Oats Tolerant of Pseudomonas syringae pv. tabaci Contain Tabtoxinine-β-Lactam-Insensitive Leaf Glutamine Synthetases

Abstract

(19,29) and for reassimilating the ammonia released by photorespiration (9). In this latter function, GS can be viewed as an ammonia detoxification mechanism (7). It is the loss of this detoxification function of GS in P. syringae pv. tabaci-infected leaves that results in the accumulation of millimolar concentrations ofammonia and the subsequent and characteristic chlorosis (6,8,27 We report that plants tolerant to P. syringae pv. tabaci do not utilize any of the first four mechanisms listed above, but rather that they contain leaf glutamine synthetases that are effectively insensitive to T,3L. We also report on the effects ofT,3L on initial ammonia assimilation in both tolerant and sensitive oat plants.T,3L3 [2-amino-4-(3 hydroxy-2-oxo-azacyclobutan-3-yl) butanoic acid] (25) is a novel amino acid produced by the soilborne tobacco pathogen Pseudomonas syringae pv. tabaci. Infection of tobacco leaves by P. syringae pv. tabaci and the subsequent production of T,BL result in the in vivo inactivation of the physiological target, glutamine synthetase (EC 6.3.2.1) (11, 26). TflL is not selective, inactivating GS obtained from a number of sources (2,10,11,14,15,26). Inactivation of GS by T,BL is irreversible, requires ATP, and is active site-directed, with one molecule of T,3L being bound per subunit of completely inactivated GS (15). GS acts in concert with glutamate synthase to catalyze the primary pathway for ammonia assimilation in higher plants (16,19,29 These cells (500 mL/flat, or 10 mL/500 g of sand) were applied to the surface of sand in which the plants were growing. Four days after inoculation, tolerant oat seedlings were identified by visual inspection for the absence of chlorosis. These selected (tolerant) seedlings were then subjected to this selection process twice more. Oat plants that survived these three pathogen challenges were allowed to set seed through self-fertilization. These plants were isolated from normal oat plants to prevent any cross-333 www.plantphysiol.org on April 1, 2019 -Published by Downloaded from

Tissue-specific changes of glutamine synthetase activity in oats after rhizosphere infestation by Pseudomonas syringae pv. tabaci. Final report

“…Although, GS polypeptide levels increase in root and leaf tissues of both SC and TC oats, only the leaf GS in TC plants remains catalyticaly active. Ion-exchange chromatography of leaf GS from TC plants has revealed that both GSI and GS2 isoforms retained activity (data not shown) and earlier in vitro inactivation studies have shown that both forms of leaf GS in ' TC plants are resistant to inactivation by TKL (Knight et al, 1988). A 43% increase in specific activity of leaf GS in TC oats over TUC oats is not in accord with the dramatic increase in levels of GS polypeptide indicating that not all the GS protein in TC oat leaves is fully active.…”

“…In vitro inactivation studies with TBL reveals that GS from 'TC leaf tissue, partially purified through ion-exchange chromatography, will lose 15 to 20% of its activity. After this initial loss of activity this GS becomes resistant to further inactivation by TBL (Knight et al, 1988). A 15 to 20 % decrease in activity could represent a limited 'TBL-GS interaction where TBL could inactivate 1 or 2 active sites.…”

“…tabaci and TRL have been obtained, so we now possess two variant oat populations: one being sensitive to rhizosphere infestation and TRL, the other being tolerant of pathogen and TRL (Knight et al, 1988). Plants of the tolerant population when challenged by P. syn'ngae pv.…”

“…Altered ammonium assimilatory capabilities in TC oats are a result of root GS remaining sensitive to inactivation by TRL , while leaf GSs are resistant to TRL inactivation. Consequently, tolerant oats challenged by the pathogen have greatly reduced root GS activities, but leaf GS specific activities increase (Knight et al, 1988).…”

Herbicide-Resistant Field Crops

“…There is considerable natural variation between species in sensitivity to glufosinate, and this variation does not appear to be based on differential sensitivity to GS (Ridley and McNally, 1985). Lines of oat that are insensitive to the GS-inhibiting glufosinate analog, tabtoxin, have a tabtoxin-resistant GS isozyme (Knight et al, 1988); however, no plants with glufosinate-resistant GS have been reported.…”