1 │ INTRODUCTION
Phytophthora melonis , a hemibiotrophic organism belongs to
oomycote class and one of the causal agents of damping-off disease, is
considered a potentially destructive disease of cucumber (Cucumis
sativus L.; 2n = 2x = 14) across Iran and other parts of the world,
that causes economic losses (Wu et al., 2014; Nazavari et al., 2016).
Therefore, it is vital to elucidate the mechanisms of plants in response
to damping-off. Typical symptoms of the disease are root and root collar
rot, stem lesions, foliar blight, fruit rot and plant death (Hatami et
al., 2013; Bagheri et al., 2020). Although current control measures forP. melonis are based on the use of fungicides (Lamichhane et al.,
2017), identification of disease-resistant genotypes may be an effective
strategy and environmentally friendly in modern crop production
(Moghaddam et al., 2019; Hashemi et al., 2019). To date, analyses of the
interaction between P. melonis and C. sativus have been
limited on screening resistant genotypes to damping-off (Nazavari et
al., 2016; Hashemi et al., 2019) and no cucumber genotype immune toP. melonis has been reported. In the never ending struggle
against pathogens, plants gradually developed a series of complex
defense mechanisms involving several factors like defense-related
enzymes and inhibitors which lead to prevent infection of pathogens
(Andersen et al., 2018). However, plant response during the P.
melonis - C. sativus interactions or, more specifically, the
underlying factors that increase susceptibility and/or resistance of the
cucumber plant is not fully understood. So, identification and
application of candidate resistance genes and defense enzymes associated
with P. melonis responses can be an efficient method to build up
the host resistance in cucumber genotypes. In 2019, three
defense-related genes of CsWRKY20 , CsLecRK6.1 andLOX1 genes were reported to be involved in the resistance to theP. melonis infection in cucumber (Hashemi et al., 2019). Complex
network of defense-related enzymes, such as peroxidase (POX;
EC1.11.1.7), superoxide dismutase (SOD; EC1.15.1.1), catalase (CAT;
EC1.11.1.6) and phenylalanine ammonia lyase (PAL; EC4.3.1.5) are
promoted the scavenging of ROS and related to resistance inducement in
plants (Prasannath and De Costa, 2015; Xie et al., 2017; Khatediya et
al., 2018). PAL has a crucial role in flavonoid productions and lignin
biosynthesis, which play a key role in the phenylpropanoid biosynthetic
pathway (Yusuf et al., 2018). PAL is one of the most extensively studied
enzymes with respect to plant responses to environmental stresses,
including pathogen infection (Huang et al. 2010; Kim & Huang 2014).
Polyphenol oxidase (PPO) and peroxidase (POX) are important oxidative
enzymes found in most plant species that catalyze the formation of
lignin and other oxidative phenols, thereby contributing to the
formation of defense barriers by structural reinforcement, to protect
against pathogens (Li & Steffens, 2002; Jiang et al., 2019). In
cucumber increasing in the activities of peroxidase were reported
following the inoculation with Fusarium oxysporum (Zhao et al.,
2012). Amaral et al (2019) reported that high activities of antioxidant
enzymes played a major role in both basal and induced resistance of
cabbage to black rot. Moreover, regarding the oxidative burst, CAT and
SOD are useful biochemical indicators of disease resistance (Su et al.,
2019). The activities of different defense-related enzymes vary in
different plants after pathogen attack and are highly complex (Siddique
et al., 2014; Su et al., 2016). To the best of our knowledge, no
comprehensive enzymic analyses have yet been accomplished on cucumber
plants to compare defence‐related enzymes in resistant and susceptible
cucumber genotypes upon inoculation with P. melonis . Therefore,
the present study estimated the activities of PAL, POX, CAT, SOD and PPO
quantitatively and investigated their roles in response to P.
melonis inoculation in the cucumber genotypes. A better insight in
cucumber defense responses can help to establish biochemical
characteristics for the selection of resistant cucumber sources and
provide a theoretical basis for disease control and breeding of cucumber
plants with higher resistance toward damping-off.