Nitric Oxide Synthesis by Plants and its Potential Impact on Nitrogen and Respiratory Metabolism

NOS-like-mediated nitric oxide is involved in Pinus thunbergii response to the invasion of Bursaphelenchus xylophilus

The content of NO and H(2)O(2) as well as the activities of nitric oxide synthase (NOS)-like and nitrate reductase (NR) were monitored in the needles of Pinus thunbergii infected by Bursaphelenchus xylophilus. The results showed that the content of NO increased significantly only 8 h after the invasion of B. xylophilus, while H(2)O(2) increased 12 h after invasion. NO donor SNP could promote and NO scavenger cPTIO could prevent the production of NO and H(2)O(2). The content of NO changed earlier than that of H(2)O(2). In addition, the symptoms appeared 9, 5 and 12 days, respectively, after the inoculation with B. xylophilus, SNP pre-treatment and cPTIO pre-treatment followed by B. xylophilus infection. After B. xylophilus infection, the content of NO in P. thunbergii changed fiercely more earlier than the appearance of external symptoms, which indicated that the content of NO was related with the appearance and the development of the symptoms. The treatment with L-NNA (NOS inhibitor) inhibited the content of NO significantly, whereas, Na(2)WO(4) (NR inhibitor) had no effect. The further analysis of NOS revealed that NO changed in consistent with cNOS activity. To sum up, NO, as the upstream signal molecule of H(2)O(2), was involved in the pine early response to the invasion of B. xylophilus and influenced the accumulation of the content of H(2)O(2). Moreover, NOS-like rather than NR was responsible for the endogenous NO generation, which was modulated by cNOS during the interaction between P. thunbergii and B. xylophilus. Key message NO is involved in early response of P. thunbergii to the invasion of B. xylophilus and NOS is the key enzyme responsible for NO generation in P. thunbergii.

In higher plants, only root mitochondria, but not leaf mitochondria reduce nitrite to NO, in vitro and in situ

At oxygen concentrations of < or =1%, even completely nitrate reductase (NR)-free root tissues reduced added nitrite to NO, indicating that, in roots, NR was not the only source for nitrite-dependent NO formation. By contrast, NR-free leaf slices were not able to reduce nitrite to NO. Root NO formation was blocked by inhibitors of mitochondrial electron transport (Myxothiazol and SHAM), whereas NO formation by NR-containing leaf slices was insensitive to the inhibitors. Consistent with that, mitochondria purified from roots, but not those from leaves, reduced nitrite to NO at the expense of NADH. The inhibitor studies suggest that, in root mitochondria, both terminal oxidases participate in NO formation, and they also suggest that even in NR-containing roots, a large part of the reduction of nitrite to NO was catalysed by mitochondria, and less by NR. The differential capacity of root and leaf mitochondria to reduce nitrite to NO appears to be common among higher plants, since it has been observed with Arabidopsis, barley, pea, and tobacco. A specific role for nitrite to NO reduction in roots under anoxia is discussed.

Nitrogen uptake in relation to excess supply and its effects on the lichens Evernia prunastri (L.) Ach and Xanthoria parietina (L.) Th. Fr

The aim of this study was to compare the physiological responses to increased nitrogen (N) supply between the nitrophytic lichen Xanthoria parietina (L.) Th. Fr. and the acidophytic lichen Evernia prunastri (L.) Ach. The two lichens were exposed to a weekly dosage of 0.05, 0.1, 0.2, 0.6 or 2.4 g N m(-2) for 2 months, administered as NH(4)NO(3) dissolved in artificial rainwater (1 l m(-2)). After the treatments, in vivo chlorophyll a fluorescence was determined to assess vitality; concentrations of total N, ammonium, nitrate and dominant amino acids, including glutamate, glutamine and arginine, were quantified in order to follow changes in N status; and the polyols ribitol, arabitol and mannitol were quantified to follow changes in the lichens' carbon (C) status. The uptake of N was quantified by labelling the fertiliser with (15)N in the ammonium position; chlorophyll a was used as an indirect marker for algal activity, and ergosterol as an indirect marker of fungal activity. Nitrogen uptake was higher in E. prunastri than in X. parietina, although the latter species may have used the mannitol reserves to obtain C skeletons and energy for N assimilation. Chlorophyll a and ergosterol concentrations remained unaltered in X. parietina irrespective of N dosage while ergosterol decreased with increasing N uptake in E. prunastri. The latter species had accumulated a large pool of ammonium at the highest N dosage, whilst in X. parietina a significant nitrate pool was instead observed. Taken together, these short-term responses to high N supply observed in the two lichens, and the differences between them, can partly explain the higher tolerance of X. parietina towards increased atmospheric N levels.