Final answer:
Neelaredoxin is constitutively expressed in anaerobic prokaryotes, and its synthesis levels remain constant even when exposed to O₂ or H₂O₂, making option 'b' the correct answer.
Step-by-step explanation:
Neelaredoxin is a 15-kDa protein commonly found in anaerobic prokaryotes with superoxide-scavenging activity that helps in protecting the cell against oxidative damage. The expression of neelaredoxin is described as being constitutively expressed, which means its synthesis occurs at all times within the cell and is independent of external stress factors such as exposure to reactive oxygen species like O2 or H2O2. Therefore, the correct answer to which statement best describes neelaredoxin synthesis is: 'b. Neelaredoxin is produced at all times; levels are constant even when exposed to O2 or H2O2'.
In comparison, superoxide dismutases, another group of antioxidant enzymes, do have their expression increased in response to pathogen attack as part of the plant's defense mechanism. This increase in expression leads to the generation of hydrogen peroxide, contributing to enhanced tolerance against pathogens. Proteins like superoxide dismutase are part of the plant's proteome, which is the full complement of proteins produced by a cell under specific gene expression.
Furthermore, studies on the proteomic profiles of plants under different stress conditions, such as exposure to aluminum or pathogens, demonstrate the differential expression of proteins including various enzymes involved in antioxidant defense, metabolism, and stress responses. This differential expression is indicative of the complex regulatory mechanisms at play in response to environmental stressors which inform the cellular response, including the production of antioxidant enzymes like neelaredoxin and superoxide dismutase.