Abstract
Hemoglobins (Hbs) corresponding to non-symbiotic (nsHb) and truncated (tHb) Hbs have been identified in rice (Oryza). This review discusses the major findings from the current studies on rice Hbs. At the molecular level, a family of thenshbgenes, consisting ofhb1,hb2,hb3,hb4andhb5, and a single copy of thethbgene exist inOryza sativavar. indica andO.sativavar. japonica, Hb transcripts coexist in rice organs and Hb polypeptides exist in rice embryonic and vegetative organs and in the cytoplasm of differentiating cells. At the structural level, the crystal structure of rice Hb1 has been elucidated, and the structures of the other rice Hbs have been modeled. Kinetic analysis indicated that rice Hb1 and 2, and possibly rice Hb3 and 4, exhibit an extremely high affinity for O2, whereas rice Hb5 and tHb possibly exhibit a low to moderate affinity for O2. Based on the accumulated information on the properties of rice Hbs and data from the analysis of other plant and non-plant Hbs, it is likely that Hbs play a variety of roles in rice organs, including O2-transport, O2-sensing, NO-scavenging and redox-signaling. From an evolutionary perspective, an outline for the evolution of rice Hbs is available. Ricenshbandthbgenes vertically evolved through different lineages, rice nsHbs evolved into clade I and clade II lineages and ricenshbs andthbs evolved under the effect of neutral selection. This review also reveals lacunae in our ability to completely understand rice Hbs. Primary lacunae are the absence of experimental information about the precise functions of rice Hbs, the properties of modeled rice Hbs and thecis-elements andtrans-acting factors that regulate the expression of ricehbgenes, and the partial understanding of the evolution of rice Hbs.
Highlights
Two decades ago Taylor and co-workers reported the cloning and sequencing of a hemoglobin (Hb) cDNA from barley[1]
This review discusses major findings from the study of rice Hbs including a historical perspective, and proposes biochemical and physiological mechanisms for rice Hbs based on information available about rice Hbs and other monocot and land plant Hbs
Immunoanalysis by Western blot and confocal microscopy using a polyclonal anti-rice Hb1 antibody revealed that Hb polypeptides exist in rice seeds and in rice leaves and roots from 2 to 14 weeks after seed germination
Summary
Two decades ago Taylor and co-workers reported the cloning and sequencing of a hemoglobin (Hb) cDNA from barley[1]. Based on gene expression (Table 1), protein localization and structural and kinetic properties of rice Hbs and data from the analysis of other plant and non-plant Hbs it is likely that Hbs play a variety of roles in rice plants growing under normal and stressed conditions. These functions may include O2-transport, O2-sensing, NOscavenging and redox-signaling. The proposed functions for rice Hbs are mostly based on the analysis of other plant and non-plant Hbs. future work should evaluate the Hb activities (e.g. the NO-binding and -detoxifying activities) in either rice organs or rice cell cultures under a variety of growing conditions. Grant information The author(s) declared that no grants were involved in supporting this work
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