Salicylic acid confers resistance against broomrape in tomato through modulation of C and N metabolism

Highlights

• Orobanche infection caused a dramatic damage in tomato growth, one of the most economically important crops.

• SA treatment improved C and N metabolites in host tissue but denied availability of the major metabolites to the parasite.

• SA treatment improved osmoregulatory status of the host plant.

• SA treatment improved fatty acids and organic acids in healthy and infected tomato.

• SA treatment seems to regulate the redox status in host plants.

Abstract

It is well known that parasitic weeds such as Orobanche (broomrape) significantly decrease crop growth and yield. Although hormonal priming is a well-known inducer of plant resistance against broomrapes (Orobanche spp.), the metabolic events associated with such resistance are poorly understood. Therefore, the current work was undertaken to elucidate the role of SA in inducing tomato resistance against Orobanche, considering its impact on carbon and nitrogen metabolism of the host. Total carbon and nitrogen and levels of carbon (sugars, organic acids and fatty acids) and nitrogen (amino acids and polyamines)-containing metabolites as well as the activities of some key enzymes involved in their metabolic pathways were evaluated. Broomrape infection significantly disrupted C/N ratio in the host roots. On contrary, SA treatment markedly induced accumulation of sugars, organic acids, fatty acids, amino acids as well as polyamines in healthy plants. Under broomrape challenge, SA mitigated the infection-induced growth inhibition by improving the level of nitrogen-containing osmoprotectants (proline, arginine and some polyamines). However, a decrease was observed in some C and N assimilates which are well known to be potentially transferred to the parasite, such as sucrose, asparagine, alanine, serine and glutamate. Interestingly, SA treatment induced the catapolism of polyamines and fatty acids in the host root. Accordingly, our study suggests that SA-induced resistance against broomrape relies on the rational utilization of C and N assimilates in a manner that disturbs the sink strength of the parasite and/or activates the defense pool of the host.

Introduction

Branched broomrape, Orobanche ramosa, is a holoparasitic plant that attacks several economic plants causing substantial crop losses (Buschmann et al., 2005). Because of the higher osmotic potential compared to its host, the parasite becomes a strong sink competing with the host sink organs (Abbes et al., 2009a; Hacham et al., 2016). Thus the presence of the parasite sink greatly disturbs assimilate partitioning in the host and consequently affects its metabolism, growth and yield (AL-Wakeel et al., 2013; Hibberd et al., 1999). On the other hand, the host plant develops various strategies for controlling broomrapes infection (AL-Wakeel et al., 2012). Therefore, understanding the complex interaction between the host plant and the parasite is needed to reduce the damage of such parasitic weed (Fernández-Aparicio et al., 2016; Krupp et al., 2019).

To cope with parasitic weeds, host plant has to disrupt the sink strength of the parasite through reliable osmoregulation strategies, which limit the flow of water and nutrients into the parasite (Abbes et al., 2009b). The process of osmotic adjustment depends on the accumulation of low molecular weight molecules, osmolytes, which act to increase the cellular osmotic pressure without harming the cell compartments (Rhodes et al., 2002). Osmolytes are mainly C and N assimilates such as sugars, sugar alcohols, some amino acids, polyamines, and quaternary ammonium compounds (Hou et al., 2016; Shao et al., 2009; Zhou and Yu, 2009). Primary metabolites, not only help in osmotic adjustment, but also participate in host resistance against the parasite. For instance, some studies have pointed to the role of the induced accumulation of some fatty acids, such as free oleic acid, in normalization of host-defense strategies including programmed cell death and sytemic acquired resistance (SAR) against invading pathogens (Li et al., 2016; Upchurch, 2008; Yaeno et al., 2004). Moreover, C/N balance was found to play a pivotal role not only in plant growth regulation, but also in plant resistance against water stress and related challenges (Chen et al., 2015; Martin et al., 2002). Therefore, modulation of C and N assimilates is of a great importance not only for disrupting the sink strength of the parasite but also for their pleiotropic roles in growth and development, stress tolerance, scavenging of free radicals, signal transduction and gene expression (Alcázar et al., 2010; Delauney and Verma, 1993; Díaz et al., 2005; Kalamaki, 2009; Liang et al., 2013; Rolland et al., 2002). Salicylic acid (SA, 2-hydroxy benzoic acid), a potent plant hormone (Raskin, 1992), has a manifold action in managing many physiological and biochemical processes such as seed germination, cellular respiration, photosynthesis, flowering and senescence (Rivas-San Vicente and Plasencia, 2011). Moreover, SA is well recognized as a key signal that mediates plant response to stressful conditions (Dianat et al., 2016; Durrant and Dong, 2004; Horváth et al., 2007). Interestingly, SA and its synthetic functional analogue, benzothiadiazol (BTH), have been reported as promising inducers of the host's defense against broomrape (AL-Wakeel et al., 2013; Kusumoto et al., 2007; Müller-Stöver et al., 2005). Unlike other types of stresses, the impact of SA on the host metabolism under broomrape challenging conditions is not exhaustively investigated (Dong et al., 2011; Hayat et al., 2010; Krasavina and Burmistrova, 2013; Németh et al., 2002; Szepesi et al., 2011; Zhi-gang et al., 2012). Owing to the considerable contribution of C and N assimilates in host defense strategies against broomrape and the regulatory role of SA on primary metabolism, we hypothesize that SA-induced resistance against broomrape is largely relying on the modulation of C and N metabolism in the host plant. To test our hypothesis, we have assessed the impact of SA on C/N ratio as well as on accumulation of individual sugars, amino acids, polyamines and fatty acids as well as the activities of some key enzymes in their metabolic pathways in both healthy and broomrape-infected tomato plants (Lycopersicon esculentum).

Priming in general provides enhanced resistance, but with less associated costs compared with direct initiation of defense system (Ahmad et al., 2012). Priming of defense is a strategy employed by plants exposed to stress to enhance resistance against future stress episodes with minimal associated costs on growth. Although the mechanisms underlying priming are poorly understood, they can provide effective crop protection through the expression of signaling proteins and transcription factors involved in inducible defense (Ahmad et al., 2012; Jisha et al., 2013). Therefore, seed priming with various bio-regulators including hormones, antioxidants and other molecules has been used to impart stress tolerance to biotic and abiotic stresses (Ahmad et al., 2012; AL-Wakeel et al., 2013; Madany and Khalil, 2017).

In recent years, many studies intensively focused on the indispensable role of salicylic acid (SA) as an internal signal arbitrating plant defense behavior against both biotic and abiotic stresses.

Under different stress conditions, it was reported that SA enhanced plant resistance by augmenting its growth (Dianat et al., 2016; Rivas-San Vicente and Plasencia, 2011). In a previous work we found that SA not only enhanced tomato growth, by improving its dry biomass as well as shoot and root heights, under Orobanche infection conditions but also it significantly reduced the severity of infection (AL-Wakeel et al., 2012). These results prompted us to ask about the potentiality of SA in enhancing plant growth as well as triggering its defense arsenal through improving C/N metabolism, especially a better understanding of how plants modulate nitrogen metabolism is essential to enrich plant growth and yield. To achieve the purpose of this study, accumulation of individual sugars, amino acids and polyamines as well as the activities of some key enzymes in their metabolic pathways were assessed in healthy and broomrape-infected tomato, as affected by SA treatment.<a name = "Line_manuscript_22"/>