Interaction of glufosinate and Colletotrichum truncatum on ammonia levels and glutamine synthetase activity in Hemp sesbania

Authors: Hoagland, Robert; Boyette, Clyde; Jordan, Robin H; Stetina, Kenneth - Ken

Submitted to: American Journal of Plant Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/26/2018
Publication Date: 10/29/2018
Citation: Hoagland, R.E., Boyette, C.D., Jordan, R.H., Stetina, K.C. 2018. Interaction of glufosinate and Colletotrichum truncatum on ammonia levels and glutamine synthetase activity in Hemp sesbania. American Journal of Plant Sciences. 9:2320-2337.

Interpretive Summary: Microbes and microbial products as bioherbicides have been studied for several decades and combinations of bioherbicides and herbicides have been examined to discover synergistic interactions to improve weed control. Scientists from the Crop Production Systems Research Unit and the Biological Control of Pests Research Unit, USDA, ARS, Stoneville, MS, cooperated to conduct bioassays to assess possible interactions of the herbicide glufosinate (Glu) with Colletotrichum truncatum (CT), a bioherbicide for hemp sesbania control. Glu inhibits glutamine synthetase (GS), elevating ammonia to toxic levels, but the mode of action of CT is unknown. The effects of Glu and CT, (applied alone and combined) on growth, GS activity and ammonia levels in hypocotyl tissues were monitored (88-h time course). Growth (elongation and fresh weight) and GS activity were inhibited in tissues in Glu and Glu plus CT treatments at 16 h, but CT did not cause substantial growth reduction or GS inhibition until after ~40 h. Generally, ammonia levels in hemp sesbania tissues under these treatments were inversely correlated with GS activity. Control tissues contained much lower ammonia levels than treatments with CT, Glu or the combination of Glu and CT. Glu exhibited toxicity toward CT by inhibiting growth and sporulation. Although no major synergistic interactions were observed in combinations of CT and Glu, further insight on the biochemical action of CT and its interaction with Glu was achieved.

Technical Abstract: The use of microbes and microbial products as bioherbicides has been studied for several decades. Researchers have also examined combinations of bioherbicides and herbicides in attempts to discover synergistic interactions applicable to weed control. Bioassays were conducted to assess possible interactions of the herbicide glufosinate [2-amino-4-(hydroxymethylphosphinyl)butanoic acid] and Colletotrichum truncatum [fungal plant pathogen and bioherbicide for hemp sesbania (Sesbania exaltata) control]. Glufosinate acts as a glutamine synthetase (GS) inhibitor leading to elevated ammonia levels, but the mode of action of C. truncatum is unknown. GS has also been implicated in plant defense in certain plant-pathogen interactions. The effects of spray applications of glufosinate (1.0 mM) and bioherbicide (8.0 x 104 conidia ml-1), (applied alone and in combination) on seedling growth, GS activity and ammonia levels in hypocotyl tissues were monitored over an 88-h time course. Growth (elongation and fresh weight accumulation) and extractable GS activity were inhibited in tissues in glufosinate and glufosinate plus C. truncatum treatments as early as 16 h, but C. truncatum treatment did not cause substantial growth reduction or GS inhibition until after ~40 h. Generally, ammonia levels in hemp sesbania tissues under these various treatments were inversely correlated with GS activity. Electrophoretic separation and localization of the hemp sesbania proteins after the treatments indicated a lack of activity 30 h in the glufosinate and lufosinate plus C. truncatum treated tissue. The untreated control tissues contained much lower ammonia levels at 24, 64, and 88 h after treatment than did treatments with C. truncatum, glufosinate or the combination of the herbicide and bioherbicide. C. truncatum alone caused elevated ammonia levels only after 64 and 88 h. Glufosinate (0.25 mM to 2.0 mM) caused a 10% to 45% reduction of colony diameter, compared to fungal growth on potato dextrose agar without glufosinate, and the herbicide also inhibited sporulation. Although no major synergistic interactions were found in combinations of C. truncatum and glufosinate, further insight on the biochemical action of this bioherbicide on this weed was attained.