Overexpression of Sirohydrochlorin Ferrochelatase Boosts Nitrogen Sulfur and Carbon Utilization in Arabidopsis thaliana

Nitrogen deficiency in the soil is a significant agronomic problem, and the application of nitrogenous fertilizers to the soil has environmental concerns, such as sodic soil and the emission of greenhouse gases. To increase the nutrient use efficiency, the cDNA of AtSirB coding for sirohydrochlorin ferrochelatase, responsible for Fe insertion to the tetrapyrrole moiety of sirohydrochlorin, was overexpressed in Arabidopsis thaliana under the control of 35S promoter for increased synthesis of siroheme. The siroheme is a cofactor for the plastidic enzymes nitrite reductase (NiR) and sulfite reductase (SiR), which reduces nitrite and sulfite to ammonium and sulfide, respectively. A three-step process including methylation, oxidation, and ferro-chelation produces siroheme from uroporphyrinogen III, an intermediate of chlorophyll (Chl) biosynthesis. The NiR and SiR gene expression and protein abundance increased in the over-expressers due to the increased AtSirB protein level. It resulted in an increase in N and S assimilation and enhanced protein content of over-expressers. Conversely, the total protein content decreased in antisense plants due to reduced NR and NiR activities. AtSirB over-expressers had higher protein and Chl contents and increased photosynthetic rate and biomass. Under N and S limitation, the protein, Chl, and photosynthetic electron transport rates in AtSirB over-expressers were higher than in WT. Results demonstrate that the SirB that hijacks uroporphyrinogen from the chlorophyll biosynthesis pathway is a crucial player in N and S assimilation. The siroheme is limiting for efficient nitrate and sulfate reduction and utilization. SirB could be genetically manipulated to increase crop productivity for sustainable agriculture.