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Sinking carbon into nitrogen metabolism
It is very interesting to read some of the predictions made by plant molecular biologists in the late 1970s. At that time, many of the hopes for plant GM (which still had not actually been developed) were unrealistic. One favourite, which we still sometimes hear from school students, was the transfer of nitrogen-fixing capabilities to non-leguminous plants. One lab famously predicted that N2-fixing wheat would be available by the year 2000. One feature that these predictions lacked was an understanding of the effect of nodulation and N2-fixation on the host plant’s physiology. The metabolic cost of establishing and maintaining nodules is significant. But, does the sink strength of nodulated roots differ between plants entirely dependent on N2-fixation, and what about plants where external nitrate is also available? These question have been part of a very detailed investigation by Voisin et al. (Dijon and Montpellier, pp. 539-546). The authors maintained pea plants in the presence and absence of nitrate, and manipulated photosynthetic rates to give a range of concentrations of available photosynthate. The carbon partitioning in the two sets of plants was very similar. They showed identical trends in terms of photosynthate going to the roots: the actual amount going to roots went up but the proportion of net photosynthate went down as photosynthetic rate increased. Furthermore, in both, the comparative pulling power of the roots declined as the plants started to flower, and had declined to a very low level during seed filling when seeds became the major sink. In some ways, these results are surprising: based on the metabolic cost of N2-fixation, we might expect that roots relying entirely on N2 would act as stronger sinks than nitrate-fed roots. All this is a clear reminder not to make assumptions; Francis Bacon’s 17th-century call to test by experiment is still very relevant.
Professor J. A. BryantUniversity of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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