Drought stress in Pinus taeda L. induces coordinated transcript accumulation of genes involved in the homogentisate pathway
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Phenylalanine is a central amino acid in plants and is the precursor of many key secondary metabolites such as lignin, phenylpropanoids, and flavonoids. Phenylalanine hydroxylase (PheH, EC 126.96.36.199) is not detected by sequence similarity in angiosperms, but it is present in gymnosperms and mosses where it is known to hydroxylate phenylalanine to produce tyrosine in vitro. However, its physiological role in gymnosperms is unclear. This study aimed to clarify the role of the gene encoding PheH in Pinus taeda (PtPheH), an economically and ecologically important tree native to the southern USA. The PtPheH gene is present as a single copy and consists of five exons and four introns. Comparison of two PtPheH alleles in P. taeda revealed a mobile element specific to the genus Pinus that was upstream of the transcriptional start site. PtPheH transcript abundance increased after introduction of exogenous phenylalanine and during water limitation. Similar highly and statistically significant shifts in transcript abundance were found for the genes involved in tyrosine synthesis and in the homogentisate pathway, but not for genes of phenylpropanoid metabolism. The coordinated accumulation of transcripts implies that under conditions of limited water availability, PtPheH may reroute phenylalanine away from lignin synthesis and into the degradative homogentisate pathway. Because PtPheH has homologs in pine, moss, and animals, it is presumably an ancient gene that has been lost in the angiosperm lineage of plants.
KeywordsPinus taeda Water availability Drought response Homogentisate pathway Phenylalanine Phenylalanine hydroxylase PtPheH Tyrosine
We thank Dr. C.L. Ribeiro and K.E. Smith for help in the pressure bomb reading and the members of the Forest Biology Research Cooperative for plant material. This project was supported by the National Research Initiative Competitive Grant no. 2008-35318-04589 from the USDA National Institute of Food and Agriculture and by an endowment from the C.V. Griffin Sr. Foundation.
OF, JMD, and ADH conceived experiments; OF and CD performed experiments; JLW did bioinformatics search in the loblolly pine genome; OF and JMD wrote the paper.
Data archiving statement
This manuscript contains no new data for public use.
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