Regulation of loblolly pine (Pinus taeda L.) arginase in developing seedling tissue during germination and post-germinative growth

After seed germination, hydrolysis of storage proteins provides a nitrogen source for the developing seedling. In conifers the majority of these reser ves are located in the living haploid megagametophyte tissue. In the develo ping loblolly pine (Pinus taeda L.) seedling an influx of free amino acids from the megagametophyte accompanies germination and early seedling growth. The major component of this amino acid pool is arginine, which is transpor ted rapidly and efficiently to the seedling without prior conversion. This arginine accounts for nearly half of the total nitrogen entering the cotyle dons and is likely a defining factor in early seedling nitrogen metabolism. In the seedling, the enzyme arginase is responsible for liberating nitroge n, in the form of ornithine and urea, from free arginine supplied by the me gagametophyte. In this report we investigate how the seedling uses arginase to cope with the large arginine influx. As part of this work we have clone d an arginase cDNA from a loblolly pine expression library. Analysis of enz yme activity data, accumulation of arginase protein and mRNA abundance indi cates that increased arginase activity after seed germination is due to de novo synthesis of the enzyme. Our results suggest that arginase is primaril y regulated at the RNA level during loblolly pine seed germination and post -germinative growth.