Crossability barriers in the interspecific hybridization between Oryza sativa and O. meyeriana

Analysis of allelic variants of RhMLO genes in rose and functional studies on susceptibility to powdery mildew related to clade V homologs

Rose has 19 MLO genes. Of these, RhMLO1 and RhMLO2 were shown to be required for powdery mildew infection, which suggests their potential as susceptibility targets towards disease resistance. Powdery mildew, caused by Podosphaera pannosa, is one of the most serious and widespread fungal diseases for roses, especially in greenhouse-grown cut roses. It has been shown that certain MLO genes are involved in powdery mildew susceptibility and that loss of function in these genes in various crops leads to broad-spectrum, long-lasting resistance against this fungal disease. For this reason, these MLO genes are called susceptibility genes. We carried out a genome-wide identification of the MLO gene family in the Rosa chinensis genome, and screened for allelic variants among 22 accessions from seven different Rosa species using re-sequencing and transcriptome data. We identified 19 MLO genes in rose, of which four are candidate genes for functional homologs in clade V, which is the clade containing all dicot MLO susceptibility genes. We detected a total of 198 different allelic variants in the set of Rosa species and accessions, corresponding to 5-15 different alleles for each of the genes. Some diploid Rosa species shared alleles with tetraploid rose cultivars, consistent with the notion that diploid species have contributed to the formation of tetraploid roses. Among the four RhMLO genes in clade V, we demonstrated using expression study, virus-induced gene silencing as well as transient RNAi silencing that two of them, RhMLO1 and RhMLO2, are required for infection by P. pannosa and suggest their potential as susceptibility targets for powdery mildew resistance breeding in rose.

Refinement of a clearing protocol to study crassinucellate ovules of the sugar beet (Beta vulgaris L., Amaranthaceae)

BACKGROUND

Clearing methods allow relatively quick processing of plant material and examination of cellular structures by rendering tissues and organs translucent. They have been adapted for plant embryology, primarily to study ovule development, megasporogenesis, megagametogenesis and embryogenesis. Such clearing methods overcome several disadvantages of the conventional embedding-sectioning techniques that are arduous and time-consuming. Although numerous protocols with different clearing solutions have been described, there have been no reports to date proposing a reliable method to clear the crassinucellate ovules of the sugar beet (Beta vulgaris L.), an economically important crop. Therefore, this study aims to find a suitable approach to improve the tissue transparency of sugar beet ovules at different developmental stages.

RESULTS

We established a methyl salicylate-based protocol that significantly improved the transparency of the B. vulgaris ovule structures, which allowed us to observe the megagameto- and embryogenesis of that species. This was achieved by (1) chemical softening of the tissues; (2) vacuum pump-assisted infiltration step; (3) shaking-assisted incubation with clearing mixtures; and (4) manual removal of the chemically softened seed coat.

CONCLUSIONS

The effectiveness of our method is due to the strategy combining various approaches at different stages of the procedure aiming at increasing the accessibility of the internal ovule structures to the clearing solution. The results of this study may be applied in sugar beet breeding programs, and it will provide a basis for further investigation of numerous aspects of the species' embryology. Moreover, that unique approach may be easily adapted to other species developing crassinucellate ovules.

Disruption of endosperm development is a major cause of hybrid seed inviability between Mimulus guttatus and Mimulus nudatus

Divergence of developmental mechanisms within populations could lead to hybrid developmental failure, and might be a factor driving speciation in angiosperms. We investigate patterns of endosperm and embryo development in Mimulus guttatus and the closely related, serpentine endemic Mimulus nudatus, and compare them to those of reciprocal hybrid seed. We address whether disruption in hybrid seed development is the primary source of reproductive isolation between these sympatric taxa. M. guttatus and M. nudatus differ in the pattern and timing of endosperm and embryo development. Some hybrid seeds exhibit early disruption of endosperm development and are completely inviable, while others develop relatively normally at first, but later exhibit impaired endosperm proliferation and low germination success. These developmental patterns are reflected in mature hybrid seeds, which are either small and flat (indicating little to no endosperm) or shriveled (indicating reduced endosperm volume). Hybrid seed inviability forms a potent reproductive barrier between M. guttatus and M. nudatus. We shed light on the extent of developmental variation between closely related species within the M. guttatus species complex, an important ecological model system, and provide a partial mechanism for the hybrid barrier between M. guttatus and M. nudatus.

Characterization of the MLO gene family in Rosaceae and gene expression analysis in Malus domestica

BACKGROUND

Powdery mildew (PM) is a major fungal disease of thousands of plant species, including many cultivated Rosaceae. PM pathogenesis is associated with up-regulation of MLO genes during early stages of infection, causing down-regulation of plant defense pathways. Specific members of the MLO gene family act as PM-susceptibility genes, as their loss-of-function mutations grant durable and broad-spectrum resistance.

RESULTS

We carried out a genome-wide characterization of the MLO gene family in apple, peach and strawberry, and we isolated apricot MLO homologs through a PCR-approach. Evolutionary relationships between MLO homologs were studied and syntenic blocks constructed. Homologs that are candidates for being PM susceptibility genes were inferred by phylogenetic relationships with functionally characterized MLO genes and, in apple, by monitoring their expression following inoculation with the PM causal pathogen Podosphaera leucotricha.

CONCLUSIONS

Genomic tools available for Rosaceae were exploited in order to characterize the MLO gene family. Candidate MLO susceptibility genes were identified. In follow-up studies it can be investigated whether silencing or a loss-of-function mutations in one or more of these candidate genes leads to PM resistance.

Loss of function in Mlo orthologs reduces susceptibility of pepper and tomato to powdery mildew disease caused by Leveillula taurica

Powdery mildew disease caused by Leveillula taurica is a serious fungal threat to greenhouse tomato and pepper production. In contrast to most powdery mildew species which are epiphytic, L. taurica is an endophytic fungus colonizing the mesophyll tissues of the leaf. In barley, Arabidopsis, tomato and pea, the correct functioning of specific homologues of the plant Mlo gene family has been found to be required for pathogenesis of epiphytic powdery mildew fungi. The aim of this study was to investigate the involvement of the Mlo genes in susceptibility to the endophytic fungus L. taurica. In tomato (Solanum lycopersicum), a loss-of-function mutation in the SlMlo1 gene results in resistance to powdery mildew disease caused by Oidium neolycopersici. When the tomato Slmlo1 mutant was inoculated with L. taurica in this study, it proved to be less susceptible compared to the control, S. lycopersicum cv. Moneymaker. Further, overexpression of SlMlo1 in the tomato Slmlo1 mutant enhanced susceptibility to L. taurica. In pepper, the CaMlo2 gene was isolated by applying a homology-based cloning approach. Compared to the previously identified CaMlo1 gene, the CaMlo2 gene is more similar to SlMlo1 as shown by phylogenetic analysis, and the expression of CaMlo2 is up-regulated at an earlier time point upon L. taurica infection. However, results of virus-induced gene silencing suggest that both CaMlo1 and CaMlo2 may be involved in the susceptibility of pepper to L. taurica. The fact that overexpression of CaMlo2 restored the susceptibility of the tomato Slmlo1 mutant to O. neolycopersici and increased its susceptibility to L. taurica confirmed the role of CaMlo2 acting as a susceptibility factor to different powdery mildews, though the role of CaMlo1 as a co-factor for susceptibility cannot be excluded.