Effects of Tabtoxinine-β-Lactam on Nitrogen Metabolism in Avena sativa L. Roots

Abstract

The effects of tabtoxinine-,8-lactam (T-,i-L) on nitrate uptake and glutamine synthetase (GS) and nitrate reductase (NR) activities in roots of Avena sativa seedlings were determined. Seven-day-old oat seedlings placed in a 10 mm KNO3 and 0.5 mm T-,B-L solution for 24 hours took up T-f,-L and lost approximately 90% of their root GS activity. 1H1T-,B-L taken up by roots of seven-day-old oat seedlings was associated with GS immunoprecipitated from the extract of these roots. Total nitrate uptake and in vivo NR activity were decreased approximately 50% in the T-,B-L treated roots. However, T-,6-L uptake did not affect the induction phases of nitrate uptake or reduction, nor did it inhibit in vitro NR activity. Thus, the decrease in nitrate uptake and reduction is a secondary effect of T-Ot-L action. Roots of seven-day-old oat seedlings were inoculated with Pseudomonas syringae pv tabaci (Tox+) and the pathogen population in the rhizosphere was estimated by dilution plate count; 6 x 10" bacteria were recovered after 3 days, as compared to the original inoculation with 7 x 10' bacteria, indicating a significant growth of the pathogen in the rhizosphere. The bacteria recovered from the rhizosphere caused chlorosis in tobacco leaves and produced T-,j-L in culture; I x 10i4 bacteria were recovered from roots of seedlings inoculated with P. syringae pv tabaci (Tox-) using the same inoculation and assay procedure as for the pv tabaci(Tox+). Extracts of surface-sterilized roots previously inoculated with P. syringae pv tabaci (Tox+) did not produce viable bacterial cultures when plated out on a complete medium. Oat seedlings growing in sand culture and inoculated with P. syringae pv tabaci (Tox+) had developed chlorosis, and root GS activity had declined to less than 10% of controls after 3 days. Conversely, seedlings inoculated with P. syringae pv tabaci (Tox-) never developed chlorosis and maintained normal levels of GS activity. All oat plants inoculated with P. syringae pv tabaci (Tox+) died within 7 days after inoculation as compared to the plants inoculated with P. syringae pv tabaci (Tox-) which grew to maturity.The phytopathogenic bacterium Pseudomonas syringae pv tabaci and several closely related pathovars produce tabtoxin which when hydrolyzed to tabtoxinine-#-lactam (18) (2-amino-4-[3-hydroxy-2-oxo-azacyclobutan-3-yl]-butanoic acid) (T-,B-L)2 is an irreversible inhibitor of GS (EC 6.3.2.1.) in vivo (7, 16). T-,B-L is not a host-selective toxin and inactivates GS purified from a variety of sources (4, 7).

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

“…Although Pseudomonas syringae pv. tabaci has a strict host specificity for tobacco, TRL has been shown to inhibit GS from a variety of plant species (Turner, 1981;Franz et al , 1982;Knight et al, 1986;Bush et al , 1987). Rhizosphere infestation of oats Avena sativa L. lodi by P. syringae pv.…”

“…Rhizosphere infestation of oats Avena sativa L. lodi by P. syringae pv. tabaci was found to be lethal to the plant (Knight et al, 1986). Plant death occurs when the bacteria, within the rhizosphere, produces T13L which is subsequently taken up by the roots resulting in inactivation of both root and leaf GS isoforms.…”

“…Plant death occurs when the bacteria, within the rhizosphere, produces T13L which is subsequently taken up by the roots resulting in inactivation of both root and leaf GS isoforms. inactivation of all plant GS leads to rapid toxic accumulations of ammonium, with symptoms being expressed as generalized chlorosis of leaf tissue followed by cell necrosis and eventually plant death (Turner and Debbage, 1982;Knight et al, 1986).…”

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

“…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).…”

“…syringae pv. tabaci can live in the rhizosphere of oat seedlings but is eventually lethal to normal oat plants (11). Inhibition of growth and seedling death were the ultimate consequences of the action of the pathogen-produced T,BL taken up by the oat roots.…”