Molecular diversity of arbuscular mycorrhizal fungi in onion roots from organic and conventional farming systems in the Netherlands

Diversity and colonization levels of naturally occurring arbuscular mycorrhizal fungi (AMF) in onion roots were studied to compare organic and conventional farming systems in the Netherlands. In 2004, 20 onion fields were sampled in a balanced survey between farming systems and between two regions, namely, Zeeland and Flevoland. In 2005, nine conventional and ten organic fields were additionally surveyed in Flevoland. AMF phylotypes were identified by rDNA sequencing. All plants were colonized, with 60% for arbuscular colonization and 84% for hyphal colonization as grand means. In Zeeland, onion roots from organic fields had higher fractional colonization levels than those from conventional fields. Onion yields in conventional farming were positively correlated with colonization level. Overall, 14 AMF phylotypes were identified. The number of phylotypes per field ranged from one to six. Two phylotypes associated with the Glomus mosseae-coronatum and the G. caledonium-geosporum species complexes were the most abundant, whereas other phylotypes were infrequently found. Organic and conventional farming systems had similar number of phylotypes per field and Shannon diversity indices. A few organic and conventional fields had larger number of phylotypes, including phylotypes associated with the genera Glomus-B, Archaeospora, and Paraglomus. This suggests that farming systems as such did not influence AMF diversity, but rather specific environmental conditions or agricultural practice

Resistance to Fusarium basal rot and response to arbuscular mycorrhizal fungi in Allium

Onion (Allium cepa L.) cultivation in low input and organic farming systems is hampered by Fusarium basal rot (FBR) and the limited ability of onion to take up nutrients like phosphorus. The symbiosis with arbuscular mycorrhizal fungi (AMF) contributes to plant acquisition of phosphorus, among other benefits. This PhD research studied the potential contributions from A. fistulosum and A. roylei to breed onion cultivars with resistance to FBR and enhanced benefit from the symbiosis with AMF. The genetic basis of these traits was studied in an A. cepa x (A. roylei x A. fistulosum) population. A collection of Fusarium isolates was analysed using AFLP markers. The most abundant species was F. oxysporum (with isolates clustered in two clades) followed by F. proliferatum. The Allium species were screened for FBR resistance using one F. oxysporum isolate from each clade, and one F. proliferatum isolate. Allium fistulosum showed high levels of resistance to these three isolates and A. roylei intermediate levels of resistance. High level of resistance from A. fistulosum was dominantly expressed in the A. roylei x A. fistulosum hybrid and the tri-hybrid population. A molecular linkage map based on AFLP markers was developed for the A. roylei x A. fistulosum hybrid. A QTL for FBR resistance from A. roylei was mapped on chromosome 2, and a QTL from A. fistulosum on chromosome 8. Each QTL separately had significant effect on FBR but did not confer complete resistance, thus more QTLs from A. fistulosum remain to be discovered. Regarding Allium-AMF relationship, a first step of research studied genetic diversity and colonization levels of naturally occurring AMF, comparing organic and conventional onion farming in the Netherlands. All plants were colonized with 60% average arbuscular colonization. Onion yields were positively correlated with colonization. AMF phylotypes were identified by rDNA sequencing. The number of phylotypes per field ranged from one to six. Two Glomus-A phylotypes were the most abundant, whereas other phylotypes were infrequently found. Organic and conventional fields had similar number of phylotypes and Shannon diversity indices. A few organic and conventional fields had larger number of phylotypes, which suggested that specific environmental conditions or agricultural practices influence AMF diversity. The genetic basis for the response to AMF in the tri-hybrid Allium population was evaluated in two independent greenhouse experiments. The weights of mycorrhizal plants were significantly larger than the non-mycorrhizal plants. Mycorrhizal Responsiveness (MR) was negatively correlated with plant weight in the non-mycorrhizal condition and was therefore considered unsuitable as an index for plant breeding purposes. Two new indices were proposed: mycorrhizal benefit (MB) and mycorrhizal breeding value (MV). Tri-hybrid genotypes showed transgressive segregation for plant weight, MB, and MV. Two QTLs from A. roylei for these traits were detected on chromosomes 2 and 3. A QTL from A. fistulosum for MV (but not MB), plant weight and the number of stem-borne roots was found on linkage group 9. Positive correlations between plant weight, rooting system and benefit from mycorrhiza were observed, which open prospects to combine these traits in the development of more robust onion cultivars. <br/

Possibilities for breeding to improve responsiveness to arbuscular mycorrhizal fungi in onion

Arbuscular mycorrhizal fungi (AMF) play an important role in the uptake of nutrients and water from soil. However, some crops, for example onion, Allium cepa L., have a poorly developed root system. As a result, onion plants need a lot of fertiliser for growth, and they are sensitive to drought. The aim of this project is to study the beneficial effects of mycorrhizal fungi on the growth and development of Allium species and to determine whether it is possible to improve onions for mycorrhizal responsiveness by breeding. Variation among Allium species indicated that selection and thus breeding for high responsiveness to AMF is possible. Two years of experiments with genotypes of a population segregating for mycorrhiza responsiveness indicated that increase in dry matter may be a more reliable trait than responsivenes

Genetic analysis of the interaction between Allium species and arbuscular mycorrhizal fungi

The response of Allium cepa, A. roylei, A. fistulosum, and the hybrid A. fistulosum × A. roylei to the arbuscular mycorrhizal fungus (AMF) Glomus intraradices was studied. The genetic basis for response to AMF was analyzed in a tri-hybrid A. cepa × (A. roylei × A. fistulosum) population. Plant response to mycorrhizal symbiosis was expressed as relative mycorrhizal responsiveness (R') and absolute responsiveness (R). In addition, the average performance (AP) of genotypes under mycorrhizal and non-mycorrhizal conditions was determined. Experiments were executed in 2 years, and comprised clonally propagated plants of each genotype grown in sterile soil, inoculated with G. intraradices or non-inoculated. Results were significantly correlated between both years. Biomass of non-mycorrhizal and mycorrhizal plants was significantly positively correlated. R' was negatively correlated with biomass of non-mycorrhizal plants and hence unsuitable as a breeding criterion. R and AP were positively correlated with biomass of mycorrhizal and non-mycorrhizal plants. QTLs contributing to mycorrhizal response were located on a linkage map of the A. roylei × A. fistulosum parental genotype. Two QTLs from A. roylei were detected on chromosomes 2 and 3 for R, AP, and biomass of mycorrhizal plants. A QTL from A. fistulosum was detected on linkage group 9 for AP (but not R), biomass of mycorrhizal and non-mycorrhizal plants, and the number of stem-borne roots. Co-segregating QTLs for plant biomass, R and AP indicate that selection for plant biomass also selects for enhanced R and AP. Moreover, our findings suggest that modern onion breeding did not select against the response to AMF, as was suggested before for other cultivated species. Positive correlation between high number of roots, biomass and large response to AMF in close relatives of onion opens prospects to combine these traits for the development of more robust onion cultivar