Changes in root morphology of wheat caused by Azospirillum inoculation

Wheat seeds (Triticum aestivum L.) were inoculated with Azospirillum brasilense Cd, Sp7, the local isolate Cd-1, and with other types of bacteria. Inoculation with 105 to 106 colony-forming units of Azospirillum caused the largest root elongation and total root surface of seedlings whereas 108 to 109 colony-forming units of Azospirillum caused inhibition of root development. Similar effects were obtained in 10 different cultivars of wheat inoculated with Azospirillum. Scanning electron micrographs of inoculated wheat root segments showed denser and longer root hairs as compared with the control inoculated with dead cells. This effect was less apparent in more mature roots. In inoculated roots bacteria were located mainly on the cell elongation area and on the bases of root hairs, but fewer bacterial cells were present on the root cap or adsorbed to root hairs. Cross sections of Azospirillum-inoculated roots showed prominent alterations of the cell arrangement in the layers of the cortex. The results suggest the existence of critical numbers of sites for Azospirillum colonization on the roots, to such an extent that root growth is affected.