Amino Acid Utilization in Seeds of Loblolly Pine during Germination and Early Seedling Growth (I. Arginine and Arginase Activity)

Abstract

The mobilization and utilization of the major storage proteins in loblolly pine (Pinus taeda L.) seeds following imbibition were investigated. Most of the seed protein reserves were contained within the megagametophyte. Breakdown of these proteins occurred primarily following radicle emergence and correlated with a substantial increase in the free amino acid pool i n the seedling; the majority of this increase appeared to be the result of export from the megagametophyte. The megagametophyte was able to break down storage proteins and export free amino acids i n the absence of the seedling.Arginine (Arg) was the most abundant amino acid among the principal storage proteins of the megagametophyte and was a major component of the free amino acid pools i n both the seedling and the megagametophyte. The increase in free Arg coincided with a marked increase in arginase activity, mainly localized within the cotyledons and epicotyl of the seedling. Arginase activity was negligible in isolated seedlings. Experiments with phenylphosphorodiamidate, a urease inhibitor, supported the hypothesis that arginase participates in Arg metabolism in the seedling. The results of this study indicate that Arg could play an important role i n the nutrition of loblolly pine during early seedling growth.

Molecular and Functional Analyses Support a Role of Ornithine-δ-Aminotransferase in the Provision of Glutamate for Glutamine Biosynthesis during Pine Germination

“…Urea generated inside the organelle is transported to the cytosol, where it could be hydrolyzed by urease (Sirko and Brodzik, 2000). The hydrolysis of urea by the enzyme urease has been reported in loblolly pine seedlings (King and Gifford, 1997;Todd and Gifford, 2002), supporting that Arg catabolism is an important source of ammonium during early seedling development. Furthermore, the role of two cytosolic GS (GS1a and GS1b), GOGAT, and AS in the metabolic fate of this ammonium has also been reported (García-Gutiérrez et al, 1995;Á vila et al, 2001;Suárez et al, 2002;Cañ as et al, 2006).…”

“…Following germination, after the G developmental stage, when most of the nitrogen storage material in the embryo is exhausted, the megagametophyte seed storage proteins are broken down and amino acids are moved to the cotyledons of the developing seedling (King and Gifford, 1997), where they are used as a source of nitrogen. At developmental stage 1 the megagametophyte only contacts with the cotyledons, whereas the hypocotyl and radicle have emerged from the seed.…”

“…In seeds of many conifers most of the nitrogen is stored in the storage proteins as Arg, the amino acid with the highest nitrogen content (Allona et al, 1994;King and Gifford, 1997). Nitrogen from Arg is mobilized via the reaction catalyzed by arginase (EC 3.5.3.1), a mitochondrial located enzyme that hydrolyzes Arg to urea and Orn (Goldraij and Polacco, 2000).…”

“…Studies with pine (Pinus spp.) germinating seeds indicate that the urea molecule is broken down by urease (EC 3.5.1.5; King and Gifford, 1997;Todd and Gifford, 2002), releasing ammonium to be assimilated into Gln by the catalytic action of Gln synthetase (GS; EC 6.3.1.2; Suárez et al, 2002). However, the metabolic fate of the Orn is still unknown.…”

Isolation and Characterisation of a Reserve Protein from the Seeds of Cereus jamacaru (Cactaceae)

“…Seed storage proteins are often rich in the amide amino acids glutamine and asparagine, but may also be rich in arginine (Shotwell & Larkins, 1989). All three of these amino acids have a high nitrogen-to-carbon ratio, making them particularly suited for nitrogen storage (King & Gifford, 1997). The overall composition of this protein bears strong resemblance to a 2S seed storage protein from Opuntia ficus-indica that we have recently characterized (Uchôa et al, 1998).…”