Are isocitrate lyase and phosphoenolpyruvate carboxykinase involved in gluconeogenesis during senescence of barley leaves and cucumber cotyledons?

The aim of this study was to investigate whether gluconeogenesis catalysed by phosphoenolpyruvate carboxykinase (PEPCK) occurs during leaf senescence, This was addressed by determining changes in the abundance and intercellul ar location of enzymes necessary for gluconeogenesis during the senescence of barley leaves and cucumber cotyledons, PEPCK was never present in barley leaves, despite the presence of large amounts of isocitrate lyase (ICL), a key enzyme of the glyoxylate cycle, and of its product, glyoxylate. Althou gh PEPCK was present in non-senescent cucumber cotyledons, its abundance de clined during senescence. Throughout senescence, PEPCK was only present in the trichomes and vasculature, whereas ICL was located in mesophyll cells. Pyruvate,P-i dikinase (PPDK) which, in concert with NAD(P)-malic enzyme, is also capable of catalysing gluconeogenesis, was present in non-senescent b arley leaves and cucumber cotyledons, but in both plants its abundance decr eased greatly during senescence. The abundance of ICL was greatly reduced i n senescing detached barley leaves by either illumination or by co-Incubati on with sucrose, and greatly increased in darkened attached barley leaves, These results argue against the large-scale occurrence of gluconeogenesis d uring senescence catalysed either by PEPCK or PPDK. In cucumber cotyledons, PEPCK may play a role in metabolic processes linked to the export of amino acids, a role in which phosphoenolpyruvate carboxylase may also be involve d. The amount of ICL was increased by starvation and during senescence may function in the conversion of lipids to organic acids, which are then utili sed in the mobilisation of amino acids from leaf protein.