Growth patterns in non-mycorrhizal and mycorrhizal short roots of Pinus sylvestris

The structure of lateral roots, non-mycorrhizal and mycorrhizal short roots of Pinus sylvestris were studied by 4',6-diamidino-2-phenylindole (DAPI) staining and 5-bromo-2-deoxyuridine (BrdU) labelling of nuclei in thin sections. The microtubule cytoskeleton of the meristematic and elongating cells of the roots was visualized by using a tubulin antibody. The orientation and shape of the BrdU-labelled nuclei and microtubule cytoskeleton was viewed by laser scanning confocal microscopy and the distance between the meristem and the first maturating xylem elements was measured. The methods used revealed that a lower number of cells is produced for the elongation zone in short roots than in lateral roots. This causes the poor elongation of the short roots. In roots with limited capacity to elongate, the slow-growing ectomycorrhizal fungus Suillus bovinus is able to compete with the root extension rate and to grow between cortical cells. In ectomycorrhiza, the elongation pattern of the short root is maintained, but the presence of fungal mycelium around the root apex seems to affect the rate and direction of divisions in the meristem.