Introduction
Cucumber (Cucumis sativus L.; 2n = 2x = 14), a member of the
family, Cucurbitaceae, is the fourth most important vegetable crop
worldwide (Ren et al. 2009; Sebastian et al. 2010). However, cucumber
like other crops is suffering from various pathogens such as different
species of oomycetes. Damping-off disease caused by the hemibiotrophic
oomycete, Phytophthora melonis , is one of the most severe
diseases of the cucurbitaceae, significantly reducing crop yield
worldwide (Erwin and Ribeiro 1996; Wu et al. 2014). The main symptoms of
cucumbers infected with P. melonis are root and root collar rot,
stem lesions, foliar blight and fruit rot and finally plant death
(McGrath 2001; Hatami et al. 2013). Damping-off can have a severe
economic impact on cucumber from seedling up to fruiting stages.
Although the use of disease-resistant genotypes is a key to
environmentally friendly and economically sustainable disease control in
modern crop production, the employment of genetic resistance to minimize
yield losses induced by P. melonis remains largely unexplored in
cucumber. Up to now, no resistant cultivar has been developed so far and
few reports are available with regard to Phytophthora damping-off
(Mansoori and Banihashemi, 1982; Hashemi et al., 2019). Therefore,
identifying the sources of resistance and studying the genetics
underlying resistance to P. melonis is pertinent to support
cucumber breeding programs. Plants respond to pathogen attack with a
multicomponent defence response that includes synthesis of antimicrobial
compounds, induction a variety of defence genes and enhancement of the
cell wall (Anil et al., 2014). As most of these defense responses can be
monitored at the transcriptional level, gene expression analysis can
provide insights into the type of defense mechanism involved in the
damping-off disease reaction and cucumber plant pathosystem.
Pathogenesis-related proteins (PRs) are one of the most commonly induced
proteins during plant defense mechanism, which have an important role in
plant immunity (Van Loon et al. 2006). PR1 (unknown biochemical
properties), PR3 (chitinases) and lipoxygenases (LOX) are strongly
induced when plants respond to infection by different types of pathogens
(Porta and Rocha-Sosa 2002; Van Loon et al. 2006). Recently some WRKY
transcription factors were found to be involved in protection
mechanisms. Therefore, these genes probably play important roles in
combating exogenous pathogens and will provide a basis for further
studies of the functional verification of them and finally, will help us
to better understand of the regulatory mechanism of plant resistance to
pathogens (Wang and Bouwmeester 2017). WRKY TFs have been implicated in
the regulation of transcriptional reprogramming associated with plant
immune responses and genetic evidence demonstrating their significance
as positive and negative regulators of disease resistance has
accumulated (Eulgem and Somssich 2007). Some of these factors appear to
affect the balance between signaling branches promoting SA-dependent and
suppressing JA-dependent responses and are required for both basal
defense and full R-gene mediated disease resistance against the oomycete
(Knoth et al. 2007, Xu et al. 2015). In several plant species, strong
and rapid induction of WRKY genes have been reported on pathogen
infection. In cucumber, among 55 WRKY genes, CsWRKY20 may
be involved in disease resistance against P. melonis (Xu et al.
2015). The LecRK gene families are another group of genes that
plays important roles under biotic and abiotic stresses in plants
(Boutrot et al. 2017). Recent findings, however, revealed the importance
of LecR Ks in plant innate immunity (Wang and Bouwmeester 2017;
Zhao et al. 2018). The CsLecRK6.1 gene among all 25 identifiedCsLecRKs genes in cucumber, was especially induced by P.
capsici and P. melonis in JSH (resistance cultivar) (Wu et al.
2014; Tingquan et al., 2014). Despite these results, the regulatory
roles of most of the identified WRKY TFs and LecRK s genes
in defense response are still largely unknown. However, knowledge about
the local (root collars) molecular defense responses compared with
systemic (leaves) defenses before and after P. melonisinoculation in cucumber is scarce. So far as we know, this is the first
study that provides information regarding root collar and leaf gene
expression level of CsWRKY20 , CsLecRK6.1 , PR3 ,PR1-1a and LOX1 genes in four contrasting genotypes of
cucumber upon inoculation with P. melonis .