Abstract:

Fusarium oxysporum f. sp. cubense (Foe), a soil-borne fungus affecting bananas (Musa
spp.), is considered one of the most devastating pathogens in agricultural history. The
fungus infects banana roots, colonises the rhizome and pseudo stem, and causes a lethal
wilting disease called Fusarium wilt. Fusarium wilt can cause losses of up to 100% in
banana fields planted with susceptible genotypes, without any known cure. Host plant
resistance to Foe, which has been identified in the Musa gene pool, is widely considered the
only feasible method to control the disease. However, conventional breeding to improve
susceptible banana varieties is hampered by male and female sterility and the long
generation period of the crop.
The inheritance of resistance in Musa to Foe race 1 in the 'SN8075F2' population,
derived from the cross of cultivar ‘Sukali Ndiizi' and the diploid banana ‘TMB2X8075-7’, was
investigated in this study. One hundred and sixty three F2 progenies were evaluated for their
response to Fusarium wilt in a screen house experiment. The test plants were inoculated by
mixing loam soil with millet grains, colonized by Foe race 1, in polythene pots. One hundred
and fifteen genotypes were categorized as susceptible and 48 as resistant based on rhizome
discolouration. Mendelian segregation analysis for susceptible vs. resistant fitted the
segregation ratio of 3:1 (X2 =1.72, P=0.81), suggesting that resistance to Fusarium wilt in the
diploid line ‘TMB2X8075-7’ is provided by a single recessive gene. The name pd1 (Panama
disease 1) has been proposed for the recessive gene responsible for resistance to Fusarium
wilt in the diploid line ‘TMB2X8075-7’.
DArT markers were identified in a segregating population following a cross between
the susceptible banana cultivar ‘Sukali Ndiizi' and a resistant diploid banana ‘TMB2X8075-7’.
The markers were in qualitative linkage disequilibrium, with 13 markers linked to resistance
and 88 markers associated with susceptibility to Foe race 1. Putative functions have been
assigned to candidate genes through in-silico database analysis including Laccase-25
(LAC25), Homeobox-leucine zipper protein (HOX32), SWIM zinc finger family protein,
Transcription factor MYB3, GDSL esterase/lipase EXL3 among others. The candidate
markers and genes closely associated with resistance/susceptibility could also be used in
genetic engineering or for marker-assisted selection (MAS) in breeding for Fusarium wilt
resistance.
The Foe race 1-banana binomial interaction of three genotypes (‘Sukali Ndiizi’ AAB,
'Mbwazirume' AAA and 'TMB2X8075-7 AA) was investigated by deep sequencing of the root
transcriptome to study Fusarium wilt resistance in bananas. A total of 299 million raw reads,
each about 100-nucleotides long, were derived from cDNA libraries constructed at four time
I
IV
k
points: 0, 48, 96 and 192 hrs after inoculation with Foe racel. From the 10136 differentially
expressed genes (DEGs), 5640 (55.7%) were uniquely up-regulated and 4496 (44.4%)
uniquely down-regulated in the libraries of ‘Mbwazirume’, ‘TMB2X28075-7’ and ‘Sukali Ndiizi
at 48, 96 and 192 hrs post inoculation. The DEGs were annotated with Gene Ontology (GO)
terms and pathway enrichment analysis, and significant pathway categories identified
included the ‘Metabolic’, ‘Ribosome’, ‘Plant-pathogen interaction' and ‘Plant hormone signal
transduction’ pathways. Salicylic acid and ethylene were stimulated in the 'Plant hormone
signal transduction’ pathways in all the three genotypes. Fifteen defence-related genes were
identified as candidate genes contributing to Fusarium wilt resistance in banana. These
candidate genes could be used to improve susceptible banana genotypes to enhance levels
of fungal disease resistance to Foe race 1.