BREEDING, INHERITANCE, AND BIOCHEMICAL STUDIES ON BRASSICA NAPUS CV. ZHONGYOU 821 :TOLERANCE TO SCLEROTINIA SCLEROTIORUM (STEM ROT)
Yunchang Li1, Jun Chen1, Richard Bennett2, Guy Kiddle2, Roger Wallsgrove2, Yongju Huang1, and Yuanhui He1
1Institute of Oil Crops Research, CAAS, XU JIA PENG, 430062,WUHAN, CHINA
2B.& P. Dept., IACR-Rothamsted, Harpenden, Hetrts., AL5 2JQ, UK
ABSTRACT
The first studies on developing improved cultivars of Brassica species, with high yields and good disease resistance, began in Institute of Oil Crops Research in 1970s. The initial developments began with the official Chinese registration of B. napus cv. Zhongyou 821. The major reason for the widespread cultivation of Zhongyou 821 was its good disease tolerance to the stem rot pathogen (Sclerotinia sclerotiorum) which is an major pathogen in China. The other reasons, which lead to its increased usage, were its consistent yields and good adaptability to different climates within China—both of which are clearly beneficial traits. Initially Zhongyou 821 occupied between 1.5—2.0 million hectares each year in the late 1980s and early 1990s (on average 25-35 percent of the total area of rapeseed in China were Zhongyou 821). Zhongyou 821 was produced by crossing a wide range of B. napus and B. campestris lines with beneficial traits. During selection the breeding material (in the progeny selection plots) was inter-cropped with radish (Raphanus sativus L.) to increase the incidence of stem rot and also to lure aphids away from the rape crop. Although there is some variation in the inheritance resistance to stem rot in Zhongyou 821 its general horizontal resistance remains high. Even though Zhongyou 821 is a double high (high seed glucosinplate and erucic acid concentrations) this has not prevented its use in China. The following paper discusses aspects of the breeding, stem rot disease resistance and biochemical defence mechanisms of Zhongyou 821.
KEY WORDS: Oilseed rape, Zhongyou 821, Stem rot, Disease tolerance/resistance, Conventional and directed breeding
INTRODUCTION
Stem rot (Sclerotinia sclerotiorum [Lib.] De Bary) is a widespread disease not only on crucifers (including Brassica species) but many other plants as well. It is a major problem on oilseed rape (Brassica L.) in some partes of the world e.g. China. The name stem rot was given because of the mode of infection i.e. from the soil into the stems of the plants (Baswani et al., 1991). Stem rot is the major creucifer pathogen in China (Liu et al., 1990). The standard strategies for control include the use of fungicides, crop rotation, limited biological control, breeding of tolerant/resistant cultivars. The production of new cultivars through directed breeding is seen as the most economical and environmental friendly. The Institute of Oil Crops Research in Wuhan first began a programme to improve rape yields in the 1970s. One of the most successful developments from these studies was the breeding of B. napus cv. Zhongyou 821. This line proved to have good stem rot tolerance, a high and consistent yield, and was also adaptable to the wide climatic conditions encountered in China (He et al., 1987). It has become a major cultivar in China - on average 25 - 35% of the total rape area each year (1.5 - 2.0 million hectares). Even though the line is a double high (high seed erucic acid and glucosinolate concentrations) it is still used extensively. This paper describes the background to the production of Zhongyou 821 and the more recent biochemical studies done on this cultivar and other Chinese lines.
BREEDING STRATEGIES FOR THE PRODUCTION OF BRASSICA NAPUS CV. ZHONGYOU 821
The production of this cultivar did not involve a standard simple crossing procedure. Many parents were used to ensure that the primary objectives of high yield, stable yield and good environmental adaptability were achieved. The second aim was to develop lines that were tolerant/resistant to S. sclerotiorum.
Figure 1: Breeding course of Zhongyou 821
A x {(B x C)F3 x [(D x E)F2 x F]f1}F1
F1
Two lines per plot of F1 interspersed with two lines of radish.
The same was done for F2 and F3 sowings (to check the stability of the required environmental characteristics in the offspring e.g. disease tolerance).
F4
At this stage the resistance to stem rot was analysed – artificial inoculation with 1 - 2 Sclerotia mm-2 in the field.
