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    

Abstract

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.

Pyrophosphate levels strongly influence ascorbate and starch content in tomato fruit

“…Additionally, it has been demonstrated to be transcriptionally regulated by sugars, nitrate, phosphate and trehalose-6-phosphate (Muller-Rober et al, 1990; Nielsen et al, 1998; Kolbe et al, 2005; Michalska et al, 2009). Moreover, it has been described that AGPase is also redox regulated (Tiessen et al, 2002; Centeno et al, 2011; Osorio et al, 2013) with malic acid potentially being a key component in this process at least in photosynthetically active tissues (Szecowka et al, 2012). …”

On the role of the tricarboxylic acid cycle in plant productivity

“…Data from kinetic profiling of wild‐type tomato (Carrari et al ; Wang et al ) and known ripening mutants (Osorio et al ) revealed that, during ripening, changes in organic acid metabolism are highly correlated with transcriptomics, metabolomics and developmental shifts. However, a comparative analysis of the climacteric fruit, tomato, versus the non‐climacteric fruit, pepper, revealed that whereas there were species‐specific patterns of network regulatory behavior, a coordinated regulation of transcripts was also observed by malate and citrate in potato (Szecowka et al ).…”

Metabolic Control of Redox and Redox Control of Metabolism in Plants

“…Intriguingly, most likely as a consequence of the altered sugar content, these lines were characterized as having elevated or reduced shelf life and inversely reduced or elevated susceptibility to post-harvest bacterial infection. More recent studies have revealed that the operation of this reverse malate shunt is context dependent and does not work in fully heterotrophic tissues such as the potato tuber (339), but that similar effects can be brought about by altering the malaterelated redox status balance in other compartments of the tomato fruit (261).…”