Photorespiratory nitrogen cycle

Tabtoxin J is a dipeptide exotoxin produced by Pseudomonas tabaci, the organism responsible for the wildfire disease of tobacco plants [141]. When hydrolyzed by peptidase, in vivo, this exotoxin releases tabtoxinine-p-lactam K, which inhibits Glutamine synthetase of the photorespiratory nitrogen cycle, causing chlorosis and death of tobacco plants [142]. 

Fig. 3. The photorespiratory nitrogen cycle. (Reproduced with permission from Keys et al., 1978.)...

Fig. 1. The photorespiratory nitrogen cycle. OSA, 2-Oxosuccinamic acid. R. D. Blackwell).

The evidence, based on the isolation of GS and glutamate synthase mutants, for the role of the glutamate synthase cycle in the photorespiratory nitrogen cycle as discussed in Section III,B,3, mitochondrial GDH-deficient mutant of Zea mays has been isolated and is being investigated by Rhodes et al. (1989). In the absence of MSO, wild-type plants have a higher shoot/root ratio and show a... 

Amongst the plant enzymes that use ammonia as substrate, glutamine synthetase (GS E.C. 6.3.1.2) has the highest affinity (Km 3-5 pm) for this nitrogen source. Ammonia is released in plant tissues by nitrite redudion and amino acid catabolism but the highest amount, up to 90%, originates in photosynthetic tissues from the photorespiratory C2 cycle [1]. 

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