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 trans formants 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 pl ants and low-nitrate reductase transformants were grown on vertical agar pl ates to investigate whether this inhibition of root growth by internal nitr ate (i) can be reversed by adding sugars to the roots and (ii) is due to sl ower growth of the main roots or to a decreased number of lateral roots. Wh en grown with a low nitrate supply, the transformants resembled wild-type p lants with respect to amino acid and protein levels, shoot-root allocation, lateral root frequency, and rates of growth. When the transformants were g rown with a high nitrate supply in the absence of sucrose they grew more sl owly 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 l ength was not affected, but the number of lateral roots per plant decreased . The slower root growth was accompanied by an increase of the concentratio n of sugars in the roots. Addition of 2% sucrose to the medium partially re versed 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 alloc ation 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.