Lateral root frequency decreases when nitrate accumulates in tobacco transformants with low nitrate reductase activity: consequences for the regulation of biomass partitioning between shoots and root1 |
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Authors: | Stitt Mark Feil Regina |
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Institution: | (1) Botanisches Institut, University of Heidelberg, Germany FAX No |
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Abstract: | Accumulation of nitrate in the shoot of low-nitrate reductase tobacco transformants leads to an increase of the shoot:root
ratio to higher values than in nitrogen-sufficient wild-type plants, even though the transformants are severely deficient
in organic nitrogen. In the present paper, wild-type plants and low- nitrate reductase transformants were grown on vertical
agar plates to investigate whether this inhibition of root growth by internal nitrate (i) can be reversed by adding sugars
to the roots and (ii) is due to slower growth of the main roots or to a decreased number of lateral roots. When grown with
a low nitrate supply, the transformants resembled wild-type plants with respect to amino acid and protein levels, shoot-root
allocation, lateral root frequency, and rates of growth. When the transformants were grown with a high nitrate supply in the
absence of sucrose they grew more slowly and had lower levels of amino acids and protein than wild-type plants, but accumulated
more nitrate and developed a high shoot:root ratio. Root length was not affected, but the number of lateral roots per plant
decreased. The slower root growth was accompanied by an increase of the concentration of sugars in the roots. Addition of
2% sucrose to the medium partially reversed the high shoot:root ratio in the transformants, but did not increase the frequency
of lateral roots. It is concluded that nitrate accumulation in the plant leads to decreased root growth via (i) changes in
carbon allocation leading to decreased allocation of sugars to root growth, and (ii) a decrease in the number of lateral roots
and a shift in the sensitivity with which root growth responds to the sugar supply.
This revised version was published online in June 2006 with corrections to the Cover Date. |
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Keywords: | nitrate signalling shoot-root allocation |
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