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Chapter 6 14-3-3 Positively regulates alkaline / neutral invertases

Plant photosynthesis produces carbohydrates necessary for growth and development, the main photosynthetic product being sucrose. Sucrose is transported from the photosynthetic areas (source) of the plant to non-photosynthetic areas (sink). Within sink tissue sucrose is converted to hexoses by two pathways in which: 1) sucrose is reversibly converted to fructose and UDP-glucose by sucrose synthases (SUS) or 2) irreversibly to fructose and glucose by invertases (INV). There are three types of invertases namely vacuolar (V-INV), cell wall (CW-INV) and cytosolic invertases (C-INV) or A/N invertases. Members of the cytosolic invertases clade are involved in overall plant development. In this study we have shown that 14-3-3 proteins physically interact with one member of the cytosolic invertases, CINV1. 14-3-3 Interaction occurs within the C-terminal region of the protein at S547, in a phosphorylation dependent manner. In vitro phosphorylation of S547 of recombinant AtCINV1 can be facilitated by CPK3, a calcium dependent kinase. Phosphorylation of S547 already has some stimulatory effect on the activity of recombinant CINV1. The subsequent binding of 14-3-3 to phosphorylated S547 further increases recombinant CINV1 activity. Our in planta data suggests that 14-3-3's are positively regulating invertase activity as demonstrated by the 14-3-3 quadruple mutant klpc, which showed a reduction in the concentration of glucose and fructose and a reduction in total A/N invertase activity by 22% when compared to WT. These results highlight the importance of 14-3-3's in sucrose conversion in Arabidopsis. Introduction Sugars are produced during photosynthesis through conversion of light energy into energy-rich carbon molecules (Rolland et al. 2006). These sugars are transported over a long distance from photosynthetic, source tissue, to non-photosynthetic, sink tissues. The major photosynthetically produced compound is sucrose (Rolland et al. 2006). Within sink tissue, sucrose can be cleaved reversibly by sucrose synthases (SUS) producing fructose and UDP-glucose, or irreversibly by invertases (INV) yielding glucose and fructose (Rolland et al. 2006; Vargas and Salerno 2010). The invertase family consists of two groups depending on their pH optimum: 1) the acid invertases (Ac-Inv) and 2) the cytosolic neutral/alkaline invertases (A/N-Inv or CINV). The vacuolar (V-INV) and extracellular or membrane bound (CW-INV) invertases belong to the Ac-INV group. The V-INV is involved in normal root growth and CW-INV is highly active after pathogen attacks and wounding (Barratt et al. 2009; Xiang et al. 2011). The hydrolysis of sucrose by SUS requires less ATP compared to sucrose conversion by INV. However, it has recently become …