Sugar Signaling Under Abiotic Stress in Plants
When plants encounter adverse (or potentially adverse) environments, primary metabolic processes, such as growth and/or photosynthesis, are, in general, rapidly affected, and the effect depends on the type and magnitude of stress (Arbona et al., 2017). For example, water deficit and low temperature directly inhibit growth, whereas low irradiance directly reduces the photosynthetic rate. The imbalance between carbon fixation and consumption leads to altered sugar levels in cells. It is thus not surprising that plants have evolved mechanisms that enable them to get information from hazardous environments through the concentration of certain sugars. Not every sugar has been attributed a signaling role in plant stress, but there is compelling evidence that at least glucose (Glc), fructose (Fru), sucrose (Suc), and trehalose-6-P (T6P) may fulfill this role (Van den Ende and El-Esawe, 2014; Li and Sheen, 2016; Sami et al., 2016; Ceusters et al., 2017). Sugar levels are perceived through sugar sensors that initiate a signaling cascade ultimately resulting in altered gene expression and protein modification. Upon these changes, plants may respond to a specific stressful condition and through this response may improve their chance of success. Many of the sugar-mediated responses do not occur in an independent manner, but rather are orchestrated with other endogenous or environmental stimuli. Two large signaling networks that essentially correspond to metabolically opposite situations have been identified. One of them corresponds to low carbon availability, this is, when C fixation is more affected than consumption or growth: the SnRK family-signaling network (Broeckx et al., 2016). The other corresponds to high C availability, when growth is more affected than photosynthesis: target of rapamycin (TOR) signaling network (Baena-González and Hanson, 2017). Variation in sugar levels is associated not only with stressful environments, but also with normal functioning (i.e., day/ night cycles). Accordingly, a large array of Arabidopsis genes (at least 10%) is sugar responsive (Cramer et al., 2011). Therefore, at first glance sugar signaling might appear as a rather unspecific way of responding to external stimuli. To solve this query, plants have evolved complex interplays with stress-related hormones, such as abscisic acid (ABA) and ethylene, as well as with direct environmental stimuli, such as light or mineral nutrients. Furthermore, not all abiotic stresses may be associated with C imbalance. Mechanical stress resulting in plant wounding may also elicit specific sugar-mediated plant responses to ameliorate plant status and to prevent pathogen infection. In this chapter, we summarize the present status of knowledge about sugar roles in plant responses to abiotic stresses, indicating main research areas and gaps still existing in this knowledge. Furthermore, potential uses of the information gathered toward obtaining crops that may be more productive under unfavorable environmental conditions are briefly discussed.