ALBERT

Notes: Tobacco seedlings were grown in nutrient agar at a range of ammonium nitrate concentrations either without added sucrose, or with 100 mol m–3 sucrose. In the absence of added sucrose, nitrogen-limited plants had increased levels of glucose, fructose and sucrose, decreased chlorophyll, decreased protein, and decreased Rubisco activity, but the level of the transcript for the small subunit of Rubisco (RbcS) did not decrease compared with nitrogen-sufficient plants. When sucrose was added to nitrogen-sufficient seedlings, there was an increase of sucrose, glucose and fructose in the leaves, growth was increased, and the chlorophyll and protein content, Rubisco activity, and the RbcS transcript level did not change. When sucrose was added to nitrogen-limited seedlings, there was a further increase of sucrose, glucose and fructose, growth was not increased, and there was a further decrease of chlorophyll, protein and Rubisco activity, and a marked decrease of the RbcS transcript level. To check that the decrease of the RbcS transcript level was not an indirect effect due to changes of nitrogen metabolites after adding sugars, glucose was added to Chenopodium cells in the presence and absence of glutamine or azaserine. Changes of glutamine that suffice to increase and decrease the level of the transcript for nitrate reductase (Nia) do not affect the RbcS transcript concentration, and glucose addition still led to a decrease of the RbcS transcript level when the internal glutamine concentration was high. Tobacco seedlings were also grown in nutrient agar at a range of phosphate concentrations either without added sucrose, or with 100 mol m–3 sucrose. Phosphate-limited seedlings did not show a decrease of chlorophyll, protein, Rubisco activity, or the level of the RbcS transcript, compared with phosphate-sufficient seedlings. The addition of sucrose to phosphate-limited plants led to a similar increase of sugars to that seen after adding sucrose to nitrogen-limited seedlings, but did not alter chlorophyll, protein, Rubisco activity, or the level of the RbcS transcript. The addition of sucrose to phosphate-limited plants led to a slight increase of the level of the transcript for nitrate reductase (Nia), increased nitrate reductase activity, and a marked increase of the amino acid content. Phosphate limitation led to an increased level of the transcript for the regulatory subunit of ADP glucose pyrophosphorylase (AgpS2), and this response was strengthened when sucrose was added. The regulation of AgpS2 expression by phosphate and sucrose was further investigated by feeding sucrose and phosphate to detached source leaves via the transpiration stream. The level of the AgpS2 transcript decreased after feeding phosphate and increased after feeding sucrose, and the effect of sucrose was antagonised by phosphate. It is concluded that the response to sugar signalling is modulated by nitrogen and phosphate in a gene-specific manner. The significance of these results for understanding the visual phenotype of nitrogen- and phosphate-limited plants, and the response of photosynthesis and starch synthesis to the plant nutrient status is discussed.