Polygenic Heritability of Rose Root Rot Disease Resistance in Offspring of Rosa multiflora

Defeng ZHUANG Lianhua LI Ce MA Fumitada NAGAOKA Yoichi ASANO Koji KAGEYAMA Masaki OCHIAI Hirokazu FUKUI

Abstract Rose root rot disease caused by Pythium helicoides Drechsler has brought heavy damage to cut rose production in Japan， and it has became apparent that Rosa multiflora showed resistance to this disease. Roses have been bred by interspecies crossing with Rosa species including of R. multiflora， and many progenies of R. multiflora have been bred as determined cluster flowered roses. However， the genetic architecture of this resistance remains. So， in this study， four kinds of R. multiflora and 21 rose cultivars that have been clarified in the cross fertilization genealogy with R. multiflora were used as plant materials， and the inheritability of resistance to root rot disease was investigated. Four kinds of R. multiflora had all high resistance， and the resistance to root rot disease in R. multiflora was inherent characteristic of species. Five varieties not less than 50% in relatedness with R. multiflora were shown as varied ‘resistant’ and ‘moderately resistant’. Therefore， the inherent resistant characteristic in R. multiflora was not controlled by single major gene， and the resistibility in R. multiflora was not cytoplasmic but nuclear heredity. Although there was significant negative relation between relative root rot severity and relatedness with R. multiflora， the variance of relative root rot severity had tendency to expand as relatedness with R. multiflora reduced. This result， therefore， indicated that the contingency of resistance expression increased as the relatedness with R. multiflora was reduced. The varieties having low degree of relatedness with R. multiflora had similar relative root rot severity with R. multiflora. These results indicated that we would be able to select resistant roses from progenies crossed with R. multiflora.

Key words Inheritance; Pythium helicoides; Resistance; Rose root rot

Rose root rot disease caused by Pythium helicoides Drechsler has brought heavy damage to cut rose production in Japan， since it makes plant die after the infection[1]. Li et al.[2] confirmed that Rosa multiflora showed resistance to this disease by bioassay. This resistibility was revealed through restraining germ tube elongation on penetration into root tissue[3] and had relation with the retardants of germ tube  elongation  contained in root[4]. Li et al.[5] suggested that the resistance to root rot disease was originated from R. multiflora， which had genetic characteristic， but this conclusion was led by the observation from only three roses which have relatedness with R. multiflora.

Roses have been bred by interspecies crossing with R. gallica， R. chinensis， R. gigantea， R. moschata， R. foetida， etc.， and R. multiflora also has been used for parent of cluster flowered roses[6]. Rosa ‘Seven sisters’， that is Chinese hybrid multiflora， has been brought to Europe from Japan or China in 1816， many progenies have been bred， and hereditary relationship with  R. multiflora  is clear in some cultivars. So， in this study， rose cultivars which have been clarified in the cross fertilization history with R. multiflora were used for plant materials， and the inheritability of resistance to root rot disease was investigated.

Materials and Methods

Plant materials

Four kinds of R. multiflora and 21 rose cultivars that have been clarified in the cross fertilization genealogy with R. multiflora were selected from the collection of Flower Festival Commemorative Park （Gifu， Japan） and were used as plant materials  （Table 1） . And Rosa ‘Nakashima 91’， which was susceptible variety， was used for control. The degree of relatedness with  R. multiflora  was calculated from the data of cross fertilization in the web site of "Help Me Find" （http：//www.helpmefind.com/rose/; 2014/July/01）. For example： the breeding process of ‘Tapis Volant’ is （R. luciae×Seedling）×（R. multiflora×Ballerina），  R. multiflora  is regarded as 100%， the degree of relatedness with R. multiflora was calculated by dividing by the seed parent and pollen parent， e.g. 0%+ 50%/2=25%. These roses were propagated by cutting and were cultivated in rock wool culture  system.

Pythium Inoculum

P. helicoides B 5， which was isolated from rotted roots and had a high pathogenicity[1]， was used for inoculation. Zoospores of P. helicoides B 5 were collected by the grass blades method[7]， and zoospore suspension of 6.0×103 zoospores/ml was used as  inoculum.

