INFECTION OF SUGAR-CANE BY THE NITROGEN-FIXING BACTERIUM ACETOBACTER-DIAZOTROPHICUS

Significant nitrogen fixation has recently been demonstrated in Brazil ian sugar cane (Saccharum officinarum) cultivars known to form associa tions with a number of diazotrophs, including Acetobacter diazotrophic us, an acid-tolerant endophytic bacterium which grows best on a sucros e-rich medium. In a series of experiments, aseptically-grown sugar can e plantlets were rooted in a liquid medium and inoculated with A. diaz otrophicus originally isolated from field-grown sugar cane. After 4, 7 , 9, and 15 d, plants were examined under light, scanning and transmis sion electron microscopes and the presence of A. diazotrophicus on and within plant tissues was confirmed by immunogold labelling. By 15 d, external bacterial colonization was seen on roots and lower stems, par ticularly at cavities in lateral root junctions. The loose cells of th e root cap at root tips were a site of entry of the bacteria into root tissues. Both at lateral root junctions and root tips, bacteria were also seen in enlarged, apparently intact, epidermal cells. After 15 d, bacteria were present in xylem vessels at the base of the stem, many connected via mucus to spiral secondary thickening. There was no obvio us pathogenic reaction to the bacteria within the xylem. From these ob servations, it is proposed that, under experimental conditions, A. dia zotrophicus firstly colonized the root and lower stem epidermal surfac es and then used root tips and lateral root junctions to enter the sug ar cane plant where it was distributed around the plant in the transpi ration stream. It is further suggested that the xylem vessels in the d ense shoots of mature plants are also a possible site of N-2-fixation by diazotrophs as they provide the low pO(2) and energy as sucrose nec essary for nitrogenase activity.