Abstract:
Exserohilum turcicum (sexual stage Setosphaeria turcica) is the hemibiotrophic causal
agent of northern leaf blight ofmaize and sorghum. This study aimed to identify the genes
involved in host colonization during the biotrophic and necrotrophic phases of infection.
It also aimed to identify race-specific differences in gene expression. RNAseq of maize
seedlings inoculated with a race 13N or 23N E. turcicum isolate was conducted before
inoculation and at 2, 5, 7, and 13 days post-inoculation (dpi). Biological replicates were
pooled per time point for each race and sequenced. A bioinformatics pipeline was used
to identify candidate effectors, and expression was validated for selected candidates.
Fungal biomass was positively correlated with the percentages of E. turcicum reads
mapped, which were low at early time points (2–7 dpi) with a significant increase at
13 dpi, indicating a lifestyle switch from biotrophy to necrotrophy between 7 and 13 dpi.
AVRHt1 is the putative E. turcicum effector recognized by the maize resistance gene Ht1.
Consistent with this, AVRHt1 was expressed in planta by race 23N, but transcripts were
absent in race 13N. In addition, specific transposable elements were expressed in 23N
only. Genes encoding the virulence-associated peptidases leupeptin-inhibiting protein
1 and fungalysin were expressed in planta. Transcriptional profiles of genes involved
in secondary metabolite synthesis or cell wall degradation revealed the importance of
these genes during late stages of infection (13 dpi). A total of 346 expressed candidate
effectors were identified, including Ecp6 and proteins similar to the secreted in xylem
(SIX) effectors common to formae speciales of Fusarium oxysporum, SIX13 and SIX5.
Expression profiling of Ecp6 and SIX13-like indicated a peak in expression at 5 and 7 dpi compared to 2 and 13 dpi. Sequencing of SIX13-like from diverse isolates of E. turcicum
revealed host-specific polymorphisms that were mostly non-synonymous, resulting in
two groups of SIX13-like proteins that corresponded to the maize or sorghum origin
of each isolate. This study suggests putative mechanisms whereby E. turcicum causes
disease. Identification of the candidate effector SIX13-like is consistent with the infection
mode of E. turcicum through the xylem of susceptible hosts.