Inoculation method and evaluation of resistance

The inoculation method of zoospore suspension to rooted cuttings was the same as in a previous report[2]. Rooted cuttings of each rose in each repetition were over 10. Root rot was evaluated 7 d after inoculation. Root rot symptoms were visually estimated  using  disease index from 0 to 3： where 0=no root rot symptom was observed （healthy and white root）， 0.5=a part of the root was slightly brown， 1=a part of the root was brown， 1.5=50% of the root was brown， 2=75% of the root was brown， 2.5=the root was completely brown but the plant survived， and 3=the root was completely brown and the plant died[2]. Root rot severity was calculated as follows：

Root rot severity=Σ （Disease index×Number of plants at each disease index） /Maximum disease index×Total number of plants

In order to reduce the error in experimental repetitions， relative root rot severity was carried out. Relative root rot severity was the ratio of root rot severity of each rose to root rot severity of ‘Nakashima 91’. Each rose was repeated over three times， and ‘Nakashima 91’ was used as control in each repetition.

Resistance was classified into three scales by following values of relative root rot severity： <0.60： resistant， 0.60-0.99： moderately resistant， >1.00： susceptible.

Results and Discussion

P. helicoides was re isolated from all infected and browned roots. The result indicated that root rot in all roots observed 7 d after inoculation was caused by infection of P. helicoides. In 26 roses， relative root rot severity varied from 0.26 to 1.52 （Table 2）.

Inheritability of Resistance to Root Rot Disease among four R. multiflora

Relative root rot severity of R. multiflora was 0.47， and  R. multiflora  had been confirmed as ‘resistant’ （Table 2）. In another three R. multiflora： ‘Carnea’， ‘Cathayensis’ and ‘Matsushima No.3’， the relative root rot severity was lower than that in  R. multiflora ， therefore， three R. multiflora might have high resistance， although there was no significant difference among four R. multiflora. From these results， the resistance to root rot disease in R. multiflora was inherent characteristic of species. Although the flower color of ‘Carnea’， ‘Cathayensis’ and ‘Matsushima No.3’ was pink， light pink to white and white， respectively， there was no obvious relation between flower color and resistance to rose root rot disease.

Inheritability of Resistance to Root Rot Disease in rose not less than 50% in relatedness with R. multiflora

In the five roses not less than 50% in relatedness with  R. multiflora ， relative root rot severity varied from 0.44 to 0.86， and those have been confirmed as ‘resistant’ and ‘moderately resistant’ （Table 2）. It was indicated that inherent resistant characteristic in R. multiflora was not controlled by single major gene. The disease resistance in rose controlled by some identified major genes has also been reported in blackspot[9] or downy mildew[10]. The genes of root rot disease will be studied in the near future by the gene analysis based on the phenotypic expression of the crossing progenies between R. multiflora with resistance to root rot disease and susceptible roses.

Dubey et al.[11] identified chloroplast DNA fragments related with downy mildew resistance in poppy. Among the five roses，  R. multiflora  was used as paternal parent in ‘Apple Blossom’ or ‘The Garland’ and was used as maternal parent in ‘Rambling Rector’ or ‘Seagull’ （Table 1）. Because of no significant  difference  in their relative root rot severity， it was estimated that the resistibility in R. multiflora was not cytoplasmic but nuclear heredity.

Inheritability of Resistance to Root Rot Disease in rose below 50% in relatedness with R. multiflora

Relative root rot severity in rose cultivars which were below 50% in relatedness with R. multiflora varied from 0.31 to 1.52 （Table 2）. Although there was significant negative relation between relative root rot severity and relatedness with R. multiflora （Fig. 1）， the variance of relative root rot severity had tendency to expand as relatedness with R. multiflora reduced. These progenies have been selected as cluster flowered roses derived from R. multiflora， and have not been exerted the selection pressure on resistance to root rot disease. The result in Fig. 1， therefore， indicated that the contingency of resistance expression increased as the relatedness with R. multiflora was reduced.

In Table 2， relative root rot severity of ‘Dance de Feu’ and ‘Veilchenblau’ having low degree of relatedness with R. multiflora （25% and 6.25%） was similar with R. multiflora. These results indicated that we would be able to select resistant roses from progenies crossed with R. multiflora.

References

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