Does Acetobacter diazotrophicus live and move in the xylem of sugarcane stems? Anatomical and physiological data

We have previously shown that the nitrogen-fixing endophyte of sugarcane, Acetobacter diazotrophicus lives in the sugar solution in the intercellular-space apoplast of the stem cortex. Various authors have claimed that it inhabits the xylem apoplast. This possibility has been investigated in the clone Ja 60-5 and shown to be most unlikely for the following reasons: (1) an adequate carbon source is lacking in the xylem sap, and cannot be supplied from the intercellular-space apoplast of the cortex. Diffusion of solutes into and out of the vascular bundles is prevented by complete lignification and suberization of the bundle sheath cell walls except at the nodes. (2) Longitudinal movement of particles as large as bacteria is severely limited at the nodes. Vessel end walls were found in 90% of vessels at each node, and only 1% of open vessels extended through two nodes. None extended as far as three nodes. In addition to vessel end walls, vessel continuity at nodes was interrupted by living cells. Dye solution in the transpiration stream in metaxylem vessels did not pass through these living cells, but accumulated in crystals (sump formation) in the vessels below the node. Only in some protoxylem vessels and cavities did dye solution move through many nodes. It is likely that selection of sugarcane clones such as Ja 60-5 for resistance to bacterial will diseases have selected for clones that have limited vessel continuity. (3) When cultured A. diazotiophicus was introduced into the transpiration stream, the xylem parenchyma reacted by secreting a bright red polymer which killed the bacteria and blocked the movement of water. We conclude that the xylem flow-apoplast of this clone of sugarcane is an unsuitable habitat for A. diazotrophicus and that additional habitats to those of the intercellular-space apoplast should be sought elsewhere. (C) 1997 Annals of Botany Company.