Figure 4.41 Nodules on the root system of a legume. A) Nodules on a root system of a broad bean. B) An infected nodule of Medicago truncatula. Rhizobia express the green fluorescent protein (GFP) and appear in green inside the nodule. New rhizobia infect the tip of the nodule through infection threads (arrow). The blue colour originates from flavonoid autofluorescence in the nodule cortex. Red fluorescence in the main root stems from chlorophyll autofluorescence, as roots were exposed to some light. Magnification bars = 1 cm in A, and 1 mm in B. (Photographs courtesy U. Mathesius)