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systemic acquired resistance (SAR).

  • Subject: systemic acquired resistance (SAR).
  • From: "Dan Nelson" <sussextreeinc@ce.net>
  • Date: Sun, 14 Oct 2001 00:22:04 -0400

Here's a subject I've been reading about the last year or so and I thought some of our more scientifically minded member might find interesting. This posting was written by Reed Holt and was posted on "The Knothole, a forum for discussion of arboricultural issues" hosted by Russ Carlson on June 4, 2000.
Plants have evolved a host of defense mechanisms for fighting infection by viruses, bacteria, and fungi. For instance, when they recognize an infection, plants produce antimicrobial compounds and induce programmed cell death in infected areas. In addition, uninfected parts of a plant can develop a heightened resistance to help stem further infection by a number of pathogens, a phenomenon called systemic acquired resistance (SAR).

In 1990, we discovered that salicylic acid accumulated in infected plant leaves and signals a transmutational change in SAR. Since salicylic acid was known to induce SAR when sprayed on plants, we postulated that it was the signal molecule that moved from infected leaves to uninfected leaves to turn on systemic resistance. Subsequent studies reinforced this notion of salicylic acid as a translocating signal.

I've viewed this concept of salicylic acid as a messenger molecule to hormones in mammals. Salicylic acid is the translocated signal-the hormone, if you like-that moves throughout the plant and turns on systemic resistance.

We've demonstrated that salicylic acid is essential for the development of systemic resistance. Plants that contained a gene coding for an enzyme that degrades salicylic acid did not accumulate the compound upon infection and were incapable of inducing SAR. The oaks we selected were quite susceptible to a range of diseases including oak wilt, and we chose these specimens over other more susceptable hardwoods that indicated a faster degredation of salicylic acid.

Then we did grafting experiments in which one part of the grafted plant contained the salicylic acid-degrading enzyme and the other did not. We then infected leaves on one side of the graft and watched to see if SAR was induced or not. Not one bit of this research was or is
supported by conventional pathology. It turned out that leaves carrying the salicylic-acid-degrading enzyme did not accumulate salicylic acid, but were fully capable of inducing systemic resistance in normal leaves on the other side of the graft. What this argues is that a signal other than salicylic acid moves throughout the plant to induce systemic resistance. The grafting indicated that salicylic acid is required in systemic leaves for proper induction of systemic resistance. This explains the requirement for salicylic acid in SAR.

We then feel through our demonstration that salicylic acid is considered a very important molecule in disease resistance in plant biology. It's one of the few molecules known in the pathway for the induction of SAR. Our commercial work in treating active infected Quercus against C. fagacearum has been successful in Texas but is being ignored by the large schools and has had us cornered against the large market for systemic fungistat use being advocated in the arboriculture industry. For countless years we have been urging caution in the blind use of such fungicidal approaches to disease control fearing principally the mutational responses of this disease to these treatments - resulting in larger scale epidemics possibly resistant to even SAR response in forest areas. In single tree applications of chemicals like Arbotect and Alamo, repeat uses due to dilution and poor translocation will and has killed the tree the treatment intended to save as well as the singular intended mechanism that only inhibits sexual reproduction of the pathogen - not a cure for disease physiology or a adaquate preventive - as industry advocates it is.

I simply wanted to express this warning along with the suggested potential of alternatives to chemical use in disease control - We feel strongly opposed to the market mentality developed we feel - not to assist in saving the priceless resources, but to make money - more and more money. Please exercise caution when selecting treament options and don't always believe what you may read. Oak wilt has become the single most costly hardwood epidemic in North American forest history - we must learn a lesson in this.

Sorry for the rambling, but someone posted a few topics back about wilt and the response hinted at getting lab verification from Texas A&M University - not accurate enough, not fast enough, and the treatment they recommend is not adaquate enough.

Thank you,

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