|Title||The genetics and mechanism of avoidance of rust infection in Hordeum chilense|
|Author(s)||Vaz Patto, M.C.|
|Source||Wageningen University. Promotor(en): P. Stam; A. Martín; R.E. Niks. - S.l. : S.n. - ISBN 9789058083975 - 121|
|Publication type||Dissertation, internally prepared|
|Keyword(s)||hordeum chilense - plantenziekteverwekkende schimmels - roestziekten - verdedigingsmechanismen - puccinia hordei - appressoria - thigmotropie - genetische merkers - genetische kartering - hordeum chilense - plant pathogenic fungi - rust diseases - puccinia hordei - defence mechanisms - appressoria - thigmotropism - genetic markers - genetic mapping|
Hordeum chilense is a perennial species occurring in Chile and Argentina. This wild barley species shows a very wide range of variation of morphological and agronomic characters and crosses easily with other members of the Triticeae tribe.
H. chilense is one of the five wild barley species in which avoidance of rust fungi as been reported. The avoidance mechanism to rust fungi is characterised by stomata overgrowth by the fungal germ tube, with no appressorium formation or penetration of the stomata, resulting in early failure of the infection process. This avoidance is an interesting mechanism of defence against rust infection, especially when it could be transferred to cultivated cereal species.
In this thesis the genetic basis of this avoidance mechanism and associations of avoidance with other characters in H. chilense were established.
We tested the hypothesis that the avoidance character occurs in a certain morphologically and molecularly distinct ecotype of H. chilense . A H. chilense collection of 88 accessions was characterised for morphological and agronomic traits, level of avoidance of Puccinia hordei , habitat of origin and AFLP fingerprint. Cluster analysis using both morphological/agronomic and AFLP fingerprint data suggested three distinct clusters of accessions. High avoidance was typical to the accessions of one of these three clusters. The accessions in the cluster with the higher levels of avoidance had been collected in humid habitats. This putative subspecific taxon was further characterised by shorter and wider spikes, more erect culms, a shorter uppermost internode until flag leaf and a greater amount of stomata density on the abaxial leaf side. We conclude that H. chilense consists of at least three rather well defined morphologically and genetically distinct subspecific taxa, one of which has a very high level of avoidance of barley leaf rust.
We studied the effect of the cuticular wax layer on the orientation of germ tube growth and on appressorium differentiation of P. hordei . Several orientation parameters and appressorium differentiation of P. hordei germ tubes were measured on H. chilense leaves with and without the wax layer.
Removal of the cuticular wax layer did not result in poor and also not in better germ tube orientation. Evidence was obtained that epidermal cell junctions rather than the wax crystals provided the landmarks to guide germ tubes along the transverse axis of the leaf. On high avoidance accessions the removal of the wax layer allowed appressoria to develop over stomata that would otherwise be overgrown. This suggests that the overgrowth of stomata on H. chilense leaves by P. hordei germ tubes is mainly due to the wax covering of the stomatal apparatus.
A molecular map of the wild barley H. chilense would greatly facilitate to map and efficiently transfer agronomic traits from H. chilense to cereal genomes. As a first step towards a map construction, we evaluated AFLP markers in H. chilense using diploid wheat ( Triticum monococcum ) and cultivated barley ( H. vulgare ) as references. H. chilense showed a higher percentage of polymorphisms than diploid wheat. It was remarkable that, based on AFLP markers generated by 12 Eco RI/ Mse I primer combinations, the cultivated barley was more similar to diploid wheat than to H. chilense . Even more surprisingly, the genetic distance between the interfertile H. chilense accessions (H1 and H7) was almost as large as the genetic distance between the non-crossable cultivated barley and diploid wheat. Eco RI/ Mse I AFLP fingerprints revealed more polymorphism than Pst I/ Mse I AFLP fingerprints for all species tested and were chosen for generating the first AFLP linkage map of H. chilense .
The mapping population consisted of 100 F 2 plants derived from a cross between two genetically distinct H. chilense accessions that were contrasting for the level of avoidance, H1 and H7. The constructed map contained 443 AFLP markers, on nine long and ten shorter linkage groups, covering a genetic distance of 714 cM. Nine of these linkage groups were assigned to H. chilense chromosomes using a set of H. chilense in T. aestivum addition and substitution lines. Strong clustering of AFLP markers was observed at putative centromeric regions. A skeletal map with a uniform distribution of markers was extracted from this linkage map. This skeletal map was applied to detect and map QTLs underlying avoidance and stoma density on the abaxial leaf epidermis.
Three QTLs were detected for avoidance and three other QTLs for stoma density. Both traits segregated independently in the F 2 .
Avoidance on H. chilense is effective to barley leaf rust, wheat leaf rust and rye leaf rust. As H. chilense shows good crossability with several cultivated cereals avoidance of leaf rusts may be introgressed into these cultivated cereals, especially wheat. Addition and substitution lines of H. chilense in Chinese Spring wheat cv. are already available or being developed as a first step towards transfer of the genes governing the avoidance trait.