Direct infection of embryogenic tissue cultures of haploidBrassica napus with resting spores ofPlasmodiophora brassicae

Differential expression of miRNAs in Brassica napus root following infection with Plasmodiophora brassicae

Canola (oilseed rape, Brassica napus L.) is susceptible to infection by the biotrophic protist Plasmodiophora brassicae, the causal agent of clubroot. To understand the roles of microRNAs (miRNAs) during the post-transcriptional regulation of disease initiation and progression, we have characterized the changes in miRNA expression profiles in canola roots during clubroot disease development and have compared these to uninfected roots. Two different stages of clubroot development were targeted in this miRNA profiling study: an early time of 10-dpi for disease initiation and a later 20-dpi, by which time the pathogen had colonized the roots (as evident by visible gall formation and histological observations). P. brassicae responsive miRNAs were identified and validated by qRT-PCR of miRNAs and the subsequent validation of the target mRNAs through starBase degradome analysis, and through 5' RLM-RACE. This study identifies putative miRNA-regulated genes with roles during clubroot disease initiation and development. Putative target genes identified in this study included: transcription factors (TFs), hormone-related genes, as well as genes associated with plant stress response regulation such as cytokinin, auxin/ethylene response elements. The results of our study may assist in elucidating the role of miRNAs in post-transcriptional regulation of target genes during disease development and may contribute to the development of strategies to engineer durable resistance to this important phytopathogen.

Transcriptome analysis of Arabidopsis clubroots indicate a key role for cytokinins in disease development

The clubroot disease of the family Brassicaceae is caused by the obligate biotrophic protist Plasmodiophora brassicae. Infected roots undergo a developmental switch that results in the formation of aberrant roots (clubs). To investigate host gene expression during the development of the disease, we have used the Arabidopsis ATH1 genome array. Two timepoints were chosen, an early timepoint at which the pathogen has colonized the root but has induced only very limited change of host cell and root morphology and a later timepoint at which more than 60% of the host root cells were colonized and root morphology was drastically altered. At both timepoints, more than 1,000 genes were differentially expressed in infected versus control roots. These included genes associated with growth and cell cycle, sugar phosphate metabolism, and defense. The involvement of plant hormones in club development was further supported; genes involved in auxin homeostasis, such as nitrilases and members of the GH3 family, were upregulated, whereas genes involved in cytokinin homeostasis (cytokinin synthases and cytokinin oxidases/dehydrogenases) were already strongly downregulated at the early timepoint. Cytokinin oxidase/dehydrogenase overexpressing lines were disease resistant, clearly indicating the importance of cytokinin as a key factor in clubroot disease development.

Toxin-based in-vitro selection and its potential application to date palm for resistance to the bayoud Fusarium wilt

Date palm (Phoenix dactylifera L.) is qualified as a 'tree' of great ecological and socio-economical importance in desert oases. Unfortunately, it is being decimated, especially in Morocco and Algeria, by a fusariosis wilt called bayoud and caused by Fusarium oxysporum f. sp. albedinis (Fao). Controlling this disease requires the implementation of an integrated management program. Breeding for resistance is one of the most promising component strategies of this program. Few naturally resistant cultivars with a mediocre fruit quality (dates) are known. Conventional and non-conventional methods are under development and have to use the simplest and easiest methods to screen for resistant individuals. The use of pathogen toxins as selective agents at the tissue culture step might be a source of variability that can lead to the selection of individuals with suitable levels of resistance to the toxin and/or to the pathogen among the genetic material available. Foa produces toxins such as fusaric, succinic, 3-phenyl lactic acids and their derivatives, marasmins and peptidic toxins. These toxins can be used bulked or separately as selective agents. The aim of this contribution was to give a brief overview on toxins and their use as a mean to select resistant lines and to initiate a discussion about the potential use of this approach for the date palm-Foa pathosystem. This review does not pretend to be comprehensive or exhaustive and was prepared mainly to highlight the potential use of Foa toxins for selecting date palm individuals with a suitable resistance level to bayoud using toxin-based selective media.

Surface Disinfestation of Resting Spores of Plasmodiophora brassicae Used to Infect Hairy Roots of Brassica spp

ABSTRACT Resting spores of Plasmodiophora brassicae were surface-disinfested by treatment with 2% chloramine-T for 20 min and then with an antibiotic solution (1,000 ppm of colistin sulfate, 1,000 ppm of vancomycin hydrochloride, and 6,000 ppm of cefotaxime sodium) for 1 day. The disinfested resting spores were used to inoculate hairy roots of cabbage (Brassica oleracea var. capitata cv. Fuji Wase), Chinese cabbage (B. pekinensis cv. Musou Hakusai), turnip (B. rapa var. rapifera cv. Wase Okabu), and rape (B. napus line Dc 119). Differences among hosts in susceptibility to clubroot in hairy roots were evident. Chinese cabbage and turnip hairy roots supported the highest percentages of root hair infection (53.3 to 80%) and the greatest production of zoosporangial groups (8.5 to 32.5 per root). Moreover, gall formation was observed only on Chinese cabbage and turnip hairy roots. The morphology of zoo-sporangia, plasmodia, and resting spores in diseased hairy roots was found to be identical to that in infected intact plants by both light and scanning electron microscopy. Pathogenicity tests confirmed the infectivity of resting spores produced in hairy roots. Thus, the hairy root culture technique should prove useful as a dual culture system for P. brassicae.

Resistance responses to Phoma lingam of plants regenerated from selected cell and embryogenic cultures of haploid Brassica napus

Resistant plants and plants with reduced susceptibility against the pathogen Phoma lingam could be regenerated from selected callus and embryogenic cultures of haploid rape (Brassica napus) previously treated with mutagens. In the two in vitro selection systems used - absence of fungus growth on the cultures after incubation with parasite spores and resistance to the toxic filtrate - the resistance to the toxin was effective. In addition, some regenerants with increased tolerance were obtained from unselected cultures. Resistance tests on regenerated plants were carried out by inoculation of whole plants in the greenhouse, reproducing as much as possible the infection mechanisms which take place under natural conditions. Preliminary results on resistance of the progeny of single susceptible and tolerant regenerants seem to indicate that the acquired resistances are of a genetic nature.

Anther culture as a breeding tool in rape : II. Progeny analyses of androgenetic lines and induced mutants from haploid cultures

Progeny analysis of androgenetic plants from inbred rape-seed (Brassica napus) shows that selective growth of microspores can occur in cultured anthers. The property of privileged growth in culture seems to be linked to such characters as flowering time and seed glucosinolate content which can be analyzed in regenerated plants. This type of selection and the fact that more variability is visible in regenerants from different microspores than in the progeny of the highly inbred anther donor line, demonstrates the higher degree of homozygosity in the doubled amphihaploids of B. napus. Furthermore, it is shown that haploid genomes of rape may be mutable. Thus it is possible to obtain several different homozygous lines from a single microspore. A system of haploid embryoids arising from single cells of the primary microspore regenerant has also been used to produce experimentally induced mutants. It is demonstrated that recessive mutations can be obtained in a homozygous state in doubled haploid regenerants from mutagenized haploid single cells.