Potential plant traits and mechanisms behind methane emissions in rice and target genes for further dissection of their roles

Rice fields are among the leading global agricultural sources of anthropogenic methane emissions, with the rice plant tissue playing a central role in the generation of methane (CH4) and the emission process. Due to the variability of methane emissions trends across studies, contrasting rice plant traits and mechanisms have often been reported to affect methane emissions. Here, we review the reported traits and mechanisms together with genes that have been identified as being associated with those traits, as a resource for future research using genome editing to pinpoint the factors affecting methane emissions in rice. Methane production in flooded soils involves several interacting agents, with the rice plant supplying root exudates to methanogenic archaea and facilitating methane transport from soil to the atmosphere through aerenchyma. Conversely, the same aerenchyma system transports atmospheric oxygen to the roots; this oxygenation of the rhizosphere supports methanotrophs which consume methane, thus mitigating emissions. Varietal differences in traits such as barriers to radial oxygen loss, root exudates, root architecture, and growth duration, have also been reported to influence methane production and emissions. Previous studies were typically conducted on lines with large differences in the genetic backgrounds; comparisons of methane emissions between lines with very similar backgrounds, for example as developed through genome editing, might help better define the effects of these traits. The genes highlighted in this review are linked to the traits and mechanisms potentially affecting methane emissions in rice and could be targeted for genome editing and subsequent comparison of methane emissions.