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 root

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.