Infection of sugar cane by the nitrogen-fixing bacterium Acetobacter diazotrophicus

Abstract

Significant nitrogen fixation has recently been demonstrated in Brazilian sugar cane (Saccharum officinarum) cultivars known to form associations with a number of diazotrophs, including Acetobacter diazotrophicus, an acid-tolerant endophytic bacterium which grows best on a sucrose-rich medium. In a series of experiments, aseptically-grown sugar cane plantlets were rooted in a liquid medium and inoculated with A. diazotrophicus originally isolated from field-grown sugar cane. After 4, 7, 9, and 15 d, plants were examined under light, scanning and transmission 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, particularly at cavities in lateral root junctions. The loose cells of the 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 obvious pathogenic reaction to the bacteria within the xylem. From these observations, it is proposed that, under experimental conditions, A. diazotrophicus firstly colonized the root and lower stem epidermal surfaces and then used root tips and lateral root junctions to enter the sugar cane plant where it was distributed around the plant in the transpiration stream. It is further suggested that the xylem vessels in the dense shoots of mature plants are also a possible site of N2-fixation by diazotrophs as they provide the low pO2 and energy as sucrose necessary for nitrogenase activity.