Decreasing the Mitochondrial Synthesis of Malate in Potato Tubers Does Not Affect Plastidial Starch Synthesis, Suggesting That the Physiological Regulation of ADPglucose Pyrophosphorylase Is Context Dependent

Modulation of the malate content of tomato (Solanum lycopersicum) fruit by altering the expression of mitochondrially localized enzymes of the tricarboxylic acid cycle resulted in enhanced transitory starch accumulation and subsequent effects on postharvest fruit physiology. In this study, we assessed whether such a manipulation would similarly affect starch biosynthesis in an organ that displays a linear, as opposed to a transient, kinetic of starch accumulation. For this purpose, we used RNA interference to down-regulate the expression of fumarase in potato (Solanum tuberosum) under the control of the tuber-specific B33 promoter. Despite displaying similar reductions in both fumarase activity and malate content as observed in tomato fruit expressing the same construct, the resultant transformants were neither characterized by an increased flux to, or accumulation of, starch, nor by alteration in yield parameters. Since the effect in tomato was mechanistically linked to derepression of the reaction catalyzed by ADP-glucose pyrophosphorylase, we evaluated whether the lack of effect on starch biosynthesis was due to differences in enzymatic properties of the enzyme from potato and tomato or rather due to differential subcellular compartmentation of reductant in the different organs. The results are discussed in the context both of current models of metabolic compartmentation and engineering.Starch is the most important carbohydrate used for food and feed purposes and represents the major resource for our diet (Smith, 2008). The total yield of starch in rice (Oryza sativa), corn (Zea mays), wheat (Triticum aestivum), and potato (Solanum tuberosum) exceeds 10⁹ tons per year (Kossmann and Lloyd, 2000; Slattery et al., 2000). In addition to its use in a nonprocessed form, extracted starch is processed in many different ways, for instance as a high-Fru syrup, as a food additive, or for various technical purposes. As a result of this considerable importance, increasing the starch content of plant tissues has been a major goal for many years, with both classical breeding and biotechnological approaches being taken extensively over the last few decades (Martin and Smith, 1995; Regierer et al., 2002).The pathway by which carbon is converted from Suc to starch in the potato tuber is well established (Kruger, 1997; Fernie et al., 2002; Geigenberger et al., 2004; Geigenberger, 2011). Imported Suc is cleaved in the cytosol by Suc synthase, resulting in the formation of UDP-Glc and Fru; the UDP-Glc is subsequently converted to Glc-1-P by UDP-Glc pyrophosphorylase. The second product of the Suc synthase reaction, Fru, is efficiently phosphorylated to Fru-6-P by fructokinase (Renz et al., 1993; Davies et al., 2005). Fru-6-P is freely converted to Glc-6-P, in which form it normally enters the amyloplast (Kammerer et al., 1998; Tauberger et al., 2000; Zhang et al., 2008), and once in the plastid, it is converted to starch via the concerted action of plastidial phosphoglucomutase, ADP-Glc pyrophosphorylase (AGPase), and the various isoforms of starch synthase (Martin and Smith, 1995; Geigenberger, 2011). Of these reactions, although some of the control of starch synthesis resides in the plastidial phosphoglucomutase reaction (Fernie et al., 2001b), the AGPase reaction harbors the highest proportion of control within the linear pathway (Sweetlove et al., 1999; Geigenberger et al., 1999, 2004). In addition, considerable control resides in both the Glc-6-P phosphate antiporter (Zhang et al., 2008) and the amyloplastidial adenylate transporter (Tjaden et al., 1998; Zhang et al., 2008) as well as in reactions external to the pathways, such as the amyloplastidial adenylate kinase (Regierer et al., 2002), cytosolic UMP synthase (Geigenberger et al., 2005), and mitochondrial NAD-malic enzyme (Jenner et al., 2001).As part of our ongoing study of the constituent enzymes of the tricarboxylic acid (TCA) cycle, we made an initially surprising observation that increasing or decreasing the content of malate via a fruit-specific expression of antisense constructs targeted against the mitochondrial malate dehydrogenase or fumarase, respectively, resulted in opposing changes in the levels of starch (Centeno et al., 2011). We were able to demonstrate that these plants were characterized by an altered cellular redox balance and that this led to changes in the activation state of the AGPase reaction. Given that starch only accumulates transiently in tomato (Solanum lycopersicum; Beckles et al., 2001) as a consequence of this activation, the fruits were characterized by altered sugar content at ripening, a fact that dramatically altered their postharvest characteristics (Centeno et al., 2011). Here, we chose to express the antisense fumarase construct in potato in order to ascertain the effect of the manipulation in an organ that linearly accumulates starch across its development. The results obtained are compared and contrasted with those of the tomato fruit and within the context of current models of subcellular redox regulation.