Rapid stalk elongation in tulip (Tulipa gesneriana L. cv. Apeldoorn) and the combined action of cold-induced invertase and the water-channel protein gamma TIP

Many bulbous plants need a low-temperature treatment for flowering. Cold, f or example, affects the elongation of the stalk, thereby influencing the qu ality of the cut flower. How the elongation of the stalk is promoted by col d and which physiological and biochemical mechanisms are involved have rema ined obscure. As in invertase has been shown to be involved in the cold-ind uced elongation of the flower stalks of tulips (Lambrechts et al., 1994, Pl ant Physiol 104: 515-520), we further characterized this enzyme by cloning the cDNA and analysing its expression in various tissues of the tulip (Tuli pa gesneriana L. cv. Apeldoorn) stalk. In addition, the role of sucrose syn thase was investigated. Since turgor pressure is an important force driving cell elongation, the role of a water-channel protein (gamma TIP) was studi ed in relation to these two enzymes. The mRNA level of the invertase found was substantially upregulated as a result of cold treatment. Analysis of th e amino acid sequence of this invertase revealed the presence of a vacuolar targeting signal. Two different forms of sucrose synthase were found, the expression of one of them appeared to be restricted to the vascular tissue while the other form was present in the surrounding tissue. Both sucrose sy nthases were present in the stalk during the entire period of bulb storage and after planting, but their activities declined during stalk elongation. The expression of the gamma TIP gene was restricted mainly to the vascular tissue and its expression profile was identical to that of invertase. Simul taneous expression of invertase and gamma TIP possibly leads to an increase in osmotic potential and vacuolar water uptake, thus providing a driving f orce for stretching the stalk cells.