Function of class one nonsymbiotic plant hemoglobins
Plants lack a circulatory system for oxygen. Thus hypoxia and anoxia are potential problems for plants. Environmental factors like flooding can make such problems even worse. At hypoxia and anoxia conditions, both nitrogen metabolism enzymes and nonsymbiotic hemoglobins in plants are up-regulated. Meanwhile, nitrite and nitric oxide are accumulated. Two physiological roles of plant nonsymbiotic hemoglobins were proposed: 1) nitric oxide scavenging during plant hypoxia using NOD function involving ascorbate and monodehydroascorbate reductase; 2) dissimilatory nitrite reduction during plant hypoxia or anoxia cooperating with other factors (since plant hemoglobin itself cannot reduce nitrite to ammonium in vitro). This thesis includes three research chapters that approached both hypotheses. Chapter 2 tested hypothesis 1) and found that even though ascorbate can slowly reduce class 1 nonsymbiotic plant hemoglobin and accomplish the reduction half of the NOD reaction, monodehydroascorbate reductase does not seem to help with this process. Chapter 3 studied two E.coli nitrite reductase mutants and showed that the E.coli nirB mutant is a proper biology screen to find plant hemoglobin cooperators in hypothesis 2). Chapter 4 tested plant s-nitrosoglutathione reductase as a potential plant hemoglobin cooperator in hypothesis 2) and found that it is not the case.