Rhizobitoxine-induced chlorosis occurs in coincidence with methionine deficiency in soybeans.
OBJECTIVE
Rhizobitoxine, produced by the legume symbiont Bradyrhizobium elkanii, inhibits cystathionine-beta-lyase (EC 4.4.1.8) in methionine biosynthesis and 1-aminocyclopropane-1-carboxylate synthase (ACC) in ethylene biosynthesis. Rhizobitoxine production by B. elkanii enhances nodulation of host legumes via the inhibition of ethylene synthesis, but causes foliar chlorosis in susceptible soybeans, though how it does so remains to be investigated. The aim of this study was to examine the physiological basis of rhizobitoxine-induced chlorosis in soybeans.


METHODS
Wild-type B. elkanii and a rhizobitoxine-deficient mutant were inoculated in Glycine max 'Lee'. Thirty days after inoculation, the upper parts of soybean shoots were analysed for amino acid contents. Chlorotic soybeans inoculated with wild-type B. elkanii were treated with methionine and ACC to assess the effects of the chemicals on the chlorosis.


RESULTS
Chlorotic upper shoots of soybeans inoculated with wild-type B. elkanii had a lower methionine content and higher accumulation of the methionine precursors than those with the rhizobitoxine-deficient mutant. In addition, the foliar chlorosis was alleviated by the application of methionine.


CONCLUSIONS
Rhizobitoxine-induced chlorosis occurs in coincidence with methionine deficiency as a result of cystathione-beta-lyase inhibition during methionine biosynthesis.
Okazaki
Shin
S
Graduate School of Life Sciences, Tohoku University, Katahira, Aoba-ku, Sendai 980-8577, Japan.
Sugawara
Masayuki
M
Yuhashi
Ken-ichi
K
Minamisawa
Kiwamu
K
D007113:Immunity, Innate / Q000235:genetics / Q000502:physiology; D008190:Lyases / Q000235:genetics / Q000378:metabolism; D008715:Methionine / Q000378:metabolism*; D010935:Plant Diseases / Q000139:chemically induced*; D018515:Plant Leaves / Q000187:drug effects / Q000378:metabolism; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism; D018520:Plant Shoots / Q000187:drug effects / Q000378:metabolism; D011412:Propanolamines / Q000494:pharmacology*; D013025:Glycine max / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*
D010940:Plant Proteins; D011412:Propanolamines; D008715:Methionine; D008190:Lyases; C019965:cystathionine beta-lyase; C041618:rhizobitoxine