GENES UP-REGULATED IN TOLERANT CAVENDISH BANANA ROOTS IN RESPONSE TO FUSARIUM OXYSPORUM F. SP. CUBENSE INFECTION

Fusarium wilt of banana, caused by Fusarium oxysporum f. sp. cubense (Foc), is regarded as one of the most devastating diseases. Cavendish cultivars (AAA genome) are highly susceptible to race 4 of the pathogen. The regulation of defence-related genes in ‘GCTCV-218’, a Fusarium wilt-tolerant Cavendish selection, was studied. A cDNA library, enriched for genes up-regulated in ‘GCTCV-218’ after Foc infection when compared to susceptible Cavendish cultivar ‘Williams’, was previously constructed using suppression subtractive hybridisation (SSH) and screened using DNA micro-array technology. Potentially induced clones (334) were selected and sequenced. Four defence-associated genes (catalase 2, pectin acetyl esterase (PAE), PR-1 and PR-3) were selected for expression profile analysis using real-time reverse transcriptase PCR (RT-PCR). ‘GCTCV-218’ showed significantly lower disease severity and incidence than the susceptible ‘Williams’ in replicated greenhouse and field trials. Several defence-related transcripts were identified by BLASTX searches, including genes coding for PR-1, PAE, xylanase inhibitor, peroxidase, catalase 2, metallothionein, response regulator 6 and tripsin inhibitor. Inverse Northern dot blot data indicated that all of the above-mentioned transcripts were up-regulated in ‘GCTCV-218’ 6 hours post inoculation (hpi) compared to ‘Williams’. Quantitative RT-PCR results confirmed that all four genes were differentially expressed in ‘GCTCV-218’ 3 and 6 hpi, confirming SSH results. PR-1 and PAE were induced very early (3 hpi) in ‘GCTCV-218’, while PR-3 and catalase 2 followed with a significant induction 6 hpi. This study concluded that ‘GCTCV-218’ is able to respond rapidly to Foc infection by activating both a biochemical and structural defence mechanism. The tolerance of ‘GCTCV-218’ was further linked to a significant increase in the induction cell wall-associated phenolic compounds.