Resistant lines were selected for multiple location tests (i.e. both regional and national testing of growth, yield and tolerance/resistance to stem rot).
Final cultivar (Registration, large - scale seed production and further testing).
Zhou et al (1996 - pers comm.) identified many materials that were used for the provision of the rape genetic resources for China, in order to further screen for stem rot tolerance/resistance. Zhou showed that Zhongyou 821 was one of the most efficient/tolerant rapeseed lines that had been produced in China to that date.
GENETICS OF DISEASE TOLERANCE/RESISTANCE TO STEM ROT
Dickson et al. (1996) had reported that the stem rot resistance in F2 and back-cross populations of oilseed rape was due to a recessive gene and that modifiers for stem rot resistance could be found in cabbage and cauliflower. Form the years of study, after the initial registration, on the cultivation practices, and the large scale disease analyses, it was shown that Zhongyou 821 had the following characteristics: it was
1. much more disease tolerant/resistant than any of the individual parents or other lines.
2. occasionally variable in its disease tolerance/resistance to stem rot - observed in the field (regional). The changes in tolerance/resistance in the field were also influenced, not unsurprisingly, by the local environmental conditions.
3. horizontally resistant to stem rot – probably due to its long cultivation time (over 13 years) and cultivation area (1.5 - 2.0 million hectares) in China; isolates of stem rot were equally effective when infection was reported in the field.
4. genetically stable for the initially selected characteristics e.g. yield and disease tolerance/resistance - large-scale cultivation for 13 years. Zhongyou 821 was continually used to back cross with other parents to produce enhanced characters whenever problems arose.
5. The mechanisms of disease tolerance/resistance were to stop invasion of mycelia and ascospore and to prevent the disease from expanding.
It was clear from these field results/observations that a logical progression in identifying disease tolerant/resistant characters would be to investigate the biochemical changes in the host i. e the potential identification of resistance markers for a more defined directed breeding programme.
BIOCHEMICAL CHANGES IN HOST PLANTS AFTER INOCULATION WITH S. SCLEROTIORUM ISOLATES
Plants can produce a number of antifungal and fungistatic secondary metabolites. Upon infection changes can occur in both constitutive compounds such as phenolics and glucosinolates and also de novo synthesis of antifungal compounds e. g. phytoalexins (see reviews by Bennett and Wallsgrove, 1994; Wallsgrove, Doughty and Bennett, 1998). It is known that certain biochemical events occur in resistant reactions to fungal infection - large increases in the activities of oxidative “burst” enzymes (lipoxygenase, superoxide dismutaseetc.) and phenylpropanoid pathway enzymes [leading to the production of lignin, phenolics (including the signal compound salicylic avid) and flavonoids]. Previously we had shown that
1. there is a very large natural variation in tissue glucosinolate content in oilseed rape lines from China (Li et al., 1999a) and that
2. systemic increases in glucosinolates were potentially a good marker for stem rot resistance (Li et al., 1999b)
Initial biochemical results have also shown that peroxidase may also be another potential resistance marker for stem rot (Fig. 2). Large local change in glucosinolates and peroxidase occur in all the lines but in resistant lines the increase are also systemic. The susceptible lines however showed a reduction in glucosinolates (Li et al., 1999b) and minimal changes in systemic peroxidase activity. Zhongyou 821 (shows intermediate resistance and therefore the systemic increases fall in between the most susceptible and resistant lines/breeding materials tested). Peroxidases have previously been shown to be a potential general resistance markers for pathogens (Hammerschmidt et al., 1982; Reuveni et al., 1990; Srivastava 1987).
A paper is in preparation detailing the biochemical changes occurring after inoculation of both European and Chinese oilseed rape lines/cultivars with stem rot - specifically local and systemic changes in glucosinolates, phenylpropanoid enzymes (PAL, CAD, peroxidase, polyphenol oxidases), oxidative burst enzymes (lipoxygenase, superoxide aismutase, catalase) and other enzymes (e.g. monoamine oxidase and myrosinase). The aims of these studies are to identify biochemical resistance markers that can be used in directed breeding programmes for rape and related Brassica species to naturally increase stem rot resistance.
Acknowledgements
This research was funded by a grant from EU (Contract No: IC18- CT97— 0173) and Hubei Science and Technology Commission.
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