Phosphoenolpyruvate carboxykinase in developing pea seeds is associated with tissues involved in solute transport and is nitrogen-responsive

The aim of this work was to investigate the occurrence of phosphoenolpyruvate carboxykinase (PEPCK) in developing pea (Pisum sativum) seeds in relation to their nitrogen supply. PEPCK was present throughout development, with the peak of PEPCK protein and activity in the seed coat and cotyledons preceding protein accumulation in the cotyledons. It showed a different developmental pattern from enzymes involved in amino acid metabolism (phosphoenolpyruvate carboxylase, glutamine synthetase and glutamate dehydrogenase). Immunolocalization showed that PEPCK was present in parts of the developing seed that are involved in the transport and metabolism of assimilates. Early in development, it was associated with the inner integument of the ovule, the endospermic cytoplasm and the outer cells of the embryo. In the middle of development, around the peak of activity, PEPCK was abundant at the outer surface of the developing cotyledons, in the embryonic axis and in the vasculature of the seed coat. Later in development, PEPCK was associated with the embryonic leaf primordia and meristem and cortex of the radicle. PEPCK protein was strongly induced in vitro in the seed coat by nitrate, ammonium and asparagine, in the cotyledons by asparagine and in planta by the supply of nitrogen, which led to an increase in asparagine secretion by empty seed coats. It is suggested that PEPCK is involved in the metabolism of nitrogenous solutes in developing pea seeds.     

Degradation of DNA and endonuclease activity associated with senescence in the leaves of pea of normal and aphyllous genotypes

Senescence and wilting of the leaves of pea (Pisum sativum L.) of normal (AfAf) and aphyllous (afaf) genotypes were accompanied by DNA degradation. In young (12th–9th) subapical leaves of AfAf plants, total DNA was high-polymeric; in the 6th leaf, DNA degradation was appreciable; and in the 4th and 3rd leaves, hydrolysis of DNA was pronounced. Similar degradation of DNA was also observed in senescing leaves of aphyllous plants, but there it started later than in the plants of normal type. The extent of DNA degradation was closely related to the elevation of total nuclease activity in pea leaves associated with the age. The leaves of plants of different genotypes distinctly differed in the activity of acid and alkaline nucleases. Senescence of the leaves was accompanied by the induction of Ca²⁺-and Mg²⁺-dependent nucleases with mol wts of 42, 37, 34, 26, and 21 kD. In different stages of leaf senescence, different sets of nucleases were detected.     

New functions of mitochondria associated membranes in cellular signaling

In all eukaryotic cells, the endoplasmic reticulum (ER) and the mitochondria establish a tight interplay, which is structurally and functionally modulated through a proteinaceous tether formed at specific subdomains of the ER membrane, designated mitochondria-associated membranes or MAMs. The tethering function of the MAMs allows the regulation of lipid synthesis and rapid transmission of calcium (Ca^2 +) signals between the ER and mitochondria, which is crucial to shape intracellular Ca^2 + signaling and regulate mitochondrial bioenergetics. Research on the molecular characterization and function of MAMs has boomed in the last few years and the list of signaling and structural proteins dynamically associated with the ER–mitochondria contact sites in physiological and pathological conditions, is rapidly increasing along with the realization of an unprecedented complexity underlying the functional role of MAMs. Besides their established role as a signaling hub for Ca^2 + and lipid transfer between ER and mitochondria, MAMs have been recently shown to regulate mitochondrial shape and motility, energy metabolism and redox status and to be central to the modulation of various key processes like ER stress, autophagy and inflammasome signaling. In this review we will discuss some emerging cell-autonomous and cell non-autonomous roles of the MAMs in mammalian cells and their relevance for important human diseases. This article is part of a Special Issue entitled: Calcium signaling in health and disease. Guest Editors: Geert Bultynck, Jacques Haiech, Claus W. Heizmann, Joachim Krebs, and Marc Moreau.