One of the biggest differences between plant cells and animal cells? Plant cells have a wall around them. But it’s more than a barrier; it’s a vital part of how plants function and survive.
An Overview of Plant Cell
Part I: Plant Cell Wall
The plant cell wall is the first line of physical defense against all kinds of pressures. These stresses can be of biotic nature or abiotic. The cell wall is a complex structure with a diverse array of functions throughout the plant lifecycle. Unlike animals, plants have evolved extra defense layers around their cells because of the constant exposure to a variety of pathogens (Fungi, bacteria, nematodes, etc). It provides structural flexibility to plant cells during cell division and resists pathogen’s advancements. Every microbe must have to cross this rigid and thick barrier to getting entry into the plant cell.
The plant cell wall entertains a variety of receptors, pores, and channels that regulates the movement of molecules, proteins, hormones, and elicitors, RNA. Being a dynamic structure and consistent with the maintenance of the cell's hydrostatic environment, defense against a wide range of pathogens, it does also play a vital role against abiotic stresses (drought, high temperature, etc.). Plant cell wall structure not only varies between plant species, but it does also vary between tissue types. And its response to microbes is also highly dependent on their lifestyle.
Cell Wall Components
The plant cell wall is composed of cellulose, hemicellulose, pectin, microfibril, and lignin. Lignin is a phenolic polymer and mainly constitute the secondary cell wall. Lignin biosynthesis pathway is also responsible for the synthesis of other phenolic compounds that are phytoalexins and flavonoids. These phenolic polymers play a vital role in defense reactions.
These components are organized into three layers—primary cell wall, middle lamella, and secondary cell wall. Upon pathogen interactions, these components of the cell wall are the main targets. Especially necrotrophs (feed on the dead host tissues), they secrete hydrolytic enzymes to macerate the cell wall.
Plant Cell Wall Response to Biotrophic Pathogens
The cell wall of plants serves as both passive and active defense barrier against microbes. The passive or preformed barrier is the first line of defense that resists the microbe entry to the cell cytoplasm. To cross this barrier, a microbe or pathogen must come with some infection strategies to lyse the integrity of the cell wall. Biotrophs need living host cells and tissues for their growth and reproduction. That’s why they form haustorium to penetrate the plant cell wall and successfully establish a feeding site. Examples of biotrophs are oomycetes, rust fungus, powdery, and downy mildews. The host plants also employed active defense against those pathogens that manipulate preformed or passive defense reinforcements through their secretions— consist of molecules or small oligomers termed as effectors. These effector molecules are perceived by plasma membrane receptors. Plasma membrane receptors recognize these invasion molecules and activate an arsenal of molecules that ward off pathogens.
Papillae Formation and Biotrophic pathogens
The plant cell wall is the first obstacle that every microbe must encounter for successful access to the protoplast. To cross this barrier, biotrophs come with a finer strategy. They establish feeding sites at the surface of the cell wall through haustorium (nutrient absorbing tube). In response, the plant actively deposits the cell wall material known as “papillae” to halt the penetration of the pathogens.
The papilla is a complex structure, forms at entry or penetrating site and accumulated with antimicrobial compounds mainly callose, components of the cell wall (cellulose, hemicellulose), phenols, and reactive oxygen species. Papillae formation is always correlated with resistance of host plant to both host-specific and non-specific pathogens.
Based on the host plant's ability to generate papillae with correct chemical depositions at right can confer the papillae mediated penetration resistance in plants against pathogens (Aist and Israel, 1997).
In comparison with biotrophs, the necrotrophs are better equipped in taking control over the programming machinery of the plant cell (chiefly plant defense responses). They have an armory of cell wall degrading enzymes (cellulase, pectinase, and hemicellulose, and also virulence factors—used to reverse the cell wall thickening process and for their own benefit).
The recognition of virulence factors or conserved effector molecules by plant cell receptors triggered defense signaling through salicylic acid, ethylene, auxin to activate defense genes. This leads to the activation of resistance reactions towards necrotrophs. If a plant is unable to identify the presence of effector molecules, it triggers susceptibility (huge release of cell wall degrading enzymes) that favors the degradation of the cell wall and movement of the pathogen to the neighboring cells.
The plant cell wall is the first obstacle that a phytopathogen encounters during the invasion of plants. It is the most complex, dynamic, and varied structure of plants that surround the plasma membrane.
It provides the plant cell a framework to maintain its cell shape with protection against all kinds of stresses: biotic and abiotic.
In response to pathogen attack, cell walls act as both: active and passive defense barriers. The cell wall resists the pathogen penetration by reinforcing the structure of its components.
Depositions of cell wall material are called papillae— formed at penetrating site is the major penetration limiting factor in plants. Papilla consists of phenolic compounds and the plant’s secondary metabolites that have antimicrobial properties.
The monomer of cell wall components acts damage-associated molecular patterns that triggered immune responses against a wide range of microbes.
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Houston, K., Tucker, M. R., Chowdhury, J., Shirley, N., & Little, A. (2016). The plant cell wall: a complex and dynamic structure as revealed by the responses of genes under stress conditions. Frontiers in plant science, 7, 984. DOI: 10.3389/fpls.2016.00984.
Underwood, W. (2012). The plant cell wall: a dynamic barrier against pathogen invasion. Frontiers in plant science, 3, 85. DOI: 10.3389/fpls.2012.00085.