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* Email: john.clarkson@warwick.ac.uk; 1 Warwick Crop Centre, School of Life Sciences, University of Warwick, Wellesbourne
Campus, Warwick CV35 9EF ; 2 East Malling Research, East Malling, Kent, ME19 6BJ
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Fusarium oxysporum is a soilborne fungus that causes root and stem rots in a wide range of crop plants. The F. oxysporum species complex includes non-pathogenic and pathogenic isolates adapted to particular hosts (Fig. 1a).
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F. oxysporum f.sp. cepae (FOC) causes a basal rot of onion bulbs (Fig. 1b) and losses in the
UK are estimated at £11M per annum.
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A BBSRC HAPI project is investigating the genetic basis of pathogenicity in FOC as well as developing markers for resistance in new onion lines (Fig 1c). Previous work on F. oxysporum in tomato has identified the importance of Secreted In Xylem’ (SIX) genes in pathogenicity.
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Fusarium sequencing and pathogenicity: Genome sequencing and direct PCR of potential pathogenicity genes has been carried out for a selection of pathogenic and non-pathogenic F. oxysporum isolates from onion.
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Transcriptome analysis: RNA sequencing is being used to explore genes expressed by pathogenic and non pathogenic F. oxysporum isolates on both resistant and susceptible onion lines.
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Onion genotyping: An onion population segregating for resistance / susceptibility to FOC is being genotyped in order to map resistance (results not shown).
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Pre-breeding onion lines and mapping populations: Further onion populations for mapping
FOC resistance are being developed as well as pre-breeding material to enable the process of introgressing resistance genes into commercial onion cultivars (results not shown).
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Figure 1: a)
isolates; b) onion basal rot; c) resistant (left) and susceptible (middle, right) onion lines.
Pathogenicity of
isolates from onion is related to presence of SIX genes
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Seven SIX genes (SIX 3, 5, 7, 9, 10, 12, 14) and three other putative effectors were detected within F. oxysporum isolates from onion by PCR and also identified in genome analysis.
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F. oxysporum isolates which had the full complement of effector genes were highly pathogenic on onion bulbs (Fig.
2, blue bars) whereas isolates where these genes were absent were non-pathogenic (green bars). One isolate (55) showed intermediate pathogenicity and had only two SIX genes and two other effectors (orange bar).
SIX genes are expressed
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An agar based system was developed for transcriptome analysis of FOC infection on onion (Fig. 3). An initial time course study showed that root penetration occurred after
24h and high levels of colonisation were apparent after 96h
(Fig. 4). SIX genes were increasingly expressed over time with SIX 3, 5 and 9 representing a high proportion of the transcripts.
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The presence of SIX genes is related to pathogenicity of
FOC. Further work will compare the transcriptome of pathogenic and non-pathogenic F. oxysporum isolates and also quantify gene expression during early infection.
Non-pathogenic
isolates Pathogenic
isolates
Figure 2: Pathogenicity of different isolates of
on onion bulbs.
Figure 3: Onions grown on agar for transcriptome studies; uninoculated control (left), inoculated with FOC (right) eight days after inoculation at 25°C.
Figure 4: Colonisation of onion root by FOC after 96h.