Using Confocal Laser Scanning Microscope to Visualize the Infection of Rice Roots by GFP-labelled Klebsiella oxytoca SA2, an Endophytic Diazotroph

The infection of rice ( Oryza sativa L.) roots by a GFP_labelled endophytic nitrogen_fixing bacterium, Klebsiella oxytoca SA2, was visualized in a living state with a confocal laser scanning microscope. Rice seedlings were gnotobiotically grown in tubes with agar medium and roots were optically sectioned after inoculation of 1, 2, 4, 8, 12, 16 and 21 d. Microscopic observation showed significantly the process that the bacteria infected the roots from the root surface to the vascular tissues in the root maturation zone. After colonization on the root surface, the GFP_labelled bacteria penetrated preferentially into the cortex of the developed lateral roots and colonized inter_ and intracellularly in the lateral root. Afterwards, they moved into the vascular cylinder at sites where the endodermis was modified by the outgrowth and the development of the lateral root. In addition, they also could penetrate into the cortex with less frequency and efficiency of the primary roots at sites where no lateral root was emerged. Cell walls of the root cells close to the invading bacteria were thickened and strong yellow_green fluorescence of them was visualized under blue light. The phenomena may represent a localized hypersensitive reaction of root cells to further restrict bacterial invasion.