Abstract:
Striga hermonthica (SH) is a parasitic weed that attacks and significantly reduces the
yield of maize in Africa. The genetic interactions responsible for resistance or
susceptibility of hosts to the parasite and the genetic differentiation that exists between
and within SH populations are not fully known. This study investigated the genetic
diversity of SH populations in the largest maize producers in Sub-Saharan Africa
(Kenya and Nigeria) and; the genetic responses of a susceptible (5057) and a resistant
(ZD05) maize genotype to SH infestation.
The SH plants were collected from farms across western Kenya (KSH) and northern
Nigeria (NSH) in October 2012 and authenticated at the Department of Botany,
University of Ibadan (UIH-22774). The plants (n=1029) were then genotyped with
1576 single nucleotide polymorphism markers and indices of genetic diversity
[effective alleles (Ne), Shannon’s information index (I), expected (He) and observed
heterozygosity (Ho)] were determined. Population structure and fixation index (Fst),
were assessed to identify genetic differentiation between and within KSH and NSH
populations. Two maize varieties (5057 and ZD05) were divided into four groups of
nine plants each and planted in rhizotrons (root observation chambers). Seven days
after planting, three groups of each maize genotype were infested with pre-germinated
SH and the fourth was used as uninfested control. Root tissue was taken at 3, 9 and 22
days post infestation (DPI) and total ribonucleic acid (RNA) was extracted using
standard method. The root transcriptome was sequenced using next-generation
sequencing. Gene expression levels of secondary metabolism, defence, and
antiapoptotic genes were determined by profiling the messenger RNA levels and
comparing the log2 fold-change (LFC) between the infested and uninfested maize
plants and between the genotypes. Data were analysed using two-way ANOVA at
α0.05.
The two populations of SH displayed high levels of genetic diversity. KSH showed
higher levels (Ne=1.41±0.01, I=0.38±0.01, Ho=0.28, He=0.25±0.0) than NSH
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(Ne=1.41±0.01, I=0.332±0.01, Ho=0.21, He=0.20±0.00). Significant genetic
differentiation (Fst=0.15) was observed between the two populations and between
three subpopulations detected within the NSH population (Fst =0.053). At 3DPI,
secondary metabolism and defence genes, benzoxazineless 1 (LFC=2.5) and chalcone
synthase 2 (LFC=3.2), were upregulated in ZD05, while in 5057, antiapoptotic genes,
bax inhibitor1 (LFC=1.4) and bcl-2 binding anthanogene-1 (LFC=1.7) were
upregulated. At 9DPI, secondary metabolism and defence genes, chalcone synthase
(LFC=-1.7) and cellulose synthase (LFC=-1.7), were downregulated in 5057, while
secondary metabolism and defence genes, chalcone isomerase (LFC=2.3), cellulose
synthase (LFC=1.5), chitinase (LFC=1.6) and phenylalanine ammonia-lyase1
(LFC=1.8) were upregulated in ZD05. At 22 DPI, secondary metabolism and defence
genes, chalcone synthase (LFC=-2.9) and phenylalanine ammonia-lyase1 (LFC=-2.9),
were down regulated in 5057, while in ZD05, secondary metabolism and defence
genes, bx13 (LFC=1.8), chalcone synthase (LFC=1.8), phenylalanine ammonia-lyase
(LFC=2.6) and antiapoptotic gene, bax inhibitor1 (LFC=1.8) were upregulated.
Striga hermonthica populations in Kenya and Nigeria are genetically distinct and
ecotypes exist within Nigeria. Genes involved in secondary metabolism and defence
were upregulated in the resistant maize genotype, but down regulated in the
susceptible genotype. The resistant line mobilized a more comprehensive response to
the parasite than the susceptible line.
