CARBOHYDRATES AFFECT DORMANCY AND GROWTH IN UNDERGROUND BUDS OF LEAFY SPURGE

Authors: Chao, Wun; Anderson, James; Horvath, David

Submitted to: Leafy Spurge News
Publication Type: Trade Journal
Publication Acceptance Date: 9/20/2005
Publication Date: 10/1/2005
Citation: Chao, W.S., Anderson, J.V., Horvath, D.P. 2005. Carbohydrates affect dormancy and growth in underground buds of leafy spurge. Leafy Spurge News. XXVII(1):9.

Interpretive Summary: The term “low-carb diet” is frequently mentioned by a health conscious society interested in reducing carbohydrate intake. What are carbohydrates? In general, carbohydrates are either simple sugars like glucose, fructose, and sucrose (table sugar) or complexes of sugars like starch. Carbohydrates are manufactured by plants during the process of photosynthesis and are a principal energy source for plants and animals. Since underground buds (crown and root buds) of leafy spurge are the primary means of vegetative reproduction, and reproduction and growth require carbohydrates, we are investigating how carbohydrates such as glucose, sucrose, and starch within underground buds affect dormancy and growth. Our current models suggest that a leaf derived signal, requiring photosynthesis for its production or transport, is involved in keeping crown and root buds of leafy spurge dormant. Our studies indicate that sugar may be the basis for the leaf-derived inhibitory signal. The growth regulating hormone gibberellic acid (GA), long known to promote the growth of buds, is now also known to overcome the leaf-derived inhibition of crown and root bud growth. Specifically, we have found that both glucose and sucrose inhibit root bud growth at concentrations as low as 30 mM, while GA concentrations at 0.015 mM overcome the inhibitory effect of sugar. In addition, crown and root buds of intact leafy spurge plants contain sucrose levels near 30 mM, which should be inhibitory to growth induction. The fact that sucrose levels decrease dramatically after removal of all above ground tissue (decapitation) supports our hypothesis that sugars play an important role associated with dormancy. Starch is the major energy reserve for plants and would be expected to decrease in crown and root buds after the removal of growth inhibiting signals. Indeed, our research indicates that starch levels are abundant in root buds of intact plants but decrease nearly 8-fold over the first 5 days after decapitation. This and other research on glucose, fructose, and sucrose levels in root buds following decapitation indicate the products of starch breakdown are rapidly utilized for metabolic reactions associated with the early growth of buds. Our model indicates that sucrose inhibits the GA response pathway or, alternatively, sucrose affects the levels of active GAs by inhibiting GA synthesis. Interactions between sugars and GA may also affect starch metabolism. Sucrose and glucose are known to repress expression of enzymes (alpha-amylases) involved in starch digestion, and GA is known to promote synthesis and activity of these enzymes. Thus, our research suggests that these simple sugars likely interact with GA to regulate the breakdown of starch in crown and root buds of leafy spurge during growth induction. The knowledge gained from these fundamental studies should enhance our potential to devise new methods for controlling the reproductive capacity of perennial weeds such as leafy spurge. Wun S. Chao, James V. Anderson, David P. Horvath, USDA-ARS, Plant Science Research Unit Biosciences Research Laboratory Fargo, ND 58105-5674 [phone]

Technical Abstract: The term “low-carb diet” is frequently mentioned by a health conscious society interested in reducing carbohydrate intake. What are carbohydrates? In general, carbohydrates are either simple sugars like glucose, fructose, and sucrose (table sugar) or complexes of sugars like starch. Carbohydrates are manufactured by plants during the process of photosynthesis and are a principal energy source for plants and animals. Since underground buds (crown and root buds) of leafy spurge are the primary means of vegetative reproduction, and reproduction and growth require carbohydrates, we are investigating how carbohydrates such as glucose, sucrose, and starch within underground buds affect dormancy and growth. Our current models suggest that a leaf derived signal, requiring photosynthesis for its production or transport, is involved in keeping crown and root buds of leafy spurge dormant. Our studies indicate that sugar may be the basis for the leaf-derived inhibitory signal. The growth regulating hormone gibberellic acid (GA), long known to promote the growth of buds, is now also known to overcome the leaf-derived inhibition of crown and root bud growth. Specifically, we have found that both glucose and sucrose inhibit root bud growth at concentrations as low as 30 mM, while GA concentrations at 0.015 mM overcome the inhibitory effect of sugar. In addition, crown and root buds of intact leafy spurge plants contain sucrose levels near 30 mM, which should be inhibitory to growth induction. The fact that sucrose levels decrease dramatically after removal of all above ground tissue (decapitation) supports our hypothesis that sugars play an important role associated with dormancy. Starch is the major energy reserve for plants and would be expected to decrease in crown and root buds after the removal of growth inhibiting signals. Indeed, our research indicates that starch levels are abundant in root buds of intact plants but decrease nearly 8-fold over the first 5 days after decapitation. This and other research on glucose, fructose, and sucrose levels in root buds following decapitation indicate the products of starch breakdown are rapidly utilized for metabolic reactions associated with the early growth of buds. Our model indicates that sucrose inhibits the GA response pathway or, alternatively, sucrose affects the levels of active GAs by inhibiting GA synthesis. Interactions between sugars and GA may also affect starch metabolism. Sucrose and glucose are known to repress expression of enzymes (alpha-amylases) involved in starch digestion, and GA is known to promote synthesis and activity of these enzymes. Thus, our research suggests that these simple sugars likely interact with GA to regulate the breakdown of starch in crown and root buds of leafy spurge during growth induction. The knowledge gained from these fundamental studies should enhance our potential to devise new methods for controlling the reproductive capacity of perennial weeds such as leafy spurge. Wun S. Chao, James V. Anderson, David P. Horvath, USDA-ARS, Plant Science Research Unit Biosciences Research Laboratory Fargo, ND 58105-5674 [phone]