Phytoglobins

Phytoglobins, also known as plant hemoglobins, are a family of heme proteins found in plants. They include leghemoglobins, which are associated with legume root nodules, and nonsymbiotic phytoglobins (nsHbs), expressed in many tissues throughout the plant.

Phytoglobins share the globin fold and bind a heme iron. Nonsymbiotic phytoglobins are typically around 140–170 amino acids in length, while leghemoglobins have similar size and properties adapted for rapid oxygen delivery. Phytoglobins are categorized into three phylogenetic classes (class I–III) based on sequence relationships, and each class can show distinct tissue distribution and ligand-binding characteristics.

Functions of phytoglobins are diverse. Leghemoglobins function in legume nodules to facilitate oxygen transport to nitrogen-fixing bacteria while keeping free O2 concentrations low enough to protect nitrogenase. Nonsymbiotic phytoglobins often participate in scavenging nitric oxide (NO) and in NO signaling. In the presence of oxygen and electron donors, some nsHbs act as NO dioxygenases, converting NO to nitrate, which helps regulate cellular NO levels and mitigate nitrosative stress. Phytoglobins may also influence reactive oxygen species and energy balance under stress conditions.

Localization and regulation vary among globs. Many nsHbs are cytosolic, but certain family members possess targeting signals that direct them to chloroplasts, mitochondria, or nuclei. Expression responds to developmental cues and environmental stresses such as hypoxia, drought, salinity, heat, and pathogen challenge.

Evolutionarily, phytoglobins constitute a multigene family widespread in vascular plants, arising through gene duplication and diversification. Functional roles can differ among species, reflecting ecological and physiological contexts.

In summary, phytoglobins participate in oxygen handling and nitric oxide metabolism, linking oxygen sensing, energy turnover, and stress responses in plants.