footnote>
In contrast, the interaction between *B. lactucae* and our progenies from *L. virosa* was similar with reaction of *Dm* genes. At 7dpi, a strong sporulation was observed on susceptible plants and often the same segregation was scored at 7 and 14 dpi. Segregating populations with these resistance were scored after inoculation with 4 isolates (Table 2). The results suggested that two tightly linked dominant genes in ViBK and two independent genes in ViCQ and one dominant gene in ViCP controlled the resistance.

The complete resistance of ViBK (315 tested plants) and ViCQ (170 tested plants) after inoculation with I3, what is Avr7, was an indication of the transfer into these genotypes of *Dm7* from the different cultivars used in this cross. The segregation for resistance to I3 scored in ViCP (ratio 350 res to 71 susc) suggested that the gene from LS238 present in ViCP could be linked to *Dm7*. Complementary tests on larger populations would be necessary to confirm these hypothesis. These loci could be designated R39 to R42 if there is no allelic gene between LS238 and LS241. Allelism tests are in progress.

## New resistance to LMV

In a collection of 116 *L. sativa*, 76 *L. serriola*, 11 *L. saligna*, 22 *L. virosa* and 6 other *Lactuca* spp. tested with LMV-E only few accessions did not show severe symptoms. These resistant genotypes were tested with LMV-0, LMV-1 and LMV-13. Only one *L. virosa* and some plants from *L. perennis* showed interesting resistance to the four isolates.

The resistance of the *L. perennis* must be fixed before any program of protoplast fusion in order to transfer it into *L. sativa* that could be a very laborious work (7). Therefore, we initiated a study of the resistant *L. virosa* (PIVT1398). An immunity was observed with the different isolates (no virus detectable in ELISA in systemic but also in infected leaves). Because of the very low fertility of the interspecific crosses with *L. virosa* (hybrid sterile, very low fertility of the BC₁), the research on inheritance was initiated in segregating population derived of a cross between two *L. virosa* (PIVT280), a susceptible one (high virus multiplication) and PIVT1398. The F₁ and F₂ generation were inoculated with LMV-0 and LMV-13 (Table 3).

After inoculation with LMV-0, the results were consistent with one dominant gene. This hypothesis was confirmed by the study of F₃ families derived from F₂ tested plants, because the progenies of all susceptible F₂ plants were homogeneous for susceptibility and the progenies of resistant F₂ plants were resistant or in segregation (data not shown). The results after inoculation with LMV-13 was different. Indeed, the expression of resistance in the heterozygous plants was sometimes irregular with often a necrotic phenotype. The same expression was observed after inoculation with LMV-E (data not shown). The results suggested an hypothesis of one incomplete dominant gene with these virulent isolates; we designated this locus *Mo3*.

In our strong test conditions (mechanical inoculation, often twice on young plant), some PIVT1398 susceptible plants were identified (2 out 30 plants inoculated with LMV-13 in greenhouse, 0 out of 114 plants inoculated with LMV-0 in growth chamber). A control of the virus multiplied in these plants revealed a LMV variant able to systemically infect PIVT1398. When these variants were inoculated to susceptible lettuce (cv. Trocadéro), they can lose their ability to overcome *Mo3*. According to the possibility of variant of LMV, for a good evaluation of *Mo3* in segregating population, the susceptibility of the plants must be checked by studying of the multiplied virus in back-inoculation on PIVT1398.
<table><caption>**Table 3.** Resistance to different LMV isolates in segregating population of *L. virosa*</caption><thead><tr><th>Genotypes</th><th>LMV isolates</th><th>Number of plants susceptible</th><th>Number of plants intermediate</th><th>Number of plants resistant</th><th>χ<sup>2</sup> 1:2:1 or 1:3</th></tr></thead><tbody><tr><td>F<sub>1</sub> (PIVT1398 x PIVT280)</td><td>LMV-13</td><td>0</td><td>10</td><td>0</td><td></td></tr><tr><td rowspan="2">F<sub>1</sub> (PIVT280 x PIVT1398)</td><td>LMV-13</td><td>3</td><td>8</td><td>7</td><td></td></tr><tr><td>LMV-0</td><td>0</td><td>0</td><td>16</td><td></td></tr><tr><td rowspan="2">F<sub>2</sub> (PIVT280 x PIVT1398)</td><td>LMV-13</td><td>17</td><td>28</td><td>13</td><td>1.14</td></tr><tr><td>LMV-0</td><td>26</td><td>4</td><td>56</td><td>1.25</td></tr></tbody></table>

Inoculation of plants cultivated in a growth chamber for LMV-0 or in a greenhouse for LMV-13 and then assay by DAS-ELISA. OD of susceptible plants were similar with inoculated PIVT280, the OD of resistant plant similar with inoculated PIVT1398 and not inoculated plants. The intermediate score corresponded to necrotic plants and/or plants with low virus multiplication

A test of $F_3$ lines were made to know whether the same gene confers the resistance to LMV-0 and to LMV-13. The 13 lines fixed susceptible to LMV-0 (11 to 74 tested plants) were susceptible to LMV-13 (5 to 28 plants tested) and the 11 lines fixed resistant to LMV-0 (34 to 88 tested plants) were resistant to LMV-13 (13 to 33 tested plants). However, in three resistant $F_3$ lines, one or two susceptible plants were identified; but a control of the virus revealed a variant of the isolate. These results suggested the same gene controlled the resistance to the both isolates. For this reason, it is possible to inoculate the plants with the common isolate (LMV-0) in breeding program in order to score *Mo3*; that is less hazardous for environment than to use a virulent isolate, especially LMV-13 that it could be seed born.

The introgression of *Mo3* into *L. sativa* were initiated (7) but it is a very laborious work due to the high mortality and to the low fertility of progeny maintained after several generations of BC with *L. sativa*.

## Conclusions

Some new resistance genes were identified in *L. virosa*. Their introgression into *L. sativa* is more or less difficult with respect to the wild resistant accession. These resistance seem due to one or two dominant major genes in each accession that could be easy to use in breeding programs. For *B. lactucae* resistance, some other genes were identified in other species easier to cross with lettuce (*L. serriola* and *L. maligna*). But the expression of these genes with some partial resistance makes them not easy to screen and need very standardized conditions for the artificial test.

The main problem for disease resistance is the durability of the efficiency of the gene. For the both diseases, we were able in our test conditions to obtain variant of
isolate that overcome some resistance (isolates of *B. lactucae* overcoming *L. saligna* resistance and variant of LMV overcoming *Mo3*). The durability of these resistance in natural conditions is not known. In absence of other resistance efficient against the new European isolates of these both diseases, we will continue these programs with complementary genetic studies as in one hand allelism tests between *B. lactucae* resistance, in the other hand the introgression of *Mo3* in diversified cultivars. A strategy to increase the durability of the resistance could be the accumulation of the resistance genes in one genotype especially if mechanisms are different: for example, resistance against *B. lactucae* from *L. saligna* with genes from *L. virosa* or *Mo3* with one allele of *mol* locus. Consequently, it will be important to identified some PCR based molecular markers of these genes; it is in progress for *Mo3*.

## Acknowledgements

We are grateful to E. Martin, F. Dauphin and M. Duteil for technical assistance with resistance tests. We wish to acknowledge the financial support to INRA provided previously by Societe Vilmorin for the generation of lines with *B. lactucae* resistance from *L. serriola* and by both Societies, Vilmorin and Tezier, for the study of LMV resistance from PIVT1398. The studies on LMV resistance of *L. virosa* were supported by AIP-INRA grant 4/4920 in 95-96.

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