Chimera of Globin/Nitric Oxide Synthase: Toward Improving Nitric Oxide Homeostasis and Nitrogen Recycling and Availability

Abstract

As a result of anthropogenic activities, there are regions suffering extreme climate changes (hot temperatures, droughts, floods), that generate serious and harmful environmental and socio-economic consequences (Hoegh-Guldberg et al., 2018). Climate change has a strong impact on agriculture, mainly by increasing soil degradation and reducing land productivity (Olsson et al., 2019). Desertification decreases soil macronutrients as organic carbon (OC), phosphorus (P) and nitrogen (N) (Shang et al., 2013; Tang et al., 2015). Considerable reductions of OC, P and N are caused by a decline in soil water, vegetation and wind erosion. It is estimated that when soil water is less than 30%, OC and N decrease approximately 50% (Shang et al., 2013). The intensity and frequency of extreme climate events predicted will increase the competition for nutrients, notably N among plants and soil microorganisms (Bennett and Klironomos, 2019; Pugnaire et al., 2019).N availability is essential for net primary production and determines changes in total vegetation biomass and soil OC (Tharammal et al., 2019). Approximately 150 Tg/yr of N is spilled to the land surface as a result of industrial activities and fossil fuel combustion (Schlesinger, 2009). Huge amounts of N fertilizers are used to increase crop productivity, but only 25%?30% are retained in plant biomass (Nadelhoffer et al., 1999; Schlesinger, 2009). The excess of N is then transported to aquatic environments resulting in eutrophication and reduction of dissolved O2, percolated to the groundwater or loss to the atmosphere, increasing greenhouse gas (GHG) emissions such as nitrogen oxides (NOx) (Schlesinger, 2009; Breitburg et al., 2018). In this context, bio/technological solutions like genetic modification of crops are required to avoid, reduce and reverse GHG emissions and water eutrophication, contributing to climate change mitigation. New biotechnological strategies developed for the agriculture to increase N use efficiency (NUE) in crops would help to fulfill this purpose. In this opinion, we will discuss some studies of nitric oxide (NO) synthases (NOS) and nitrate reductase (NR) enzymes playing a collaborative role with globin (Hb) proteins, leading to NO and/or nitrate (NO3-) homeostasis in different organisms. We propose that chimeric globin-NOS such as the NOS from Synechococcus PCC 7335 (SyNOS) may have evolved in photosynthetic microorganisms contributing to a more efficient N recycling and sustaining growth in N fluctuating conditions. Engineered crops that maximize NUE would result in a better adaptation to changing climatic conditions with less N fertilization, preserving aquatic ecosystems and atmosphere.