Phytoglobin: a novel nomenclature for plant globins accepted by the globin community at the 2014 XVIII conference on Oxygen-Binding and Sensing Proteins

Hemoglobin (Hb) is a heme-containing protein found in the red blood cells of vertebrates. For many years, the only known Hb-like molecule in plants was leghemoglobin (Lb). The discovery that other Hb-like proteins existed in plants led to the term “nonsymbiotic Hbs (nsHbs)” to differentiate them from the Lbs. While this terminology was adequate in the early stages of research on the protein, the complexity of the research in this area necessitates a change in the definition of these proteins to delineate them from red blood cell Hb. At the 2014 XVIII Conference on Oxygen-Binding and Sensing Proteins, the group devoted to the study of heme-containing proteins, this issue was discussed and a consensus was reached on a proposed name change. We propose Phytoglobin (Phytogb) as a logical, descriptive name to describe a heme-containing (Hb-like) protein found in plants. It will be readily recognized by the research community without a prolonged explanation of the origin of the term. The classification system that has been established can essentially remain unchanged substituting Phytogb in place of nsHb. Here, we present a guide to the new nomenclature, with reference to the existing terminology and a phylogenetic scheme, placing the known Phytogbs in the new nomenclature.

Hemoglobin (Hb) is a heme-containing protein found in the red blood cells of vertebrates 1 . Hemoglobin-like proteins are also found in other tissues of vertebrates where they are given tissue-specific names that help to identify their locations and distinguish them from red blood cell Hb 2,3 . For many years, the only known Hb-like molecule in plants was leghemoglobin (Lb), a protein induced as a result of the symbiotic relationship between legume plants and nitrogen-fixing bacteria 4 . The discovery that other Hb-like proteins existed in plants not capable of symbiotic relationships led to the term "nonsymbiotic Hbs (nsHbs)" to differentiate them from the Lbs 5 . While this terminology was adequate in the early stages of research on the protein, the complexity of the research in this area necessitates a change in the definition of these proteins to delineate them from red blood cell Hb, in keeping with the terminology for other Hb-like proteins, such as myoglobin in muscle, neuroglobin in neuron tissue and cytoglobin in vertebrate cell cytoplasm 2,3 . In 2001 Hunt et al. 6 classified plant Hbs as globin (GLB)0, GLB1, GLB2, GLb3 and GLBS corresponding to undetermined (mostly liverwort and moss) nsHbs, angiosperm nsHbs class/type 1 and nsHbs class/type 2, truncated Hbs and symbiotic Hbs (which included Lbs), respectively. However, an epithet for plant Hbs was absent in this nomenclature and distinctive characteristics for each category were not fully defined resulting in an incomplete classification system.
At the 2014 XVIII Conference on Oxygen-Binding and Sensing Proteins, the group devoted to the study of heme-containing proteins, the above issue was discussed and a consensus was reached on a proposed name change. Phytoglobin (phyto, plant; globin, hemecontaining protein folding structurally similar to the sperm whale myoglobin structure whose heme-Fe is invariably coordinated at the proximal site by His F8), abbreviated as Phytogb, was proposed as a logical, descriptive name to describe a heme-containing (Hb-like) protein found in plants. It will be readily recognized by the research community without a prolonged explanation of the origin of the term, as is the case for 'nonsymbiotic hemoglobin'.
The classification system that has been established can essentially remain unchanged substituting Phytogb in place of nsHb. A guide to the new nomenclature, with reference to the existing terminology, is given in Table 1. A more detailed phylogenetic scheme, placing the known Phytogbs in the new nomenclature, is shown in Figure 1.  Figure 1) and are localized in non-legume an legume nodules and apparently play a role in symbiotic N 2 -fixation.  Table 1 for explanation). Also, we propose that acronym for the species-specific Phytogbs corresponds to the first three binomial (i.e. genus and species) letters followed by the Phytogb type and phytogb number of copy. For example, the acronym for rice (Oryza sativa) Phytogb1.1 (see Table 1) corresponds to OrysatPhytogb1.1.
Author contributions RDH conceived the proposal. RDH, MSH and RAP discussed and proposed the novel nomenclature, prepared the first draft of the manuscript, revised the draft manuscript and have agreed to the final content.

Competing interests
No competing interests were disclosed.

Grant information
The author(s) declared that no grants were involved in supporting this work.
"Myogloblin in a cyanobacterium" (Potts 1992) , but plant Hbs were poor relatives in this family. The et al necessity of new terminology for plant hemoglobins appeared and such thoughts apparented and were discussed. The proposition of the new terminology "phytoglobin" (Phb) was already once stated. It was made in Bach Institute of Biochemistry (Moscow, Russia) in the book by Kretovich and this term was even used in the published article (Topunov 1994) . This idea was not further developed at that time and in the next paper (Topunov 1995) the old word "leghemoglobin" was employed again but using of the "phytoglobin" terminology in the article was referred.
In conclusion: it is the right desire to revive this terminology for plant hemoglobins and it has to be promoted and continued. There is the one difference between these two propositions of the "phytoglobin" term. In 1990s it was used as the general terminology for all plant hemoglobins (from both leguminous and non-leguminous plants) , so "leghemoglobin" term could be excluded. Now it is proposed for plant hemoglobins aside from Lb, and it looks more pragmatic because scientists are accustomed to the "leghemoglobin" term and there is no need to exclude it from the scientific practice.