Salicylic acid and defense responses in plants

Abstract

Salicylic acid (SA) is a simple phenolic compound distributed in a wide range of plant taxa. Depending on the plant species, developmental stage, and growth conditions, it can be synthesized from cinnamic acid produced by phenylalanine ammonia-lyase in the cytosol or from isochorismic acid generated by isochorismate synthase in chloroplasts. However, a fully defined SA biosynthetic pathway is still unavailable in plants. Besides its role in regulating various aspects of plant growth and development, SA is a plant immune signal essential for both local defense response and systemic acquired resistance. Significant progress has been made recently in understanding SA-mediated defense signaling networks including identification of SA receptors and elucidation of the crucial role of NPR1 (nonexpressor of pathogenesis-related genes 1) in SA signal execution. Understanding of SA-mediated plant defense has facilitated the development of disease-resistant crops through genetic manipulation of the SA signaling pathway. Although the use of NPR1 and its orthologs in developing broad-spectrum transgenic disease resistance has been successfully extended to a variety of crop species, commercial application of these transgenic crops has been hampered by ethical concerns. In this regard, cisgenesis may hold the potential for application of bioengineered disease-resistant crops in agriculture.