Journal of Plant Physiology 168 (2011) 1114–1122 Contents lists available at ScienceDirect Journal of Plant Physiology journal homepage: www.elsevier.de/jplph A survey on basal resistance and riboﬂavin-induced defense responses of sugar beet against Rhizoctonia solani Parissa Taheri ∗ , Saeed Tarighi Department of Crop Protection, Faculty of Agriculture, Ferdowsi University of Mashhad, P.O. Box 91775-1163, Mashhad, Iran article info Article history: Received 28 September 2010 Received in revised form 29 December 2010 Accepted 4 January 2011 Keywords: Beta vulgaris Oxidative burst Peroxidase Phenylalanine ammonia-lyase Rhizoctonia diseases summary We examined basal defense responses and cytomolecular aspects of riboﬂavin-induced resistance (IR) in sugar beet-Rhizoctonia solani pathsystem by investigating H 2 O 2 burst, phenolics accumulation and analyzing the expression of phenylalanine ammonia-lyase (PAL) and peroxidase (cprx1) genes. Riboﬂavin was capable of priming plant defense responses via timely induction of H 2 O 2 production and phenolics accumulation. A correlation was found between induction of resistance by riboﬂavin and upregulation of PAL and cprx1 which are involved in phenylpropanoid signaling and phenolics metabolism. Application of peroxidase and PAL inhibitors suppressed not only basal resistance, but also riboﬂavin-IR of sugar beet to the pathogen. Treatment of the leaves with each inhibitor alone or together with riboﬂavin reduced phenolics accumulation which was correlated with higher level of disease progress. Together, these results demonstrate the indispensability of rapid H 2 O 2 accumulation, phenylpropanoid pathway and phenolics metabolism in basal defense and riboﬂavin-IR of sugar beet against R. solani. © 2011 Elsevier GmbH. All rights reserved. Introduction Rhizoctonia solani anastomosis group (AG) 2-2 IV, the causal agent of root rot and foliar blight diseases (Matsumoto and Matsuyama, 1999) is one of the most destructive pathogens of sugar beet with high yield losses worldwide. Because of the high variability in the Rhizoctonia populations, its wide host range and long-term survival in soil, management of diseases caused by this necrotrophic fungus is difﬁcult (Taheri et al., 2007). The pathogen is not efﬁciently controlled by resistance breeding and there is no cul- tivar completely resistant to R. solani in any plant species. Although several lines of sugar beet appear to be partially resistant to the fun- gus (Nagendran et al., 2009), the basis for resistance is principally unknown. Furthermore, an intensive use of other crop protection strategies such as application of chemicals seems to be necessary to limit the disease damage. The growing concern about negative environmental effects of fungicides and appearance of fungicide- resistant pathogen strains is motivating research for alternative protection methods. Among such novel strategies, induced resis- Abbreviations: AOPP, -aminooxy--phenylpropionic acid; DI, disease index; dpi, days post inoculation; dpt, days post treatment; ET, ethylene; IR, induced resistance; ISR, induced systemic resistance; JA, jasmonic acid; LOX, lipoxygenase; PAL, phenylalanine ammonia-lyase; PR, pathogenesis-related proteins; qRT-PCR, quantitative reverse transcription-polymerase chain reaction; ROS, reactive oxygen species; SAR, systemic acquired resistance. ∗ Corresponding author. Tel.: +98 511 8795612; fax: +98 511 8787430. E-mail address: p-taheri@um.ac.ir (P. Taheri). tance using environmentally safe compounds such as vitamins and understanding basal defense mechanisms to plan effective disease control methods have emerged as potential supplements in crop protection measures. Plants naturally express variable levels of resistance against dif- ferent groups of pathogens. This kind of primary defense response is known as basal resistance that is controlled by several genes. It is obtained by cooperation of multiple molecular and cellular defense responses and involvement of various signaling pathways. The role of plant hormones such as salicylic acid (SA), jasmonic acid (JA), and ethylene (ET) has been broadly investigated in basal resistance of several plants to various pathogens. Application of metabolic inhibitors for blocking each of the signaling pathways, or using plant genotypes that are impaired in their response to these signaling molecules showed higher susceptibility to pathogens (Hamiduzzaman et al., 2005). Induced resistance, which was initially considered to be restricted to infection with necrotizing pathogens, is referred to as systemic acquired resistance (SAR) and is associated with SA accu- mulation in plants (Loake and Grant, 2007). Recently, however, this concept has been expanded to include resistance induced by a wide range of biotic and abiotic agents. Induced resistance mediated by the biotic agents such as plant growth-promoting rhizobacte- ria (PGPR) has been studied extensively in different pathosystems and is referred to induced systemic resistance (ISR), which is dis- tinguished from SAR because it mainly functions independently of SA but requires responsiveness to JA and ET (Pieterse et al., 2001). Induction of plant defense responses by abiotic agents, for 0176-1617/$ – see front matter © 2011 Elsevier GmbH. All rights reserved. doi:10.1016/j.jplph.2011.01.001