**5. Distribution of carbon fluxes between primary and secondary metabolism during stress conditions and regulation**

The influence of exogenous/endogenous biotic and abiotic stresses influence the plant to make a trade off between growth & reproduction with defense mechanism intended to protect the plant. This feature in plants attracts special focus in plant ecophysiology. Primary metabolism which provides carbon skletons for the synthesis of PSMs including phenolics requires large amounts of available resources. The stress

#### *Effect of Biotic and Abiotic Stresses on Plant Metabolic Pathways DOI: http://dx.doi.org/10.5772/intechopen.99796*

condition leads to several biochemical and molecular mechanism triggering the adaptive response. As a part of this strategy cells try to divert the available carbon fluxes between primary and secondary metabolism and other limited resources. A considerable quantity of photosynthates are diverted to the production of phenolics and other PSMs. Lattanzio *et al.*, (2015) [20] proposed that there is a link between primary and secondary metabolism couples the accumulation of proline, a stress metabolite with energy transfer towards phenyl propanoid biosynthesis via the oxidative pentose phosphate pathway. Accordingly, some of the transduction pathways that involve a) proline redox cycle, b) pentose phosphate pathway are biased towards PSMs synthesis.

Phosphoenol pyruvate (PEP), a metabolite from glycolytic pathway is shared by four different metabolic routes leading to the formation of 1) TCA cycle, ATP generation and amino acid synthesis, 2) Methyl erythritol 4-phosphate pathway for the formation of isoprene units, 3) Shikimate-phenyl propanoid pathway for the formation of phenolic compounds, 4) Another anaplerotic route to refill PEP with PEP carboxylase. The stress conditions which favors phenolic formation increases the gene expression of shikimate dehydrogenase, phenyl ammonia lyase, chalcone synthase and PEP carboxylase specific enzymes involved in their production [21].

Biotic stress induced by *Amphibolis michoacaensis* induces gall formation in *Quercus castanea* wherein there is upregulation of phenolic related genes Phenylalanine ammonia lyase (PAL) at the intermediate and late growth stages; Phenyl propanoid genes at the intermediate stage and lignin genes at late stage. There is differential regulation of molecular switches related to secondary metabolites production during different stages of gall formation [22].

Infestation of rice by brown planthopper (BPH) in rice is observed to increase expression of *OsPAL6* and *OsPAL8* for the synthesis of phenylalanine ammonia lyase (PAL) through direct up-regulated by OsMYB30, an R2R3 MYB transcription factors [23].

Plants possess an effective immune system to combat most microbial attackers . There is an analogous immune system in plants like in animals to combat the microbial infestation. Salicylic acid is one of the hormone which is triggered in response to biotic stress. The immune response elicited leads to massive transcriptional reprogramming, cell wall strengthening, production of secondary metabolites and antimicrobial proteins [24].

Another mode of immune response to stress conditions is to influence epigenetic modifications in the development of stress memory. This is particularly of significance in high temperature heat shock stress. These modifications in turn can activate heat shock responsive genes and transcriptional factors by providing conceptual frame work for understanding molecular mechanisms behind the 'transcriptional stress memory' as potential memory tools in the regulation of plant heat stress response (HSR) [25].

### **6. Phenolics as antioxidants**

Aerobic metabolism induces the formation of reactive oxygen and nitrogen species (RONS) radicals whose levels are expected to increase in various types of stress conditions. Plant phenolics are powerful antioxidants that can mediate scavenging of harmful reactive oxygen species (ROS). The antioxidant activity of phenolics is based on number of hydroxyl groups, the presence of alkyl chains and the number of unpaired electrons. Phenolic acids form stable phenoxyl radicals in reaction with radical molecules. Ellagic acid is a powerful antioxidant as it is high in hydrogen bonds so that they can act as electron acceptors and hydrogen donors. Hydroxycinnamic acids (HCAs) are more effective antioxidants than hydroxybenzoic acids (HBAs). Some of the compounds like ferulic acid not only act as antioxidants but also inhibit enzymes involved in free radical generation and activate other scavenging enzymes [26, 27]. The type of antioxidants and their quantity is dependent on the type of endogenous or exogenous stress and to exploit these antioxidants for human use and commercialisation induction can be achieved by exposing the plants to the selected exogenous stress conditions.
