Systemic acquired resistance

Systemic acquired resistance (SAR) is a "whole-plant" resistance response that occurs following an earlier localized exposure to a pathogen. SAR is analogous to the innate immune system found in animals, and although there are many shared aspects between the two systems, it is thought to be a result of convergent evolution.[1] The systemic acquired resistance response is dependent on the plant hormone, salicylic acid.

Discovery

While, it has been recognized since at least the 1930s that plants have some kind of induced immunity to pathogens, the modern study of systemic acquired resistance began in the 1980s when the invention of new tools allowed scientists to probe the molecular mechanisms of SAR.[2] A number of 'marker genes' were characterized in the 80s and 90s which are strongly induced as part of the SAR response. These pathogenesis-related proteins (PR) belong to a number of different protein families. While there is substantial overlap, the spectrum of PR proteins expressed in a particular plant species is variable.[2] It was noticed in the early 1990s that levels of salicylic acid (SA) increased dramatically in tobacco and cucumber upon infection.[2] This pattern has been replicated in many other species since then. Further studies showed that SAR can also be induced by exogenous SA application and that transgenic Arabidopsis plants expressing a bacterial salicylate hydroxylase gene are unable to accumulate SA or mount an appropriate defensive response to a variety of pathogens.[2]

The first plant receptors of conserved microbial signatures were identified in rice (XA21, 1995)[3] and in Arabidopsis (FLS2, 2000).[4]

Mechanism

Plants use pattern-recognition receptors to recognize conserved microbial signatures. This recognition triggers an immune response. Plants also carry immune receptors that recognize highly variable pathogen effectors, these include the NBS-LRR class of proteins. SAR is associated with the induction of a wide range of genes (so called PR or "pathogenesis-related" genes), and the activation of SAR requires the accumulation of endogenous salicylic acid (SA). The pathogen-induced SA signal activates a molecular signal transduction pathway that is identified by a gene called NIM1, NPR1 or SAI1 (three names for the same gene) in the model genetic system Arabidopsis thaliana.

Use in disease control

Unusually, the synthetic fungicide acibenzolar-S-methyl is not directly toxic to pathogens, but rather acts by inducing SAR in the crop plants to which it is applied. It is a propesticide — converted in-vivo into 1,2,3-benzothiadiazole-7-carboxylic acid by methyl salicylate esterase.[5] Field trials have found that acibenzolar-S-methyl (also known as BSA) is effective at controlling some plant diseases, but may have little effect on others, especially fungal pathogens which may not be very susceptible to SAR.[6]

See also

References
  1. Ausubel FM (October 2005). "Are innate immune signaling pathways in plants and animals conserved?". Nature Immunology. 6 (10): 973–9. doi:10.1038/ni1253. PMID 16177805. S2CID 7451505.
  2. Ryals, J., U. Neuenschwander, M. Willits, A. Molina, H. Steiner, and M. Hunt. 1996. Systemic Acquired Resistance. Plant Cell 8:1809–1819.
  3. Song WY, Wang GL, Chen LL, Kim HS, Pi LY, Holsten T, Gardner J, Wang B, Zhai WX, Zhu LH, Fauquet C, Ronald P (December 1995). "A receptor kinase-like protein encoded by the rice disease resistance gene, Xa21". Science. 270 (5243): 1804–6. doi:10.1126/science.270.5243.1804. PMID 8525370. S2CID 10548988.
  4. Gómez-Gómez L, Boller T (June 2000). "FLS2: an LRR receptor-like kinase involved in the perception of the bacterial elicitor flagellin in Arabidopsis". Molecular Cell. 5 (6): 1003–11. doi:10.1016/S1097-2765(00)80265-8. PMID 10911994.
  5. Jeschke, Peter (2016). "Propesticides and their use as agrochemicals". Pest Management Science. 72 (2): 210–225. doi:10.1002/ps.4170. PMID 26449612.
  6. Vallad, Gary E.; Goodman, Robert M. (2004). "Systemic Acquired Resistance and Induced Systemic Resistance in Conventional Agriculture". Crop Science. 44 (6): 1920–1934. doi:10.2135/cropsci2004.1920. ISSN 1435-0653. Retrieved 2020-11-27.

Further reading
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