How Plants Protect Themselves Against Diseases?
We all know that plants are sedentary and cannot ‘run away’ from adversary caused by both biotic (living organisms such as bacteria, fungi, virus, nematodes) or abiotic (environmental such as moisture or nutrient deficit, temperature, drought, salinity or alkalinity in soil, pollution etc) stresses. They have to stand up to the challenges and face them either by countering them or sometimes by adapting to evade them. How plants achieve this has been an intriguing phenomenon and continues to be an interesting area of research.
In case of plants, disease is any physiological abnormality or significant disruption in the “normal” health of a plant. To protect themselves from damage caused by disease(s), plants have developed a wide variety of constitutive (that is continuous and present in all tissues) and inducible (triggered only when required) defense mechanisms. Constitutive defenses include many preformed structural barriers such as cell walls, waxy epidermal cuticles, and bark that not only protect the plant from invasion but also give the plant strength and rigidity. In addition to preformed barriers, all living plant cells have the ability to detect invading pathogens and respond with inducible defenses including the production of toxic chemicals, pathogen-degrading enzymes, and deliberate cell suicide. Plants use good surveillance system to detect the pathogens before embarking on the task of producing cost-involving toxic chemicals or defense-related macromolecules such as proteins.
Talking about the pre-formed structural barriers, all plant tissues contain them to limit pathogen attachment, invasion and infection. The cell wall, a wooden box encasing cellular components, is not only an excellent fort like structure but also incorporates a wide variety of chemical defenses that can be rapidly activated when the cell detects the presence of potential pathogens. All plant cells have a primary cell wall, which provides structural support and many cells also form a secondary cell wall that develops inside of the primary cell wall after the cell stops growing. Many cell walls also contain lignin (primary component of wood), that make cell wall highly impermeable to pathogens. Cutin, suberin, and waxes are fatty substances that may be deposited in either primary or secondary cell walls (or both) and outer protective tissues of the plant body, including bark and in many cases do not permit establishment of the pathogens by not allowing moisture (essential for germination of spores and growth of pathogens) to stay on the plant surface. It is also essential here to mention that plant cell wall houses some enzymes that trigger ‘induced defense response’ when the surveillance system detects pathogen through a mechanism that senses ‘Microbe associated molecular patterns’ and communicates these ‘enemy attack’ signals. Epidermis (outer most cell layer of plant surfaces) also contains specialized appendages like trichomes and hairs that act both as mechanical/physical barriers (as in soybean and beans) and sometimes as chemical factories that produce repelling oils.
These pre-formed physical barriers are not enough for the plants to protect themselves. They have evolved immunity system (not exactly in lines with what we perceive in higher animals or humans) which is basically a chemical warfare that is propelled in a cascade manner and develops into an insurmountable barrier for the pathogens to establish and cause damage. Chemical barriers, most often the second line defense, include production of ‘secondary metabolites’ which per se are not needed for plant growth and development (unlike primary metabolites like sugars, proteins, nucleic acids that are essential). These secondary metabolites usually belong to one of the three large chemical classes: terpenoids, phenolics, and alkaloids. Terpenoids (terpenes) occur in all plants and represent the largest class of secondary metabolites with over 22,000 compounds described! Some good examples include menthol produced by mint, pyrethrins produced by Chrysanthemum, Turpentine produced by pines, gossypol produced by cotton and azadirachtin produced by neem. Phenolics are another large class of secondary metabolites produced by plants to defend themselves against pathogens and primarily include compounds flavonoids, anthocyanins, phytoalexins, tannins, lignin, and furanocoumarins. These toxic molecules disrupt pathogen metabolism or cellular structure and are often pathogen specific in their toxicity. Examples include medicarpin produced by alfalfa (Medicago sativa), rishitin produced by both tomatoes and potatoes (the Solanaceae family), and camalexin, produced by Arabidopsis thaliana.
Apart from secondary metabolites, there are several proteins and enzymes produced by plants in a bid to self-protect. Most of these are produced specifically against the invading pathogens through a well orchestrated process of gene regulation and we will look into the details of these mechanisms in our next articles. Thus, we understand that plants have evolved many arsenals to fight diseases and they are just not silent victims when invaded.
Dr. V. Dinesh Kumar
Principal Scientist (Biotechnology)
Hyderabad 500 030
- Reference: http://scienceindia.in/home/view_article/73
- Image courtesy: http://2blades.org/the-unmet-need/