Title: The Impact of Microbes in Plant Immunity: Sustainable Approach for Crop Protection

One of the biggest demanding situations for food security in the 21 st century is to enhance crop yield stability through the improvement of diseases-resistant crops. Managing plant health is a major challenge for modern food production and compounded by the lack of common ground among the many disease control disciplines involved. All plants simultaneously engage with billions of microbes which can be collectively referred to as the plant microbiome. Most microbes inside the plant microbiome are harmless or even beneficial to the plant as they promote plant growth or provide protection in opposition to diseases. However, some of these microbes also cause disease with devastating effects on crop yields. To prevent pathogen infection, plants have evolved an advanced innate immune system that recognizes conserved cell surface molecules that most pathogen possesses. Activation of the plant immune system stops the invading pathogen, however this comes with fitness cost that significantly reduces plant growth and leads to yield penalty. Apart from their innate immune system controlling pre-programmed defense reactions, plants can also increase the responsiveness of their immune system in response to selected environmental signals. This phenomenon is known as “defense priming”. Although defense priming rarely provides full protection, its broad-spectrum effectiveness, low-fitness cost, long‐lasting durability and inherited to future generations make it attractive for sustainable crop protection.


Introduction
Plant pathogens can destroy up to 30% of worlds agricultural output (Savory et al., 2019), and hence, there is therefore an urgent need to develop disease-proof cropping systems.
Plants are colonized by way of a massive variety of micro organisms which could reach cell densities much more than the number of plant cells (Mendes et al., 2013). The rhizosphere is highly complex surroundings that consist of the narrow area of nutrient-rich that surrounds plant roots and is influenced. It is densely populated by various microorganisms which include fungi, bacteria, protists, nematodes and invertebrates. Plant roots secrete an assortment of primary metabolites (e.g., organic acids, carbohydrates, and amino acids) and secondary metabolites (e.g., alkaloids, terpenoids, and phenolics) which are believed to shape, signal, interfere with, or in some way affect the rhizosphere microflora (Venturi and Keel, 2016). This release or exudation in the rhizosphere of a large assortment of chemicals comes at a significant cost of carbon and nitrogen for the plant, with the ultimate benefit of attracting and promoting beneficial microorganisms while combating pathogenic or otherwise harmful ones (Venturi and Keel, 2016). This release or exudation in the rhizosphere of a large assortment of chemicals comes at a significant cost of carbon and nitrogen for the plant, with the ultimate benefit of attracting and promoting beneficial microorganisms while combating pathogenic or otherwise harmful ones (Venturi and Keel, 2016  Plants generally overcome the threats caused by pathogenic microbes by their innate ability to perceive signals from potential pathogens. Thereafter, the plants reprogram their defense systems appropriately to overcome these threats (Jain et al., 2012). Rhizosphere microbiome performs a considerable function in reprogramming the defense responses of plants (Spence et al., 2014). One of the maximum mentioned limitations of plant defense is the absence of acquired immunity that allows immunological memory, which may be activated to remove re-infecting pathogens (Sharrock and Sun, 2020). As a result improving disease control management via by focusing on plant immunity gives restrained possibilities as innate resistance genes ought to be slowly constructed into the genome through breeding, while pathogens can easily overcome the resistance due to their noticeably faster rate of evolution.
Alternative procedures to supplement the missing plant immune functions through genetic engineering (Dong and Ronald, 2019) or by using extensive amounts of pesticides are problematic due to lack of legal framework, public acceptance and direct adverse consequence on soil health (Raman, 2019;Hawkins et al., 2019). Thus, the situation arises where production must increase without relying heavily on the use of pesticides. Therefore, the sustainable preservation of agricultural productivity calls for new strategies for crop protection. In order to improve the plant disease management, one solution is to shift away from the reductionist view, where the plant health is studied by focusing on individual components in isolation, to a more holistic framework where the plant immunity is considered to emerge as a result of interactions with plant-associated microorganisms and environmental conditions (Teixeira et al., 2019).
Priming is an adaptive approach that improves the defensive capability of plants. This phenomenon is marked by way of a progressed activation of induced defense mechanisms.
Stimuli from pathogens, beneficial microbes, or arthropods, spider mites as well as chemicals and abiotic cues, can trigger the establishment of priming by acting as warning signals A major limitation of plant immunity is the lack of adaptive immunity. Unlike animals or even bacteria, plants do not have immunological memory that would enable them to recognize and trigger a robust secondary response against a pathogen previously found.
Nevertheless, when microbiome components are included, rhizosphere immunity can also be regarded as an adaptive mechanism in which immunological memory is provided by pathogen-suppressive microbes than can restrict re-infecting pathogens within and between plant generations (Wei et al., 2020).
The ability of plants to respond to aggressive environments by sensitizing the immune system in response to stress signals has evolved to make their immune system even more fascinating. This is known as priming, a phenomenon that is defined by psychologists as the implicit memory effect in which exposure to a stimulus influences the response to a subsequent stimulus (Gulan and Valerjev, 2010). In evaluation/comparison with the animal immune system, primes of defence often describes as plant vaccination.

