Plant (ethephon) and JA (methyl jasmonate) shoots application

Plant defense mechanismNaturally, plants express a wide array of defence responses to protect them from abiotic and biotic stressors. Plant pathogenic nematodes trigger plant defense. However,  (De Vos et al., 2005) pointed out that the infection stage, age and type of plant tissue are important factors for  determining the nature of the specific defense response. Comprehensive investigations have been done to study the role of of plant hormones such as salicylic acid (SA), jasmonic acid (JA), and ethylene (ET) in plant’s basal resistance against several pathogens (Taheri & Tarighi, 2011). Traditionally,JA and ET exhibit a synergistic interaction in activating plant defense against wounding and necrotrophs, while SA protects plants from biotrophs. SA and JA/ET pathways are mutually antaganostic (Glazebrook, 2005). These pathways have crucial role in host plant defense responses to root-knot nematodes (RKNs) and CNs. Nahar et al. (2011) comprehensively investigated SA, JA and ET role in inducing rice resistance against Meloidogyne graminicola and data showed the synergistic interaction between JA and ET. Exogenous ET (ethephon) and JA (methyl jasmonate) shoots application induced a strong systemic defense response in the roots, verified by a major up-regulation of OsPR1a and OsPR1b, a pathogenesis-related genes. On the other hand, SA analog BTH (benzo-1,2,3-thiadiazole-7-carbothioic acid S-methyl ester) exhibited a lesser potent systemic defense inducer from shoot to root. Additional analysis by using JA biosynthesis mutants and ET-insensitive transgenics was done. Analysis indicated that intact JA pathway is prerequisite for ET-induced systemic defense, while impairment of ET will not affect JA function. Moreover, quantitative real-time reverse transcription-polymerase chain reaction (qRT-PCR) revealed that shoots application of ET triggers JA biosynthesis in the roots (Nahar et al.,2011).Generation of reactive oxygen species (ROS) during oxidative burst is an important mechanism in plant defense. H2O2 is the most stable ROS and has the capacity to act as signaling molecule to  promotes resistance (Asselbergh et al., 2007; Kang, 2008). A study showed that thiamine-treated rice triggers H2O2 and phenylpropanoid mediated lignin production, which hinders root knot nematode infection and supporting H2O2  as an effective defense mechaninsm (Huang et al., 2016). H2O2 is  involved in cell wall reinforcement by enhancing protein cross-linking and with integration of  phenolic compounds in the cell wall (Asselbergh et al., 2007; Kang, 2008).Vitamins are one group of molecules that can induce plant defense. Riboflavin (vitamin B2)  treated pineapples show reduction of  M. javanica egg production by 60% to 64% compared to the non-treated control. Function of ribofIavin has been observed by (Dong & Beer, 2000) in riboflavin-treated tobacco against  Tobacco mosaic virus (TMV) and Alternaria alternata. It highlighted riboflavin triggers PR genes expression and the same authors  suggested the ability of riboflavin to activate signal transduction which elicits systemic acquired resistance (SAR) in plants. SAR is a type resistance response that occurs throughout the plant’s tissues resulting from an earlier localized exposure to a pathogen. This kind of resistance conferred is long-lasting protection and effective against broad-spectrum of microbial pathogens such as bacteria, fungi and viruses (Ryals et al.,1996; Sticher et al.,1997).The function of phytohormones in plant defence against nematodes depends on feeding site and nematode development, the host, the nematode species and the phase of parasitism (Kammerhofer et al., 2015). Abscisic acid (ABA) can have different effects depending on the individual plant pathogen interaction. (Yasuda et al., 2008) reported that ABA treatment acted antagonistically with systemic acquired resistance (SAR)  signaling in Arabidopsis. On the other hand, Adie et al (2007) reported higher infection rate of Pythium irregulare in ABA deficient mutant  plants. Nahar et al. (2012)  stated that exogenous ABA application at 50 µM 3 weeks after inoculation on Nihonmasar and Nipponbare rice cultivars, triggered rice seedlings susceptibility to Hirschmaniella oryzae. Further gene analysis by qRT-PCR showed antagonistic interaction between ABA and the SA/JA/ET-dependent basal defense system. This result may explain the disease-inducing effect of ABA in rice.Priming and its significancePriming is a mechanism that results in a physiological condition  in which a plant responds faster and or more accurately to biotic and abiotic stress (Prime-A-Plant Group et al., 2006). Priming improves the  generation of defense  signals, and is more energy-efficient compared to the direct and constitutive activation of defense. During pathogen attack or under environmental perturbation a stronger defense response is triggered in a primed plant in comparison with an unprimed plant. Conrath et al. (2006) described the potential of chemical priming by utilizing  ?