Biotic Stress

Abstract

In nature plants seldom grow in isolation. Their growth and development are greatly influenced by abiotic and biotic factors to which they are continuously exposed. Unlike animals they are rooted in the soil, and consequently they cannot escape adverse environmental conditions in their vicinity (Fig. 32.1). Rather they have to develop the strategies to adapt themselves to the hostile conditions in order to survive and grow. Biotic interactions of plants are not always harmful but they can be beneficial too. Plant-pollinator interactions, rhizobia-legume interactions, or mycorrhizal interactions are examples of mutualism in which both the partners of the association are benefitted. In rhizobia-legume interactions, host plant provides ecological niche and nutrients which are required for the growth of prokaryotes, while the prokaryotes provide nitrogen to the host plant. Some plants are not able to excrete protons or organic acids in the soil required for uptake of nutrients like phosphates. They do so through symbiotic associations with mycorrhizal fungi. Associations, in which one of the partners is benefitted and the other one is not affected, are called commensalism. In some cases, the association may become beneficial as the benefitting partner may stimulate defense mechanism in the other organisms. There may also be harmful biotic interactions among plants and other organisms. These include interactions of plants with pathogens, with the plant pests and parasitic associations between the plants. Plant pathogens are the organisms that spend a part of their life cycle or complete their life cycle inside the plant. These include the microbial pathogen such as viruses, bacteria, or fungi. On the contrary, plant pests include the herbivores, such as insects, nematodes, or mammals which cause damage to plants by eating their vegetative tissue, fruits, or seeds (Fig. 32.2). It is interesting to note that plants growing in the wild rarely develop disease. However, domestication of economically important species leads to the development of monocultures, thereby resulting in genetic uniformity which makes them more susceptible to infections. Understanding the interactions of plants with their biotic environment greatly helps in reducing the use of agrochemicals which, in turn, would facilitate reduction in pollution and also in the cost of energy required for their production. Allelopathic interactions occur between plants growing nearby. Growth of some plants is inhibited because of chemicals produced by the adjacent plants which results in a sort of a chemical warfare. In this chapter plant’s interactions with biotic factors will be dealt with at physiological and molecular level.

Multiple-Choice Questions

• 1. Interaction among plants, herbivores, and their natural enemies is known as:

  • (a) Biotrophic interactions

  • (b) Pathogenic interaction

  • (c) Tritrophic interactions

  • (d) Allelopathic interactions

• 2. A non-protein amino acid which has a role in defense is:

  • (a) Homoserine

  • (b) Canavanine

  • (c) Citrulline

  • (d) Ornithine

• 3. Systemin is an example of:

  • (a) Pattern-associated molecular pattern

  • (b) Damage-associated molecular pattern

  • (c) An effector molecule

  • (d) A molecule which is translocated away from the damaged site of the plant

• 4. Elicitor refers to the molecules:

  • (a) Produced by the plant in response to pathogenic infection

  • (b) Produced by the pathogen at the time of infection

  • (c) Present on the surface of plant cells by which pathogens are recognized

  • (d) By which plant is able to identify “self” from “nonself”

• 5. HAMPs are:

  • (a) Components from the plant feed which are modified in the guts of the insects

  • (b) Molecules produced in plants in response to the damage caused by herbivores

  • (c) Molecular pattern on the plant surface recognized by the insects

  • (d) Compounds in herbivores identified by the plant

• 6. An example of HAMP is:

  • (a) Linoleic acid

  • (b) Volicitin

  • (c) Glutamine

  • (d) Caeliferins

• 7. Holoparasites are the plants which:

  • (a) Contain chlorophyll and have little photosynthetic capacity

  • (b) Have lost their capacity to complete their life cycle without the host

  • (c) Get complete nourishment from the host plant

  • (d) Are dependent on the host plant for the supply of water and minerals

• 8. Strigol is a (tick mark which is not true):

  • (a) Compound which inhibits germination of the seeds of Striga sp

  • (b) Stimulant for the germination of cotton seeds

  • (c) Secondary metabolite released from the roots of host plant

  • (d) Sesquiterpene released from the roots of cotton plant

• 9. Sticky substance produced by the host plant when induced by Cuscuta include (tick mark which is not correct):

  • (a) Arabinogalactan proteins

  • (b) Polysaccharides

  • (c) Parahydroxyacids

  • (d) Pectins

• 10. Parasitic plants have:

  • (a) Lower water use efficiency than the host plants

  • (b) Higher water use efficiency than the host plants

  • (c) Lower transpiration rates than the host plants

  • (d) Maintain high temperature around their leaves

• 11. Catechin, a toxic secondary metabolite, is produced by:

  • (a) Eichhornia crassipes

  • (b) Alliaria petiolata

  • (c) Centaurea maculosa

  • (d) Juglans nigra

• 12. An interaction when both partners of the association are benefitted is known as:

  • (a) Commensalism

  • (b) Mutualism

  • (c) Amensalism

  • (d) Parasitism

• 13. Necrotrophy is the pathogenic infection in plants when the cells:

  • (a) Are killed prior to infection

  • (b) Remain alive throughout infection

  • (c) Are immediately killed after the infection

  • (d) Are killed much later on after the infection

• 14. An effector molecule is:

  • (a) Produced by pathogens which triggers basal defense in plants

  • (b) Present on the surface of pathogen by which it is identified

  • (c) A molecule made by pathogen to overcome basal defense of the plant

  • (d) Produced by the plant in response to pathogenic infection

• 15. Effector-triggered immunity refers to:

  • (a) Defense strategies by host plant in response to the molecular pattern at the pathogen’s surface

  • (b) Suppression of host-triggered immunity by the effector molecules produced by pathogens

  • (c) Triggering of defense response in plant as a result of incompatible interaction of factors produced by pathogens and R gene products

  • (d) Immunity in host plants in response to elicitors

• 16. Fusicoccin is a non-specific toxin which:

  • (a) Inhibits histone acetylase thus affecting gene expression involved in defense against pathogen

  • (b) Mimics action of jasmonic acid isoleucine and helps in bacterial infection by activating JA signaling pathway

  • (c) Acts by damaging plant cell walls

  • (d) Inactivates membrane-bound H+-ATPase proton pump

• 17. Toxicity of tabtoxin (a dipeptide) is due to:

  • (a) Release of its hydrolysis product an amino acid threonine

  • (b) Release of tabtoximine as a result of its hydrolysis, a potent inhibitor of glutamine synthetase

  • (c) Due to pores created in the membrane resulting in leakage of nutrients

  • (d) Irreversible opening of stomata which leads to wilting of plants

• 18. Effector molecules are:

  • (a) Proteins required to develop Type III system in bacteria

  • (b) Products of R genes in host plants

  • (c) Molecules produced by pathogens that trigger infection of the host plant

  • (d) Products of avr genes of pathogens

• 19. Systemic acquired resistance is:

  • (a) Due to accumulation of defense proteins not only at the site of infection but also systemically in uninfected tissues and/or uninfected plants

  • (b) Due to suppression of infection by disease-causing pathogens by providing systemic resistance by antagonism

  • (c) Resistance in the host plant at the infected tissues by pathogens

  • (d) Very specific and long lasting

Answers

1. c	2. b	3. b	4. d	5. a	6. b	7. b
8. d	9. c	10. a	11. c	12. b	13. a	14. c
15.c	16. d	17. b	18. d	19. a