Priming: an alternative to direct activation of defense
In everyday language, to prime means to prepare or make ready. In-plant defense, priming is a physiological system by using which a plant prepares to extra quick or aggressively responds to future biotic or abiotic stress. The circumstances of readiness executed by means of priming have been termed the "primed state" (Conrath et al., 2006). Defense Priming has been reported for a range of plant taxa, including wild species and cultivated varieties, and from herbaceous to long-lived woody plants (Hilker et al., 2015). Defense priming is postulate to an adaptive, low-cost defensive measure because defense responses are not, or only slightly and transiently, activated by means of a given priming stimulus. Instead, defense responses are deployed in a faster, stronger, and/ or more sustained manner flowing the perception of a later challenging signal (triggering stimulus); that is, in times of stress (Conrath et al., 2015) (Fig. 1). Because priming initiates a state of readiness that does not confer resistance per se but rather permits for accelerated induced resistance as soon as an attack occurs. One presumed gain of priming is that it does not impose the costs related to complete implementation of induced defense response. In comparison with the animal immune system, priming of plant defence is often described as plant vaccination. Priming a powerful enhanced basal resistance, that's controlled by a multitude of genes; therefore, priming of basal resistance is effective towards a broad range of biological threats (

Priming: Green Vaccination
Priming is an effective strategy to combat biotic and abiotic stresses, and it therefore represents a potential approach to enhance plant protection in agricultural systems (Walters et al., 2013). As there is an urgent need for new techniques that do not rely on pesticides or single resistance genes, the exploitation of the potential of the plant immune system in combination with other strategies may maintain the ability to achieve higher safety of crops.
The elegance of priming for agricultural safety is also associated with the fact that this phenomenon, unlike the direct activation of defences, does not incur major developmental costs (van Hulten et al., 2006). There has already been a considerable translation of knowledge from the laboratory to the field (Walters et al., 2013).
Plant defense priming provides broad spectrum resistance against pests and pathogens, and is also durable. Once induced, priming can be maintained all through the life of a plant and so primed crops should require fewer pesticide applications in order to reach similar levels of protection. By reducing pesticide inputs, integration of transgenerational priming into existing crop protection schemes could provide multiple benefits to both growers and to the environment (Singh and Roberts, 2015). In agriculture, transgenerational priming of plant defences has the capability to make a contribution to sustainable intensification. An efficient induction of TGIP would allow poor farmers to collect their own seed stocks of more resistant crop varieties, thereby making their food production less vulnerable to outbreaks of pests and disease.
Priming would assist to develop new crop types that are better suited to modern agriculture. I argue that crop improvement efforts must include using elicitors to prime or activate induced resistance in the field and, above all, to select for triggered heritable epigenetic states in progeny that is primed for defense.

Conclusion
The plant immune system prevents most pathogens from entering the root or reaching levels that are harmful to the plant. Irrespective of whether or not the association is harmful, neutral, or beneficial to the plant, microbes can avoid and intrude with the plant immune system. A good method for crop disease management is the use of microbes capable of antagonistic behaviour against pathogens to induce systemic resistance in plant. Also, the application of elicitors on plants is yet another method of pathogen control. However, to achieve full defense of plants against pathogens, an integrated approach to disease management and control involving the use of microbes, their metabolites, synthetic chemicals, and plant extracts formulation which will applied simultaneously to the plant, will allow farmers win the war against plant pathogens, increase crop yields, and achieve a sustainable agricultural production. Plant defense priming seems an attractive strategy to achieve this (Fig. 2).
Induced resistance by means of defence priming is durable. Once induced, priming can be maintained during the life of a plant and inherited epigenetically to subsequent generations.
In addition to this, the activation of priming and the selection of cultivars with transgenerational defence priming holds many benefits to breeding programs for the development of beneficial new traits in crops. The capacity to enhance resistance to pests and diseases through this mechanism provides a new mechanism via reliance on chemicals can be reduced without having to change the genetic make-up of our elite crop varieties. This could similarly provide a valuable tool for reducing the residues of chemical pesticides in the fruits and also additionally generate valuable knowledge for aid programmes in India, where poor infrastructure and limited economic ability demand a small-scale and self-sustaining mode of agriculture. Under these circumstances, crop seed stocks are commonly maintained by means of farmers themselves. An efficient induction of TGIP would allow poor farmers to collect their own seed stocks of more resistant crop varieties, thereby making their food production less vulnerable to outbreaks of pests and disease.
In addition induced resistance may not provide the "normal" degree of protection that we generally observe after the application of pesticides; however, priming can be used in combination with pesticides, microbes, biological control, resistance breeding, or any other integrated disease management strategy. However, before it could be able to commercially applied, several aspects, starting with the priming mechanisms have to be elucidated.
Assessments of priming treatments' persistence (duration of priming effects) and the range of biotic and abiotic stresses they may protect against are also warranted. Elucidation of mechanisms behind these diverse effects may provide highly interesting insights and opportunities in sustainable agriculture. I think it will not be exaggerating to conclude that plant defence priming/green vaccination is a smart plant health care for human welfare and could be a sustainable approach for crop protection with and broad effectiveness. We stand at the beginning of an exciting new road of research, wherein the mechanisms, ecological significance and potential applications of transgenerational plant defence just starting to be revealed.

References:
1. Ahmad, S., Gordon-Weeks, R., Pickett, J., and Ton, J. Natural variation in priming of basal resistance: from evolutionary origin to agricultural exploitation. Molecular Plant Pathology, 2010, 11, 817-827.   Figure 1. Diagram illustrating the principle of priming for defence. Leaves previously exposed to a priming signal are somehow able to responds more effectively to biotic attack, and consequently generate higher levels of resistance.

Figure 2.
Schematic overview of the integration of priming and rhizosphere immunity as described in the main text for sustainable approach for crop protection.