The 1/24/97 issue of Science has what I believe is a very significant
article documenting for the first time clear evidence that ethylene
biosynthesis plays a direct, causal role in plant disease response. The
article is "A Legume Ethylene-Insensitive Mutant Hyperinfected by Its
Rhizobial Symbiont." The authors are R. Varma Penmetsa and Douglas R. Cook,
Dept. of Plant Pathology and Microbiology, Crop Biotechnology Center, Texas
A+M. While the study focuses on rhizobium and nitrogen fixation -- a
positive consequence of infection -- the authors point out the mechanisms
and impacts of ethylene are likely the same relative to damaging root
pathogens, and hence the broader significance of their findings.
For years plant pathologists and other root-health experts have
hypothesized that ethylene plays some role in plant defense mechanisms. But
this is the first solid evidence of a direct role. If ways could be found
to stimulate ethylene production, either by the plant or soil microorganisms
or maybe both???, especially when plants are young and immune systems and
responses are being shaped (i.e. the early stages of systemic acquired
resistance), the season-long beneficial consequences for the plant could be
sizable – better root development, more efficient N and water uptake,
greater capacity to ward off pathogens and insect pressure later in the
season, better ability to compete with early-season weeds, etc. If this
proves to be true, scientists and farmers will have a viable new method to
determine which soils are most likely to be disease suppressive, and to
study how to make them more suppressive through management.
I would be interested to know if any researchers have done studies
of the levels of ethylene in soils that are disease-suppressive versus prone
to root infections.
Also, someone please help out us non-experts -- what soil
microorganisms, if any, produce ethylene, or play a symbiotic role in the
production of ethylene?
My guess is that rather straight-forward field and/or soil column
studies could be carried out quickly and cheaply to quantify the impact of
management practices and production inputs on ethylene production and levels
in the rhizosphere. I am far from an expert on soil microbiology and
pathogenesis, but I would expect that even a 15% to 20% increase sustained
over a period of 48 hours to a few days could provide plant roots enough
added protection against some pathogens to account for a part, if not a
significant part of the yield boost we see in some cropping systems, for
reasons that remain hard to explain. Could someone develop a simple assay
for ethylene biosynthesis, suitable for use in assessing the impacts of
management practices on soil/plant health? Maybe someone already has?
I will post this message on <http://www.pmac.net, in "Cutting Edge
Science and Technology Issues">, along with a synopsis of the Science
article, any responses over SANET or received directly. I am going to
pursue the literature some as well, and will post pertinent information.
Charles Benbrook 202-546-5089 (voice)
Benbrook Consulting Services 202-546-5028 (fax)
409 First Street S.E. email@example.com [e-mail]
Washington, D.C. 20003 http://www.pmac.net