Microbial activity is greatly influenced by root. The ability of plant root to secrete diverse compounds in rhizosphere is remarkable metabolic feature. The major source of substrate for microbial activity are composed of exudates, lysate, mucilage, secretion, dead cells materials and gases including respiratory CO2 (Lynch and Whipps 1990). Plant root release large number of border cells (BCs) into the rhizosphere which has a role in plant health and may also secrete a complex mixture of proteins that may protect the root tip from infection (Wen et al., 2007). BCs are released from root tip as individual cells or group of cells and form an interface between root and the soil environment. The separation of BCs involves dissociation of individual cells from each other and root tissue. This process requires the activity of cell wall degrading enzymes that solubilize the cell wall connection between cells (Driouich et al., 2007). Border cells become detached from the root and enmesh in the mucilage surrounding the root surface (Hawes et al., 1998). One way in which plant can cope with the attack by an array of soil-borne pathogen is by production of detached living cells. Border cells release a diverse range of biological chemicals that influence the behaviour of fungi and bacteria (Hawes et al., 1998).
Root border cells along with released secretions form a protective structure termed the root extracellular trap (RET) that plays a role in root interaction with soil-borne microorganisms. Ropitaux et al.(2020) studies reveal that RET prevented Phytophthora parasitica zoospores from colonizing root tips by blocking their entry into the root tissues and inducing their lysis. Vast majority of zoospores could not penetrate the zone of mucilage and the few that were able to reach the zone or penetrated the mucilage underwent a rapid lysis and death. RET induce killing of Phytophthora parasitica zoospores may be due to the combined effects of histones and exDNA found in soybean RET (possibly originating from dead BCs). Yet the possibility is that the other anti-microbial molecules occurring in soybean RET may contribute in killing of the zoospores (Ropitaux et al., 2020). The dynamic of signal exchange leading to the development of mantle may play a role in fostering plant health by protecting root meristems from pathogenic invasion (Gunawardena et al., 2005). Border cells can repel pathogenic bacteria by their secreted mucilage (Driouich et al., 2010). The roots of pea seedling were infected with Nectria haematocca, the observation revealed that mantle comprised of fungal hyphae intermeshed with border cells covered the tips of most roots and on physical detachment of mantle, the roots were found to be free of infection (Gunawardena and Hawes 2002).
The secreted mucilage contain pectin, cellulose, antimicrobial proteins, extracellular DNA (exDNA), histones and two hemicellulose polysaccharides, xyloglucan and heteromannan (Driouich et al., 2013; Ropitaux et al., 2020). This exDNA-based matrix appears to function in root defense in similar manner to that of neutrophil extracellular traps in mammalian cells (Hawes et al., 2011; Driouich et al., 2013). Extracellular DNases are abundant in soil-borne pathogens and preliminary evidence show that these enzymes may act as virulence factors during infection (Hawes et al., 2016). The exDNA is exported into the surrounding mucilage which attracts, trap and immobilizes pathogen in a host specific manner. When this exDNA is degraded the resistance of root cap to infection is lost (Wen et al., 2009; Hawes et al., 2012). The exDNA is a bactericidal component of the extracellular matrix that surrounds root BCs and the root caps, protecting root tips from infection by trapping or blocking pathogenic microbes (Hawes et al., 2016).
Rhizosphere dynamic is recognition and each of these activities is host specific and genotype specific (Hawes et al., 2003). Root cap cells of two cotton species elicited a chemotactic response in zoospores of Pythium dissotocum (Goldberg et al., 1989). The zoospores encyst and penetrate the cells within minutes and can fully digest a thousand of BCs within an hour (Hawes et al., 2003). Whereas, Pythium catenulatum exhibiting chemotactic response to Agrostis palustris (Bentgrass) were neither attracted nor did they infect cotton seedlings or isolated root cap cells (Goldberg et al., 1989). The interactions are completely inert because the genes required for recognition response are absent. Thus there is no attraction, no encystment, no penetration and killing of BCs and no germination and growth of fungus (Hawes et al., 2003). The zoospores will continue to swim in the same vessel as the BCs for days because at molecular level, in the absence of appropriate genotypes, the plant and pathogen are functionally invisible to each other. Finally the zoospores starve to death despite nutrients being available. The nutrient delivered to the rhizosphere are packaged in living cells and living cells have the capacity to resist invasion and digestion by microorganisms (Hawes et al., 2003). Similarly the nutrient available in the polysaccharide mucilage secreted by root cap are only available to those microorganisms that have appropriate enzymes to solubilize and digest it but sometime the extracellular plant enzymes may degrade the mucilage into component sugars which then is available as signals and nutrient to those microorganisms that have appropriate enzymes to utilize them (Hawes et al., 2003).
The BCs within extended sheaths on compost water extract treated roots were found to be coaggregated with bacterial population Acinetobacter species (Curlango-Rivera et al., 2013). These bacteria have the potential to act as biocontrol agent. Qing-Yun et al.(2009) report Acinetobacter sp. strain is used as biocontrol agent against Ralstonia solanacearum causing bacterial wilt in tomato.
Root border cells production may be stimulated to increase the synthesis and release of defense molecules in the rhizosphere which may contribute in controlling the pathogenic invasion.
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