Abstract
This study aimed to directly elucidate cultivar variation in disease susceptibility and disease responses in relation to hormonal status in the interaction of Brassica napus cultivars and Xanthomonas campestris pv. campestris (Xcc), the causal agent of black rot disease. Fully expanded leaves of six B. napus cultivars (cvs. Capitol, Youngsan, Saturnin, Colosse, Tamra, and Mosa) were inoculated with Xcc. At 14 days post-inoculation with Xcc, cultivar variation in susceptibility or resistance was interpreted with defense responses as estimated by redox status, defensive metabolites, and expression of phenylpropanoid synthesis-related genes in relation to endogenous hormonal status. Disease susceptibility of six cultivars was distinguished by necrotic lesions in the Xcc-inoculated leaves and characterized concurrently based on the higher increase in reactive oxygen species and lipid peroxidation. Among these cultivars, as the susceptibility was higher, the ratios of abscisic acid (ABA)/jasmonic acid (JA) and salicylic acid (SA)/JA tended to increase with enhanced expression of SA signaling regulatory gene NPR1 and transcriptional factor TGA1 and antagonistic suppression of JA-regulated gene PDF 1.2. In the resistant cultivar (cv. Capitol), accumulation of defensive metabolites with enhanced expression of genes involved in flavonoids (chalcone synthase), proanthocyanidins (anthocyanidin reductase), and hydroxycinnamic acids (ferulate-5-hydroxylase) biosynthesis and higher redox status were observed, whereas the opposite results were obtained for susceptible cultivars (cvs. Mosa and Tamra). These results clearly indicate that cultivar variation in susceptibility to infection by Xcc was determined by enhanced alteration of the SA/JA ratio, as a negative regulator of redox status and phenylpropanoid synthesis in the Brasica napus–Xcc pathosystem.
Highlights
Oilseed rape (Brassica napus L.), grown for the production of vegetable oil, animal feeds, and alternative fuel, is one of the major agro-economic crops
Accumulation of reactive oxygen species (ROS), such as O2−, H2O2, and hydroxyl radical, and the induction of their scavenging enzymes are basic responses to plant stresses caused by a wide range of environmental stresses (Lee et al, 2009) and pathogen infection (Silva et al, 2004; Finiti et al, 2014)
Due to the impairment of ROS-scavenging system, enhanced lipid peroxidation (MDA) occurs, which is associated with necrosis of plant tissues (Venisse et al, 2001)
Summary
Oilseed rape (Brassica napus L.), grown for the production of vegetable oil, animal feeds, and alternative fuel, is one of the major agro-economic crops. Campestris (Xcc), the causal agent of black rot, has become a major threat to Brassica species (Velasco et al, 2013). Pathogen invasion induces different plant–pathogen defense reactions, including susceptibility, resistance, or nonhost reactions (O’Donnell et al, 2003; Aires et al, 2011). One of the earliest physiological responses to pathogen infection is rapid reactive oxygen species (ROS) production (Venisse et al, 2001). ROS can reduce pathogen viability via direct antibacterial activity, and is implicated in the oxidative damage of challenged plant cells through lipid peroxidation (Lee et al, 2007, 2013). Glutathione (GSH) is one of the major nonenzymatic antioxidants present in a plant cell, which maintains the intracellular redox homeostasis by reducing cellular disulfide bonds (Finiti et al, 2014)
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