Table 1. System and characteristics of the accepted nomenclature for plant (algae + land plants) Phytoglobins (Phytogb).

Former plant globin name and abbreviation (in parenthesis)	New nomenclature a	Plant origin	Distinctive characteristics b
Nonsymbiotic hemoglobin (nsHb)	Phytogb0	Algae c+bryophytes+ gymnosperms	Heme-Fe either penta- or hexacoordinate. Moderate to high affinity for O 2. Localized in any plant organ.
Class/type 1 nonsymbiotic hemoglobin (nsHb-1)	Phytogb1	Angiosperms	Heme-Fe predominantly hexacoordinated by a distal amino acid. Extremely high affinity for O 2 mostly due to a very low O 2-dissociation rate constant ( k off). Localized in any plant organ.
Class/type 2 nonsymbiotic hemoglobin (nsHb-2)	Phytogb2	Angiosperms	Heme-Fe predominantly pentacoordinated. Moderate to high affinity for O 2. Localized in any plant organ.
Symbiotic hemoglobin (symHb)	SymPhytogb	Non-legume N 2-fixing plants d	Heme-Fe predominantly pentacoordinated. Moderate to high affinity for O 2. Specifically localized in N 2-fixing nodules of actinorhizal plants or any other non-legume land plant
Leghemoglobin (Lb)	Lb	N 2-fixing legumes d	Heme-Fe predominantly pentacoordinate. Moderate to high affinity for O 2. Specifically localized in legume N 2-fixing nodules.
Class/type 3 nonsymbiotic hemoglobin/ Truncated hemoglobin (tHb)	Phytogb3	Algae c+land plants	Globin-domain amino acid sequence and structure (i.e. folding into the 2/2-fold) similar to those of bacterial tHbs. Heme-Fe either penta- or hexacoordinate. Moderate to high affinity for O 2. Localized in any plant organ.

^aNumerical classification corresponds to that previously proposed by Hunt et al. ⁶. Proteins coded by multiple phytogb gene copy numbers within the same plant species should be indicated as the number of copy after the Phytogb numerical classification. For example, rice ( Oryza sativa) Phytogbs 1 and 2 (corresponding to the former nsHbs-1) should be indicated as rice Phytogb1.1 and Phytogb1.2, respectively (see text for a description on the species-specific Phytogbs acronym).

^bHeme-Fe coordination and affinity for O ₂ correspond to those from moss Phytogb0 ^{7– 9}, barley ¹⁰, rice ¹¹ and Arabidopsis ¹² Phytogb1, Arabidopsis Phytogb2 ¹², Casuarina SymPhytogb ¹³, soybean Lb ^{14, 15} and Arabidopsis Phytogb3 ¹⁶ representative of Phytogb0, Phytogb1, Phytogb2, SymPhytogb, Lb and Phytogb3, respectively.

^cAmino acid sequence of algal globins analyzed so far ^{17– 19} is similar to that of land plant Phytogb0 and Phytogb3, hence algal globins can be classified as Phytogb0 or Phytogb3, respectively.

^dSome SymPhytogbs and Lbs (such as the Parasponia ²⁰ and Casuarina ²¹ and Chamaecrista ²² globins, respectively) are intermediate between Phytogbs1 and Phytogbs2 and SymPhytogbs and Lbs ^{22, 23} because they exhibit amino acid sequence similarity to Phytogbs1 and Phytogbs2 ( Figure 1) and are localized in non-legume an legume nodules and apparently play a role in symbiotic N ₂-fixation.