Plants, being sessile, are often exploited as a source of food and shelter by a wide range of organisms, including microbes and animals. Plants, unlike animals, do not have a circulatory immune system to combat biotic stress. However, plants have a more generalized defense strategy comprising either preformed or inducible mechanisms to resist pathogen invasion. The role of salicylic acid in the induction of defense response and the involvement of phenylalanine ammonia lyase (PAL), a key enzyme in the phenyl propanoid pathway during pathogenesis is well-documented. In the present study, two species of Casuarina—C. equisetifolia and C. junghuhniana—were analyzed for their differential expression of PAL and phenol during acetyl salicylic acid (ASA) stress. The rooted cuttings of both species were incubated in ASA containing media and the PAL and phenol content in the root and needle tissues of both species were estimated for 24 h. A substantial increase in PAL activity and phenol content was observed in both species during signaling; however, in C. junghuhniana, both PAL and phenol content were found to sustain at higher levels even after 24 h, while in C. equisetifolia, the levels of both PAL and phenol decreased by 16 h of treatment. The results indicate that the higher level of biotic stress tolerance in C. junghuhniana in comparison to C. equisetifolia may be attributed to the sustenance of high level of phenol and PAL in tissues during stress conditions.

All living organisms are involved in a constant struggle with and against other organisms to exploit their environment. Being sessile, plants are constantly attacked by pathogens, and as a result, plants have evolved a plethora of constitutive and induced basal defenses to defend against pathogens. Some defense mechanisms are constitutive, whereas many are induced by pathogen attack, and together they produce an integrated response that protects the plant not only at the site of infection but also throughout the plant. This systemic protection, which is known as systemic acquired resistance (SAR), protects the plant from secondary infection both by the original infecting pathogen and also by other pathogens (Ryals et al., 1994). At molecular level, SAR is characterized by the increased expression of a large number of pathogenesis-related genes (PR genes) in both local and systemic tissues.

Salicylic acid (SA), ethylene and jasmonic acid (JA) are the important phytohormones involved in the signaling response of plants during biotic stresses involving pathogens and insect pests (Pieterse and Van Loon, 1999; Spoel et al., 2003; and Thaler et al., 2004). SA is predominantly associated with resistance against biotrophic and hemibiotrophic pathogens and trigger systemic acquired resistance (Grant and Lamb, 2006). SA has been proposed as systemic signal because SA levels in the phloem increase significantly following pathogen infection (Métraux et al., 1990; Rasmussen et al., 1991; and Yalpani et al., 1991). The physiological and molecular mechanisms of SA-mediated plant response are not completely known, but its role in defense reactions is universally accepted (Pieterse and Van Loon, 1999; and Grant and Lamb, 2006).

Genetics & Evolution Journal, Casuarina Equisetifolia, Casuarina Junghuhniana, Biotic Stress, Phenylpropanoid pathway, Systemic acquired Resistance, Phenylalanine Ammonia Lyase, PAL, Acetyl Salicylic Acid, ASA, Jasmonic Acid, Polyphenolic Parenchyma, Chlorogenic Acid, Trichosporium Vesiculosum, Casuarina Equisetifolia, Phenylalanine Ammonia Lyase Activity, Polyphenol Oxidase, Phytopathologia Mediterranea, Dichloroisonicotinic, Benzothiadiazole.