Single-cell transcriptome atlases of soybean root and mature nodule reveal new regulatory programs that control the nodulation process

The soybean root system is complex.In addition to being composed of various cell types,the soybean root system includes the primary root,the lateral roots,and the nodule,an organ in which mutualistic symbiosis with N-fixing rhizobia occurs.A mature soybean root nodule is characterized by a central infection zone where atmospheric nitrogen is fixed and assimilated by the symbiont,resulting from the close cooperation between the plant cell and the bacteria.To date,the transcriptome of individual cells isolated from developing soybean nodules has been established,but the transcriptomic signatures of cells from the mature soybean nodule have not yet been characterized.Using single-nucleus RNA-seq and Molecular Cartography technologies,we precisely characterized the transcriptomic signature of soybean root and mature nodule cell types and revealed the co-existence of different sub-populations of B.diazoefficiens-infected cells in the mature soybean nodule,including those actively involved in nitrogen fixation and those engaged in senescence.Mining of the single-cell-resolution nodule transcriptome atlas and the associated gene co-expression network confirmed the role of known nodulation-related genes and identified new genes that control the nodulation process.For instance,we functionally characterized the role of GmFWL3,a plasma membrane microdomain-associated protein that controls rhizobial infection.Our study reveals the unique cellular complexity of the mature soybean nodule and helps redefine the concept of cell types when considering the infection zone of the soybean nodule.

Putting the brakes on chloroplast stress signaling

As sessile organisms,plants must be able to sense their surroundings and adjust.One way plants do this is by using their energy-producing organelles(chloroplasts and mitochondria).During environmental stress,these organelles experience metabolic changes that induce signals for acclimation.While many metabolites have been proposed as signaling factors,reactive oxygen species(ROS)are known to play prominent roles.In the chloroplast,the ROS singlet oxygen(^(1)O_(2))is naturally produced during impaired photosynthesis and can lead to retrograde signaling to the nucleus(to control the expression of hundreds of genes),chloroplast degradation,and cell death.