Haem oxygenase (HO): an overlooked enzyme of plant metabolism and defence

G S Shekhawat,K Verma

doi:10.1093/jxb/erq074

Abstract

Haem oxygenase (HO) degrades free haem released from haem proteins with the generation of ferrous iron (Fe2+), biliverdin-IXalpha (BV-IXalpha), and carbon monoxide (CO). The mechanism of haem cleavage has been conserved between plants and other organisms even though the function, subcellular localization, and cofactor requirements of HO differ substantially. The crystal structure of HO1, a monomeric protein, has been extensively reported in mammals, pathogenic bacteria, and cyanobacteria, but no such reports are available for higher plant HOs except a predicted model for pea HO1. Along with haem degradation, HO performs various cellular processes including iron acquisition/mobilization, phytochrome chromophore synthesis, cell protection, and stomatal regulation. To date, four HO genes (HO1, HO2, HO3, and HO4) have been reported in plants. HO1 has been well explored in cell metabolism; however, the divergent roles of the other three HOs is less known. The transcriptional up-regulation of HO1 in plants responds to many agents, such as light, UV, iron deprivation, reactive oxygen species (ROS), abscisic acid (ABA), and haematin. Recently the HO1/CO system has gained more attention due to its physiological cytoprotective role in plants. This review focuses on the recent advances made in plant HO research involving its role in environmental stresses. Moreover, the review emphasizes physiological, biochemical, and molecular aspects of this enzyme in plants.

Highlights

Haem oxygenase (HO; EC 1.14.99.3) is a ubiquitous, sensitive, and highly active enzyme, which catalyses the stereospecific cleavage of haem to BV-IXa with the release of iron (Fe2+) and carbon monoxide (CO) (Fig. 1)
This review focuses on the recent advances made in plant HO research involving its role in environmental stresses
In higher plants and other photosynthetic organisms such as algae and cyanobacteria, HOs have a variety of roles: (i) biosynthesis of the bilin chromophore for photosynthetic antennae or photoreceptor synthesis; (ii) release of iron from available cell components under deprivation conditions; and (iii) a key role as an antioxidant in the cell protection machinery of higher plants

Summary

Introduction

Haem oxygenase (HO; EC 1.14.99.3) is a ubiquitous, sensitive, and highly active enzyme, which catalyses the stereospecific cleavage of haem to BV-IXa with the release of iron (Fe2+) and carbon monoxide (CO) (Fig. 1). Genes encoding HOs have been isolated from a wide variety of organisms including mammals, higher plants, red algae, cryptophytas, cyanobacteria, and pathogenic bacteria (Muramoto et al, 1999; Ortiz de Montellano and Wilks, 2001; Terry et al, 2002; Balestrasse et al, 2008).

Results

Conclusion