A NITROGEN-FIXING ENDOPHYTE OF SUGARCANE STEMS - A NEW ROLE FOR THE APOPLAST

The intercellular spaces of sugarcane (Saccharum officinarum L.) stem parenchyma are filled with solution (determined by cryoscanning micros copy), which can be removed aseptically by centrifugation. It containe d 12% sucrose (Sue; pH 5.5.) and yielded pure cultures of an acid-prod ucing bacterium (approximately 10(4) bacteria/mL extracted fluid) on N -poor medium containing 10% Suc (pH 5.5). This bacterium was identical with the type culture of Acetobacter diazotrophicus, a recently disco vered N-2-fixing bacterium specific to sugarcane, with respect to nine biochemical and morphological characteristics, including acetylene re duction in air. Similar bacteria were observed in situ in the intercel lular spaces. This demonstrates the presence of an N-2-fixing endophyt e living in apoplastic fluid of plant tissue and also that the fluid a pproximates the composition of the endophytes's optimal culture medium . The apoplastic fluid occupied 3% of the stem volume; this approximat es 3 tons of fluid/ha of the crop. This endogenous culture broth consi sting of substrate and N-2-fixing bacteria may be enough volume to acc ount for earlier reports that some cultivars of sugarcane are independ ent of N fertilizers. It is suggested that genetic manipulation of apo plastic fluid composition may facilitate the establishment of similar symbioses with endophytic bacteria in other crop plants.