infected cells of nodules treated with 250 mg/L of nZnO were at the
early stage of infection. Most Rhizobia were just being released from
the infection droplets and were being differentiated into mature bacteroids
(Figs. 6b, 7c). These newly developing bacteroids were small and
being surrounded by active endoplasmic reticulum (ER), mitochondria
(M), and a large number of small vesicles, which delivered a large
amount of membrane material to eventually form the mature symbiotic
membrane (Fig. 7c). The infection threads were abnormally enlarged
with excess matrix wrapped around the Rhizobia (Fig. 7d).
At 14 dpi, the lightmicroscopic image of controlled-infected nodules
showed clearly four distinct zones: a meristem zone, an invasion zone,
an interzone, and a large nitrogen fixation zone. However, a few