Journal of Experimental Botany, Vol. 55, No. 394, pp. iv, January 1, 2004
© 2004 Oxford University Press
Preface |
Preface to Plant Carbon--Nitrogen Interactions from Rhizosphere to Planet
In recent years, the numerous interactions between plant carbon and nitrogen metabolism have been intensively studied at multiple levels of complexity and plant anatomy. Within the plant, primary nitrogen assimilation necessitates extensive co-operation between different cell compartments (chloroplasts, peroxisomes, cytosol, and mitochondria), while changes in carbon and nitrogen status influence organ physiology and root/shoot relationships. However, carbon--nitrogen interactions are not just whole plant phenomena: they also have a significant dynamic interface and impact on the environment and hence on the biosphere. The six articles that comprise this theme were presented at a two-day session at the 2003 SEB meeting titled Plant Carbon-Nitrogen Interactions from Rhizosphere to Planet. This highly complex and wide-ranging topic involves many aspects of plant biology from gene regulation to plant--environment interactions. Together, these articles provide a brief but up-to date insight into key issues related to these processes, describing the very latest developments in our understanding of how plants co-ordinate carbon, nitrate and ammonium assimilation into the organic compounds required for growth on an organ, plant and global scale, together with emerging concepts of metabolic crosstalk, transport and signalling. In their article, Mantelin and Touraine discuss the positive effect of bacteria on plant growth. The rhizosphere and rhizoplane bacteria are crucial determinants of plant nutrient availability and hence growth. It is surprising, therefore, that so much still remains to be resolved concerning the factors governing the bacterial/root interaction.
Nitrate, and its successive reduction to nitrite and ammonia, are not only important in relation to primary N metabolism and associated signalling pathways, but they also have roles in stress signalling through the associated production of reactive oxygen species (ROS) and reactive nitrogen species such as NO. Haem oxygenases fulfil protective roles in mammals and, in their article, Baudouin et al. report the presence of a haem oxygenase in Medicago sativa nodules, suggesting that it might fulfil a similar role in nodules, protecting bacterial N-fixation enzymes from oxidative damage, since ROS are involved in the nodulation process and in nodule senescence. On a similar theme, Taylor et al. describe the effects of oxidative stress on components of the tricarboxylic acid cycle and showing how this might impair the carbon–nitrogen interaction.
Mitochondria fulfil key roles in photosynthetic N metabolism, including the oxidation of glycine and other substrates that produce organic acids for N assimilation. Noctor et al. review evidence obtained from analysis of the Nicotiana sylvestris mutant (CMSII) that lacks functional mitochondrial complex I, showing the importance of redox shuttles between organelles in optimizing photosynthesis and nitrogen assimilation as well as ameliorating antioxidant defences. Complex I functions as an important electron sink in the optimization of net CO2 fixation under photorespiratory conditions and also in co-ordinating the rate of leaf N assimilation with organic acid production. These authors suggest that plants have a system of general amino acid control, a view supported and elaborated in the article by Halford et al. who describe a new regulatory plant protein, GCN2, that may be important in initiating changes in expression of genes involved in general amino acid control. Finally, the article by Raven and Handley presents a global approach to understanding C/N metabolism, starting with the `big bang', and discussing the possibilities and limitations for improvement of the C/N ratio in agricultural systems.
The different lines of research and the processes described in these articles interact in many ways that contribute to nitrogen use efficiency on a global scale that impacts on agriculture and biodiversity. Disentangling these multiple interactions will provide experimental challenges for the next decade at least. These articles demonstrate the intricacies of this rapidly evolving subject where state-of-the-art technologies are being applied to maximum benefit. We hope that these articles provide useful, interesting and thought-provoking insights that will prompt further experimentation and breakthroughs in this exciting and expanding field.
Christine H. Foyer
Caroline Bowsher
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