The of which is ‘to excite’. Hormones

The word ‘hormone’ is derived from a Greek word and the literary meaning of which is ‘to excite’. Hormones are explicit chemicals and play role in the coordination of the different parts of organisms as well as act as a signaling networks.

Hormones are produced in one part of the organisms in small amounts and then transport to other parts of organisms in triggered response.  In the natural environment, plants interact with different types of pathogenic organisms such as bacteria, fungi, viruses, parasites, browsing herbivores and nematodes. Not all pathogens are able to cause diseases to plants, only a few of them are liable to cause the disease to plants.

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Plants have developed their defense mechanisms to protect from the attack of different pathogens. The defense mechanism performed by plants is complex which comprises of multiple layers of defense which are effective against the diverse array of the pathogen. Plants hinder pathogen infections through utilization of their own physical and chemical barriers. In addition to this, plants have developed their induced defense mechanism that is triggered upon pathogen infection which are called as hormones. These defenses may include complex molecular reactions, biochemical and morphological changes such as oxidative burst, resistant gene expression, production of antimicrobial compounds or programmed cell death (van Loon et al. 2006).

The innate immune system is the peculiar characteristics of plants which helps in the defense mechanisms of the plants against potential pathogens. Plants including food crops are likely to be affected by biotic and abiotic stress because of their immobile growing habit. Biotic stress include bacteria, fungi, viruses nematodes and herbivore pests  (Atkinson, 2012). These biotic agents in the presence of host plant and favorable environmental condition cause disease which leads consequent losses. This loss range from 20 to 40 % (Savary, 2012).

These biotic agents help to generate hormones in host plants like jasmonate, salicylic acid and ethylene and their level directly influenced by disease development. There is a co-evolutionary arms race between plant and their enemies that helps to provide plants with a highly sophisticated defense system that, similar to the animal innate immune system, recognizes the signals from injured cells, and responds by activating an effective immune response against the invader encountered (Verhage et al. 2010). Plant hormones play pivotal roles in the defense mechanisms by cross-communicating hormones to the plants through signaling network (Spoel and Dong, 2008; Pieterse et al., 2009). Plant enemies have a capacity to hijack the plant’s defense signaling network for their own benefit through antagonists effect against the host immune response of plants (Grant and Jones, 2009).

The mechanism of plant immune system is well described by the zigzag model in which the perception of pathogen-associated molecular pattern (MAMPs or PAMPs) by host-encoded pattern recognition receptors (PRRs) results in PAMP-triggered immunity (PTI). Pathogens which are able to cause damage to plants secrete effectors that suppress PTI and thus induce disease resulting in effectors triggered susceptibility (ETS) (Bari et al. 2009). For the counter defense strategy, plants recognize a given effectors either directly or indirectly and activate effector-triggered immunity (ETI) resulting in disease resistance (Chisholm et al. 2006; Jones and Dangl 2006). The activation of PTI or ETI enhances plant disease resistance and restricts pathogen growth.

Plants have an ability to produce a wide range of hormones, which include auxins, gibberellins (GA), abscisic acid (ABA), cytokinins (CK), salicylic acid (SA), ethylene (ET), jasmonates (JA), brassinosteroids (BR) and peptide hormones. Recently, strigolactones are identified as a new class of plant hormones (Gomez-Roldan et al. 2008; Umehara et al. 2008). Plant hormones play important roles in diverse growth and developmental processes as well as various biotic and abiotic stress responses in plants. The functions of these plant hormones in complex defense signaling network by the plant and the interactions of the attackers with plant hormones and mechanism for regulation of induced systemic resistance (ISR) by these hormones is not well understood.

To explain scientific knowledge on these specific hormones, this review is carried out to elaborate both positive and negative crosstalk on the molecular pathway interaction by these hormones and their role in defense mechanism against bacterial and fungal diseases as biotic stress. The search result of online database Scopus using keywords, ‘role of Jasmonic acid and salicylic acid in plant defense, ‘host defense’, mechanism of defense by SA and JA, and ‘biotic stress’ are included. The present knowledge on the role of salicylic and jasmonic acid to host defense against bacterial and fungal pathogen will be highlighted in the first section of the paper. The second section will discuss the salicylic acid and jasmonic acid signaling and its crosstalk with special reference to biotroph and necrotroph pathogen.

The future prospects, home messages for phytopathologist, research gaps and some suggestions will be provided at the end. This paper will highlight the role of salicylic and jasmonic acid in plant defense, nematologist, and molecular breeder will get the insight knowledge on resistance breeding by the introduction of salicylic and jasmonic acid-responsive gene or manipulation of the desirable signaling pathway for the excretion of these hormones.  

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