**1. Introduction**

Every living organism shares some basic features like order, sensitivity or response to the environment, reproduction, growth and development, regulation, energy processing, and evolution with adaptation [1]. A basic concept in classical genetics emphasizes that the phenotype of the organism is based on the interaction of genotype with the environment. The emergence of specific natural products is dependent on the highly ordered interaction between plants with the biotic and abiotic environments around them [2]. Plants are sessile organisms and respond to the stress conditions by changing the expression levels of certain genes involved in the production of metabolites which are secreted in response to its interaction with its environment [3]. The various

metabolic pathways produce different types of metabolites and based on the pathway they are classified as primary and secondary, although a strict demarcation is difficult to draw between them. The primary metabolites are essential to the cellular growth and reproduction whereas the secondary metabolites although not required directly for the same, these are the compounds that are synthesized in response to any biotic or abiotic stress which may be exogenous or endogenous the cell [3].

The production of secondary metabolites is infact influenced by primary metabolites. Some of the C and N fluxes can be diverted for the production of secondary metabolites during the stressful conditions and there is always a dynamic balance maintained between the two based on the cellular needs. In comparison with the primary metabolites the concentration of secondary metabolites is low and the type of secreted plant secondary metabolite (PSM) is based on the type of stressful physiology induced by biotic and abiotic stress condition. Some of the secondary metabolites are acts as regulators of development, growth and defense. Some of these compounds can be reintegrated into plant primary metabolism [4] *senu lato*.

The regulation of production of PSM involves extensive cross talk and signaling pathways with the key roles played by molecules like salicylic and jasmonic acids, calcium, abscisic acid, spolyamines and nitric oxides [5–8].

Over 2,14,000 types of secondary metabolites are known and are commonly classified according to their structure, function and biosynthetic pathway. Plant secondary metabolites can be classified into four major classes: i) Terpenoids, ii) Phenolic compounds, iii) Alkaloids and iv) Sulfur-containing compounds [9].
