Overexpression of sirohydrochlorin ferrochelatase boosts nitrogen and carbon assimilation and overcomes nutrient deficiency in Arabidopsis thaliana

• AtSirB encodes sirohydrochlorin ferrochelatase, the terminal enzyme involved in biosynthesis of siroheme, the cofactor of chloroplastic nitrite reductase and sulfite reductase involved in N and S assimilation. • Overexpression of AtSirB in Arabidopsis thaliana leads to increased NiR and SiR expression nitrite reductase activity, higher protein and chlorophyll contents and enhanced photosynthetic efficiency. • AtSirB overexpression improves both NUE and SUE in Arabidopsis thaliana under N- and S-limiting conditions. • Enhancing AtSirB expression can potentially boost crop productivity, alleviate N and S deficiencies, and reduce the need for N-based fertilizers. Nitrogen (N) deficiency in soil results in the reduction of plant productivity and emission of greenhouse nitrogenous gases from the applied nitrogenous fertilizers and formation of sodic soil. To overcome soil N-deficiency the rate of N-assimilation was augmented by overexpression of AtSirB coding for sirohydrochlorin ferrochelatase, responsible for Fe insertion to the tetrapyrrole moiety of sirohydrochlorin 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 reduce nitrite (NO 2 - ) and sulfite (SO 3 2- ) to ammonium (NH 4 + ) and sulfide (S 2- ) respectively. Increased gene and protein expression of AtSirB resulted in heightened NiR and SiR gene expression and protein abundance, boosting NiR and nitrate reductases (NR) activities that resulted in increased protein and chlorophyll (Chl) content in Arabidopsis thaliana . Conversely, antisense plants showed decreased protein levels due to reduced NR and NiR activities. AtSirB overexpressors due to their higher protein and Chl contents had increased photosynthesis, and biomass. Under N and sulfur (S) limitations, these overexpressors outperformed wild type (WT) and antisense plants. These results demonstrate the crucial role of AtSirB and siroheme in N and carbon (C) assimilation. Furthermore, they indicate the potential of SirB to enhance crop productivity, alleviate N and S deficiency across different cropping regimes.