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 .