Alteration of the Interconversion of Pyruvate and Malate in the Plastid or Cytosol of Ripening Tomato Fruit Invokes Diverse Consequences on Sugar But Similar Effects on Cellular Organic Acid, Metabolism, and Transitory Starch Accumulation    

Abstract

The aim of this work was to investigate the effect of decreased cytosolic phosphoenolpyruvate carboxykinase (PEPCK) and plastidic NADP-dependent malic enzyme (ME) on tomato (Solanum lycopersicum) ripening. Transgenic tomato plants with strongly reduced levels of PEPCK and plastidic NADP-ME were generated by RNA interference gene silencing under the control of a ripening-specific E8 promoter. While these genetic modifications had relatively little effect on the total fruit yield and size, they had strong effects on fruit metabolism. Both transformants were characterized by lower levels of starch at breaker stage. Analysis of the activation state of ADP-glucose pyrophosphorylase correlated with the decrease of starch in both transformants, which suggests that it is due to an altered cellular redox status. Moreover, metabolic profiling and feeding experiments involving positionally labeled glucoses of fruits lacking in plastidic NADP-ME and cytosolic PEPCK activities revealed differential changes in overall respiration rates and tricarboxylic acid (TCA) cycle flux. Inactivation of cytosolic PEPCK affected the respiration rate, which suggests that an excess of oxaloacetate is converted to aspartate and reintroduced in the TCA cycle via 2-oxoglutarate/glutamate. On the other hand, the plastidic NADP-ME antisense lines were characterized by no changes in respiration rates and TCA cycle flux, which together with increases of pyruvate kinase and phosphoenolpyruvate carboxylase activities indicate that pyruvate is supplied through these enzymes to the TCA cycle. These results are discussed in the context of current models of the importance of malate during tomato fruit ripening.

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).…”

Overexpression of the phosphoenolpyruvate carboxykinase gene (<i>SlPEPCK</i>) promotes soluble sugar accumulation in fruit and post-germination growth of tomato (<i>Solanum lycopersicum</i> L.)

“…However, the fruit soluble sugar and organic acid contents were altered in the 35Spro::SlPEPCK OX and E8pro::SlPEPCK OX lines ( Figures 5 and 6). In our previous study on RNAi transgenic tomato, SlPEPCK suppression resulted in a reduced sugar and an increased malate level in red-ripe fruits (Huang et al 2015;Osorio et al 2013). In contrast to the results from the RNAi plants, the transgenic lines overexpressing SlPEPCK tended to exhibit increased sugar and reduced malate contents during fruit ripening (Figures 5 and 6).…”

“…However, the physiological role of PEPCK in plant and fruit development remained to be elucidated in tomato. Recent studies on RNAi transgenic tomato plants revealed that PEPCK suppression leads to reduced sugar contents accompanied by an increase in malate in ripening fruit (Huang et al 2015;Osorio et al 2013). These results indicate the involvement of PEPCK and gluconeogenesis in sugar accumulation and dissimilation of organic acid metabolism during fruit ripening.…”

“…In tomato, a high expression level of SlPEPCK was reported in ripening tomato fruit (Bahrami et al 2001;Saito et al 2008;Yin et al 2010). Recent studies on tomato lines in which endogenous SlPEPCK was suppressed by RNA interference (RNAi) proved the involvement of PEPCK in soluble sugar accumulation and dissimilation of organic acids during fruit ripening (Huang et al 2015;Osorio et al 2013). Furthermore, systemic suppression of SlPEPCK by the constitutive 35S promoter resulted in suppression of post-germination growth of transgenic tomato seedlings (Huang et al 2015), suggesting that PEPCK and gluconeogenesis also participate in carbohydrate supply during tomato germination through a similar mechanism to that which occurs in fat-storing seed plants.…”

Network Analyses Reveal Shifts in Transcript Profiles and Metabolites That Accompany the Expression of SUN and an Elongated Tomato Fruit

“…Tomato (Solanum lycopersicum) is an important vegetable that is extensively studied for aspects related to plant and fruit development, fruit ripening, and quality (de Jong et al, 2011;Matas et al, 2011;Goulet et al, 2012;Ruan et al, 2012;Mazzucato et al, 2013;Osorio et al, 2013;Pan et al, 2013;Zhong et al, 2013;van der Knaap et al, 2014). As a member of the Solanaceae family, tomato is also an excellent genetic system because of its diploid and highly inbred nature as well as the availability of genetic and genomic resources, such as a reference genome sequence (Tomato Genome Consortium, 2012).…”