-aminobutyric acid (BABA) and benzo-(1,2,3)-thiadiazole-7-carbothioic acid s-methylester (BTH) to induce plant defense to hamper different plant pathogens. In sugar beet, priming with thiamine is effective against Rhizoctonia solani by timely induction of H2O2 and accumulation of phenolic compounds (Taheri & Tarighi, 2011). Ascorbic acidAscorbic acid (Aa / vitamin C) is a well-studied water soluble antioxidant that is universally distributed in higher  plants. As one of the major plant metabolites, Aa plays numerous biochemical functions such as mitigation of excessive ROS through enzymatic and non-enzymatic detoxification (Mittler, 2002). Study reveals that reduction of Aa biosynthesis in plants leads to higher sensitivity under environmental perturbation such as salinity, high light intensity, low or high temperature and drought  (Muller-Moule et al., 2004). Moreover, transgenic tobacco plants  overexpressing the thylakoid-bound APX gene show increased temperate stress tolerance. Analysis showed that there was a remarkable increase in  APX activity  and the transgenic plants had less H202  and malondialdehyde than wild-type plants (Sun et al., 2010). Aa is also believed to be involved in plant cell growth and development, through various mechanisms (Esaka, 1998; Smirnoff & Wheeler, 2000). Hidalgo et.al (1989) described that MDHA, generated from Aa by AO in the apoplast, stimulates cell growth through enhanced vacuolization and ion uptake, caused by depolarization of the plasma memebrane. Aa is essential in inducing plant resistance against several plant-parasitic nematodes. (Arrigoni et al., 1979a) described the role of Aa in triggering the resistance of tomato against Meloidogygne incognita. When Aa at 45 Mm applied exogenously in Roma, a susceptible line, root infection was reduced by 96.5% compared to untreated plants. The same author measured whether nematode invasion influenced Aa concentrations in the plant roots. Findings indicated that 12 days after inoculation, a high amount of Aa was detected in resistant line roots while the Aa concentration of susceptible roots was unaffected by the nematode invasion. Aa biosynthetic ability of the tomato after nematode invasion differs between resistant and susceptible lines after Aa exogenous application. They postulated that Aa is utilized for mitochondrial hydroxyproline proteins synthesis to permit development of the cyanide resistant-respiration (CRR) which is the metabolic process initiated by the cell to counteract the effects of the nematode. CRR is an alternative electrotransport pathway to cytochrome respiration which is conferred by a protein, the alternative oxidase (AOX), embedded in the inner mitochondrial membrane (Lambers et al., 2008). There is a direct electron transfer from ubiquinone pool to oxygen, bypassing complex III and cytochrome c oxidase, two sites of energy conservation in the cell. Complex III is a phase in electron transport chain which transfers electrons from ubiquinone to cytochrome c. Unlike the cytochrome pathway, the transport of electrons from ubiquinol to O2 through the alternative path is not associated to proton extrusion, and therefore not coupled to energy conservation. This leads to lower yield of ATP compared with cytochrome path is used. CRR has physiological roles. Under stress conditions, electron transport flow is linked to ROS generation (Viega et al,2003). At the complex III, several ROS are formed, and by surpassing electrons immediately from ubiquinone to AOX would avoid ROS formation. In addition, AOX is involved in catalyzing the reduction of molecular oxygen by ubiquinol which leads in the production of H2O and not superoxide or H2O2.Zacheo et al. (1995) also studied the effect of Aa in pea root infected by Heterodera goettingiana. Aa concentrations were measured after nematode invasion and ascorbic free radical (AFR)-reductase activity was assayed. Higher concentration of Aa was present in the root of resistant pea lines than in the susceptible lines. Increase of Aa concentration by 180% were noted in a resistant lines , which correlated with a very low number of nematodes in the roots. In addition, AFR-reductase assay following nematode attack reaveled an increased by 51%- 230% for resistant line, while susceptible line decreased by 60% -72%. AFR-reductase is the major enzyme which maintains Aa in reduced form. This study demonstrated the capacity of resistant lines to biosynthesize and utilize Aa. Ascorbate biosynthesis is well-described in higher plants to occur through the D-mannose/L-galactose pathway (Smirnoff-Wheeler pathway), where through GDP-sugar intermediates, conversion of D-mannose to L-galactose takes place (Wheeler et al., 1998).  Another means of Aa synthesis works via Uronic acid intermediates  such as D-galacturonic acid (Isherwood et al., 1954). This pathway involves the reduction of D-galacturonic acid  to L-galactonic acid  by galacturonic acid reductase, which is subsequently converted to L-galactono-1,4-lactone. Further oxidation of L-galactono-1,4-lactone to Aa involves the aid of the L-galactono-1,4-lactone dehydrogenase (GALDH) enzyme (Siendones  et al., 1999; Smirnoff, 2001). Other potential Aa biosynthesis pathways have been proposed such L-gulose pathway where L-gulose and L-gulono-1,4-lactone are necessary intermediates (Davey et al., 1999). But there is lack of characterization in defining the intermediate steps of this pathway in the higher plants. Recently, experiments showed the involvement of the D-galacturonic acid pathway in converting D-galacturonic acid, a result from cell pectin’s degradation to Aa via L-galactono-1,4-lactone (Agius et al., 2003; Badejo et al., 2009; Cruz–Rus et al., 2011). The myoinositol based pathway was also considered but (Endres & Tenhaken, 2009) emphasized its insignificant role in plant ascorbate biosynthesis.Ascorbate oxidedase (AO) and ascorbate peroxidase (APX) are two enzymes that facilitates in the oxidation of ascorbate. AO is an apoplastic enzyme that involved in catalyzing the oxidation of of Aa tounstable molecule monodehydroascorbate (MDHA) which is converted to dehydroascorbate (DHA) if not reduced again to Aa by the help of monodehydroascorbate reductase (MDHAR). Recycling of DHA to Aa takes place by using reduced glutathione (GSH) as a reducing substrate, a reaction which is catalyzed by DHA reductase (DHAR) (Smirnoff, 1996; Noctor & Foyer, 1998). 

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