Sucrose/cytokinin interaction in Sinapis alba at floral induction: a shoot‐to‐root‐to‐shoot physiological loop

In plants of Sinapis alba induced to flower by exposure to a single long day (LD), previous work demonstrated that the movement of a shoot-to-root signal early during the photoextension period of the LD was essential for flowering. Interrupting this movement by bark ringing (girdling) of the stem inhibited the floral response of plants to the LD. In the present work we show that (a) the girdling treatment decreases the soluble sugar level in the roots of induced plants, and (b) the inhibitory effect on the floral response caused by girdling can be completely overcome by supplying the roots with sucrose directly at appropriate times. Thus, we demonstrate that sucrose moving in the phloem is the shoot-to-root signal essential for flowering. We have also found that one of the major effects of the extra-sucrose on the roots is to stimulate the root-to-shoot movement of [9R]Z, the predominant cytokinin of the xylem sap in Sinapis. The importance of this upward movement of [9R]Z for flowering is indicated by the observation that (a) the floral response to the LD is inhibited by growing plants in an atmosphere saturated with water (impairing upward movement of xylem sap) during the LD itself, and (b) the inhibitory effect on the floral response caused by girdling, which markedly reduces [9R]Z export from roots, is relieved by direct application of BA, a cytokinin, to the apex. Other possible effects of the shoot-derived sucrose on roots in relation to flowering are also discussed. Our results show that a shoot-to-root-to-shoot physiological loop is essential for flowering in intact Sinapis plants.

[1]  J. Sheen,et al.  Sugars as signaling molecules. , 1999, Current opinion in plant biology.

[2]  M. Stitt,et al.  Nitrate regulation of metabolism and growth. , 1999, Current opinion in plant biology.

[3]  P. Tillard,et al.  Are phloem amino acids involved in the shoot to root control of NO-3 uptake in Ricinus communis plants? , 1998 .

[4]  T. Chiou,et al.  Sucrose is a signal molecule in assimilate partitioning. , 1998, Proceedings of the National Academy of Sciences of the United States of America.

[5]  T. Schmülling,et al.  Conditional transgenic expression of the ipt gene indicates a function for cytokinins in paracrine signaling in whole tobacco plants. , 1997, The Plant journal : for cell and molecular biology.

[6]  N. Chua,et al.  Sucrose control of phytochrome A signaling in Arabidopsis. , 1997, The Plant cell.

[7]  C. Beveridge,et al.  The shoot controls zeatin riboside export from pea roots. Evidence from the branching mutant rms4 , 1997 .

[8]  T. G. Prasad,et al.  Hormone Signals from Roots to Shoots of Sunflower (Helianthus annuus L.). Moderate Soil Drying Increases Delivery of Abscisic Acid and Depresses Delivery of Cytokinins in Xylem Sap , 1996 .

[9]  K. Koch.  CARBOHYDRATE-MODULATED GENE EXPRESSION IN PLANTS. , 1996, Annual review of plant physiology and plant molecular biology.

[10]  N. Crawford,et al.  Nitrate: nutrient and signal for plant growth. , 1995, The Plant cell.

[11]  B. Touraine,et al.  N Demand and the Regulation of Nitrate Uptake , 1994, Plant physiology.

[12]  G. Bernier,et al.  Cytokinins in Phloem and Xylem Saps of Sinapis-alba During Floral Induction , 1994 .

[13]  A. Bano,et al.  Abscisic Acid and Cytokinins as Possible Root-to-Shoot Signals in Xylem Sap of Rice Plants in Drying Soil , 1993 .

[14]  G. Bernier,et al.  Physiological Signals That Induce Flowering. , 1993, The Plant cell.

[15]  G. Bernier,et al.  Cation fluxes in the saps of Sinapis alba during the floral transition , 1993 .

[16]  B. Muller,et al.  Inhibition of NO⊟3 Uptake by Various Phloem-Translocated Amino Acids in Soybean Seedlings , 1992 .

[17]  H. D. Cooper,et al.  Cycling of Amino-Nitrogen and other Nutrients between Shoots and Roots in Cereals—A Possible Mechanism Integrating Shoot and Root in the Regulation of Nutrient Uptake , 1989 .

[18]  G. Bernier,et al.  Cytokinin Fluxes during Floral Induction in the Long Day Plant Sinapis alba L. , 1988, Plant physiology.

[19]  G. Bernier,et al.  Partial floral evocation by exogenous cytokinin in the long‐day plant Sinapis alba , 1986 .

[20]  M. Bodson,et al.  Elevation in the Sucrose Content of the Shoot Apical Meristem of Sinapis alba at Floral Evocation. , 1985, Plant physiology.

[21]  G. Bernier,et al.  Partial floral evocation by high irradiance in the long‐day plant Sinapis alba , 1983 .

[22]  K. Bradford,et al.  Xylem Transport of 1-Aminocyclopropane-1-carboxylic Acid, an Ethylene Precursor, in Waterlogged Tomato Plants. , 1980, Plant physiology.

[23]  M. Jackson.  Hormones from roots as signals for the shoots of stressed plants , 1997 .

[24]  G. Bernier,et al.  Sucrose levels in leaf exudate as a function of floral induction in the long day plant Sinapsis alba , 1991 .

[25]  William J. Davies,et al.  Root Signals and the Regulation of Growth and Development of Plants in Drying Soil , 1991 .

[26]  G. Bernier,et al.  Cytokinins in flower initiation. , 1990 .