Expression of asparagine synthetase in response to carbohydrate supply in model callus cultures and shoot tips of asparagus (Asparagus officinalis L.)

Sugar uptake and metabolism were studied in callus cultures and shoot tips of asparagus. Asparagus callus cultures were used to model senescence in sh oot tips. Callus cultures absorbed glucose from a nutrient medium, and accu mulated sucrose, glucose and fructose. This uptake of glucose by the callus cultures down-regulated expression of asparagine synthetase and beta -gala ctosidase transcripts that otherwise accumulated when sugar was withheld. W hen 80 mm-long asparagus shoots were excised from growing plants and placed in 2% and 8% sucrose solutions, endogenous concentrations of sucrose, gluc ose, fructose, UDPglucose, and glucose-6-phosphate declined in the 30mm-lon g meristematic tip regions. At the same time, asparagine and asparagine syn thetase gene transcripts began to accumulate in these tips. When 10 mm-long asparagus shoot tips were placed on glucose- or fructose-containing agar, the tips accumulated sucrose, glucose and fructose, and asparagine accumula tion and expression of asparagine synthetase were marginally reduced. We co ncluded that in callus cultures, asparagine synthetase expression was sugar regulated, but that sugar regulation was not as pronounced in asparagus sh oot tips. This may be due in part to slower rates oi sugar uptake into shoo t tips and in part to compartmentation of sugars in the tips. We suggest th at callus cultures are not a suitable model for metabolic studies in aspara gus shoot tips.