Exploring the Potential of Methanotrophs for Plant Growth Promotion in Rice Agriculture

Abstract

Rice fields are one of the important anthropogenic sources of methane emissions. Methanotrophs dwelling near the rice roots and at the oxic–anoxic interface of paddy fields can oxidize a large fraction of the generated methane and are therefore considered to be important. Nitrogen fixation in rice root-associated methanotrophs is well known. Our aim in this study was to explore the potential of methanotrophs as bio-inoculants for rice and the studies were performed in pot experiments in monsoon. Ten indigenously isolated methanotrophs were used belonging to eight diverse genera of Type Ia, Type Ib, and Type II methanotrophs, including the newly described genera and/or species, Methylocucumis oryzae and Methylolobus aquaticus, as well as Ca. Methylobacter oryzae and Ca. Methylobacter coli. Additionally, two consortia (Methylomonas strains and Methylocystis-Methylosinus strains) were used. Nitrogen fixation pathways or nifH genes were detected in all of the used methanotrophs. Plant growth promotion (PGPR) was seen in terms of increased plant height and grain yield. Nine out of twelve (seven single strains and two consortia) showed positive effects on grain yield (6–38%). The highest increase in grain yield was seen after inoculation with Ca. Methylobacter coli (38%) followed by Methylomonas consortium (35%) and Methylocucumis oryzae (31%). Methylomagnum ishizawai inoculated plants showed the highest plant height. Methylocucumis oryzae inoculated plants showed early flowering, grain formation, and grain maturation (~17–18 days earlier). In all the pot experiments, minimal quantities of nitrogen fertilizer were used with no additional organic fertilizer inputs. The present study demonstrated the possibility of developing methanotrophs as bio-inoculants for rice agriculture, which would promote plant growth under low inputs of nitrogenous fertilizers. Although the effect of methanotrophs on methane mitigation is still under investigation, their application to reduce methane emissions from rice fields could be an added advantage.