Variation und Vererbung von Glucosinolatgehalt und muster in Grünmasse und Samen von Raps (Brassica napus L.) und deren Zusammenhang zum Befall mit Rapsstängelschädlingen

Abstract

Today oilseed rape (Brassica napus L.) is the third largest oil crop worldwide. One of the reasons for its importance lies in the breeding of cultivars with canola quality and low contents of glucosinolates (GSL). This achievement of plant breeders made it possible to use both the oil for consumption and rapeseed cake as co-product in animal feeding. GSL are sulfur-rich secondary plant compounds and a characteristic of the plant family Brassicaceae. The function of the GSL inside plants, together with the hydrolyzing enzyme myrosinase, is believed to be a constitutive plant defense mechanism against unspecific herbivores: The so called glucosinolate-myrosinase system. Nevertheless, oilseed rape is also infested by pests that only accept Brassicaceae as hosts. For some specialized pests, it could be shown that GSL or their degradation products can influence the behavior of pests regarding the choice of host plant, oviposition or feeding. Thus, the possibility exists that GSL in green tissue can mediate a quantitative resistance and variation of GSL represents a natural source of resistance. Contrary to the seeds, the inheritance of GSL in leaf and stem tissues is not studied well. Objectives of this study were on the one hand to evaluate GSL as potential resistance factor for the two specialized pests rape stem weevil (Ceutorhynchus napi) and cabbage stem weevil (Ceutorhynchus pallidactylus) and, on the other hand, a genetic analysis of GSL contents in leaves and stems. Therefore, a series of field trials was carried out for three years. 28 genetically diverse genotypes, comprising 15 resynthesized lines and 13 old and new breeding lines were tested with regard to their variation of GSL contents and composition as well as their susceptibility to stem weevils. Data of pest infestation were scored parallel by the section of entomology and originate from the dissertation of Schäfer-Kösterke (2015). In order to investigate the possibility of selecting genotypes with differing GSL contents in seed and green matter, the inheritance of GSL was studied in a second experiment with a DH population. The main question of this QTL mapping experiment was to what extent genome regions involved in the GSL metabolism differed between leaf, steam and seed tissues. Regarding GSL variation, the analysis of the first experiment showed a large genetic variation with high heritabilities for the eleven identified GSL. The growth stage of the plants proved to have a big impact on GSL contents: The average GSL content of the 28 genotypes decreased from 18 µmol during early bolting to 4 µmol at the beginning of flowering. Furthermore, seeds had on average 47 µmol higher GSL contents than green matter samples and stems had 3 µmol higher contents than leaves. Also the GSL composition differed significantly between green matter and seeds, but not between the two sampling dates. In addition, location and year had an influence on GSL contents. In 2012 and 2013 the effect of the location was larger than the effect of the year. Between leaf and stem material a high correlation of 0.96 for total GSL contents was found. Between total GSL contents of seed and green matter this correlation was less pronounced with 0.60, and for indolic GSL with 0.14 not significantly different from zero. The complex temporal and spatial distribution of GSL inside plants is discussed with regard to transport processes. The analysis of the relation between stem weevils and GSL showed that the natural infestation pressure was low with 2.6 rape stem weevil larvae per plant and 2.8 cabbage stem weevil larvae per plant. Hence, a statistically significant differentiation between genotypes was found only at one location in 2012 and 2013 and only for number of rape stem weevil larvae. For cabbage stem weevil no such significant differentiation was given. In the two remaining environments no significant correlation could be determined between number of rape stem weevil larvae per plant and either the contents of total GSL, sum of aliphatic GSL, sum of indolic GSL or each of the eleven single GSL. Furthermore, principal component analysis and comparisons between groups of low- and high-infested genotypes did not indicate a relation between GSL contents or GSL composition and the host plant preference or the relative feeding damage of rape stem weevil larvae in stems. However, the resynthesized line S30 was notable with low numbers of larvae per plant as well as low percentages of larvae feeding inside stems in both environments. Nevertheless, the GSL contents and composition of S30 did not show specific characteristics. For the objective of mapping QTL involved in the GSL metabolism, GSL were determined in leaf, stem and seed tissues from 120 DH lines of the DH-Population ‘L16 x Express’. The two parents L16 and Express do not only differ in their GSL content in seeds (L16 59,0 µmol vs. Express 26.4 µmol) and green matter (L16 1,1 µmol vs. Express 6,2 µmol), but also in their composition of aliphatic and indolic GSL (L16 ~31 % Indol-GSL vs. Express ~10% Indol-GSL). Averaged over two locations GSL contents of the population at bud stage were as low as 5.4 µmol in stems and 3.7 µmol in leaves. However, seeds at maturity had an average content of 48.6 µmol. Heritabilities of GSL traits with significant genotypic variation ranged between 0.64 and 0.86 in stems, between 0.55 and 0.89 in leaves and between 0.70 and 0.98 in seeds. The correlation of total GSL contents between stem and leaf material was 0.95. Between stem (leaf) and seed this correlation was lower with a correlation coefficient of 0.52 (0.53). The constructed linkage map comprised a total of 4003 SNP markers, which covered 2050 cM distributed over 19 linkage groups (LG). The average distance between two markers was 2 cM. In total 115 QTL were mapped, of which 49 were responsible for GSL contents in seeds, 35 in stems and 31 in leaves. For aliphatic GSL three major regions on linkage groups A03, C02 and C09 could be found. While QTL on LG A03 and C09 were assigned to all three plant tissues, QTL on LG C02 affected specifically GSL contents of seeds. For indolic GSL in leaves and stem QTL were localized on LG A02 and C07, which did not correspond to those from seeds (A03, C02 and C05). The results indicate that 1) accumulation of aliphatic and indolic GSL were regulated by different regions of the genome, 2) GSL in stem and leaves were often controlled by identical genome regions and 3) GSL accumulation in seeds was partly controlled by the same QTL as in leaves and stems, but also partly by seed specific QTL.