Plant sugar-response pathways. Part of a complex regulatory web.

In addition to playing a central role in metabolism, soluble sugars such as Glc and Suc help regulate many developmental and physiological processes in plants (for review, see Koch, 1996; Smeekens, 1998; Sheen et al., 1999; Yu, 1999). For example, sugar levels have been postulated to play an important role in determining the time at which some plant species flower. Treatments that induce flowering can also lead to increased transport of carbohydrates from leaves to shoot apical meristems (Corbesier et al., 1998). This increased sugar transport takes place prior to the rise in metabolic activity that occurs during the transition to flowering, suggesting that sugar levels do not simply rise in response to greater metabolic demand and that sugars may be acting to signal the transition to flowering (for review, see Bernier et al., 1993). Recent findings that Arabidopsis can be induced to flower under conditions of complete darkness by supplying the aerial portions of the plant with exogenous Suc also suggest a role for sugar levels in regulating floral induction (Roldán et al., 1999). A role for sugars in seed germination is suggested by reports that exogenous sugars allow wild-type seeds to germinate in the presence of abscisic acid (ABA; Garciarrubio et al., 1997; Finkelstein and Lynch, 2000). Other developmental processes affected by soluble sugar levels include tuber formation by potatoes (Müller-Röber et al., 1992) and control of root to shoot ratios in a variety of plant species (for review, see Wilson 1988). Sugars are also thought to help control key metabolic processes such as photosynthesis (Krapp et al., 1993) and starch synthesis and breakdown (for review, see Koch, 1996). Strong evidence for the importance of sugars in controlling plant processes is also provided by reports that sugars help regulate the expression of a significant number of plant genes (for review, see Koch, 1996). Whereas sugars have been implicated in control of many plant processes, the molecular mechanisms by which sugars act remain largely unknown. In contrast, sugar-response pathways have been relatively well characterized in the yeast Saccharomyces cerevisiae (for review, see Johnston, 1999). Information obtained from studies on yeast may be used to make predictions regarding plant sugar-response pathways. For example, as yeast mediate sugar responses via multiple signal transduction pathways, plants may also be expected to utilize several sugarresponse pathways. In addition, plant sugarresponse pathways may employ homologs of some of the components of yeast sugar-response pathways. The available evidence suggests that some of the most important factors thought to act in yeast sugarresponse pathways, such as hexokinase (Hohmann et al., 1999) and SNF1 protein kinase (for review, see Hardie et al., 1998), also play important roles in plant sugar-response pathways. However, although plant and yeast sugar-response pathways are likely to share many features, plants utilize some factors not involved in yeast sugar-response pathways. For example, a calcium-dependent protein kinase acts in sugar-regulated gene expression in plants, but is not known to play a similar role in yeast (Ohto and Nakamura, 1995). Despite the importance of soluble sugars in regulating plant development and physiology, many fundamental questions regarding plant sugar responses have barely begun to be addressed. For example, precisely which plant processes are sugar regulated remains controversial. In addition, which plant processes are affected by sugars acting in metabolism, as opposed to in signaling, has yet to be determined in most cases. In fact the identities of the molecule(s) that trigger sugar-response pathways remain in question. Although Glc or Suc may act directly as signaling molecules in some sugar-response pathways, other pathways may sense the level of a different sugar or sugar metabolite. In addition, some sugarresponse pathways may sense the rate of flux through a particular metabolic pathway, rather than the absolute levels of sugars or sugar metabolites (Krapp et al., 1993). The pathways by which plants respond to sugars as signaling molecules remain to be elucidated. Characterization of these pathways is complicated by the fact that they form part of a complex regulatory web that also includes phytohormone and environmental-response pathways. Several approaches are being employed to address these issues. The remainder of this review focuses on a description of these approaches and some of the results obtained using them. 1 This work was supported by the U.S. Department of Energy, Energy Biosciences Program (grant no. DE–FG03– 00ER15061). * E-mail [email protected]; fax 713–348 –5154.