The polyphenol oxidase gene family in land plants: Lineage-specific duplication and expansion.

Lan T Tran,C Peter Constabel,John S Taylor

doi:10.1186/1471-2164-13-395

Abstract

BackgroundPlant polyphenol oxidases (PPOs) are enzymes that typically use molecular oxygen to oxidize ortho-diphenols to ortho-quinones. These commonly cause browning reactions following tissue damage, and may be important in plant defense. Some PPOs function as hydroxylases or in cross-linking reactions, but in most plants their physiological roles are not known. To better understand the importance of PPOs in the plant kingdom, we surveyed PPO gene families in 25 sequenced genomes from chlorophytes, bryophytes, lycophytes, and flowering plants. The PPO genes were then analyzed in silico for gene structure, phylogenetic relationships, and targeting signals.ResultsMany previously uncharacterized PPO genes were uncovered. The moss, Physcomitrella patens, contained 13 PPO genes and Selaginella moellendorffii (spike moss) and Glycine max (soybean) each had 11 genes. Populus trichocarpa (poplar) contained a highly diversified gene family with 11 PPO genes, but several flowering plants had only a single PPO gene. By contrast, no PPO-like sequences were identified in several chlorophyte (green algae) genomes or Arabidopsis (A. lyrata and A. thaliana). We found that many PPOs contained one or two introns often near the 3’ terminus. Furthermore, N-terminal amino acid sequence analysis using ChloroP and TargetP 1.1 predicted that several putative PPOs are synthesized via the secretory pathway, a unique finding as most PPOs are predicted to be chloroplast proteins. Phylogenetic reconstruction of these sequences revealed that large PPO gene repertoires in some species are mostly a consequence of independent bursts of gene duplication, while the lineage leading to Arabidopsis must have lost all PPO genes.ConclusionOur survey identified PPOs in gene families of varying sizes in all land plants except in the genus Arabidopsis. While we found variation in intron numbers and positions, overall PPO gene structure is congruent with the phylogenetic relationships based on primary sequence data. The dynamic nature of this gene family differentiates PPO from other oxidative enzymes, and is consistent with a protein important for a diversity of functions relating to environmental adaptation.

Highlights

Plant polyphenol oxidases (PPOs) are enzymes that typically use molecular oxygen to oxidize ortho-diphenols to ortho-quinones
Our results show that the PPO gene family has recently expanded in some species, but is reduced or absent in others
Genomic identification of PPO genes in land plants Our TBLASTX search uncovered over 130 candidate PPO genes in 18 of the 25 genomes analyzed (Table 1; Additional files 1 and 2), representing four distantlyrelated lineages of land plants (bryophytes, lycophytes, monocotyledonous anthophytes and eudicotyledonous anthophytes)

Summary

Introduction

Plant polyphenol oxidases (PPOs) are enzymes that typically use molecular oxygen to oxidize ortho-diphenols to ortho-quinones. These commonly cause browning reactions following tissue damage, and may be important in plant defense. Some PPOs function as hydroxylases or in cross-linking reactions, but in most plants their physiological roles are not known. Polyphenol oxidases (PPOs) are dicopper enzymes that oxidize ortho-diphenols to ortho-diquinones using molecular oxygen. PPO genes have been identified in green plants as well as in animals and fungi, where they are often referred to as tyrosinases and appear to be predicted to be localized in the chloroplast while their phenolic substrates accumulate in the vacuole. There is strong evidence for a defensive role of PPO in some plants, for example in tomato and poplar. The evidence is mixed [1,5]

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