Rhizobium nod factors reactivate the cell cycle during infection and nodule primordium formation, but the cycle is only completed in primordium formation

Rhizobia induce the formation of root nodules on the roots of leguminous plants. In temperate legumes, nodule organogenesis starts with the induction of cell divisions in regions of the root inner cortex opposite protoxylem poles, resulting in the formation of nodule primordia. It has been postulated that the susceptibility of these inner cortical cells to Rhizoblum nodulation (Nod) factors is conferred by an arrest at a specific stage of the cell cycle. Concomitantly with the formation of nodule primordia, cytoplasmic rearrangement occurs in the outer cortex. Radially aligned cytoplasmic strands form bridges, and these have been called preinfection threads. It has been proposed that the cytoplasmic bridges are related to phragmosomes. By studying the in situ expression of the cell cycle genes cyc2, H4, and cdc2 in pea and alfalfa root cortical cells after inoculation with Rhizobium or purified Nod factor, we show that the susceptibility of inner cortical cells to Rhizobium is not conferred by an arrest at the G₂ phase and that the majority of the dividing cells are arrested at the G₀/G₁ phase. Furthermore, the outer cortical cells forming a preinfection thread enter the cell cycle although they do not divide.