M. truncatula root responses to S. meliloti. Time points correspond to those used for harvesting RNA from inoculated roots for production of cDNAs for probing microarrays. A, A root hair with swollen tip (arrow) at 1 hpi. B, A root hair with an asymmetrically growing tip (arrow) at 6 hpi. C, Root hair deformation and branching (arrow) at 12 hpi. D, A curled root hair at 24 hpi. E, Cell divisions in the root inner cortex forming a nodule primordium (arrow) at 48 hpi. F, A curled root hair with a nascent infection thread (arrow, blue) at 48 hpi. G, A nodule primordium (arrow) with infection threads (blue) penetrating the dividing cells at 72 hpi. Bars in A, B, C, D, and F are 25 μm, and in E and G are 100 μm.