GROWTH AND MICROTUBULE ORIENTATION OF ZEA-MAYS ROOTS SUBJECTED TO OSMOTIC-STRESS

Previous work has shown that microtubule (MT) reorientation follows th e onset of growth inhibition on the lower side of graviresponding root s, indicating that growth reduction can occur independently of MT reor ientation. To test this observation further, we examined whether the r eduction in growth in response to osmotic stress is correlated with MT reorientation. The distribution and rate of growth in maize roots exp osed to 350 mOsm sorbitol and KCl or 5 mM Mes/Tris buffer were measure d with a digitizer. After various times roots were processed for indir ect immunofluorescence microscopy. Application of sorbitol or KCI had no effect on the organization of MTs in the apical 2 mm of the root bu t resulted in striking and different effects in the basal region of th e root. Sorbitol treatment caused rapid appearance of oval to circular holes in the microtubular array that persisted for at least 9 h. Betw een 30 min and 4 h of submersion in KCl, MTs in cortical cells 4 mm an d farther from the quiescent center began to reorient oblique to the l ongitudinal axis. After 9 h, the alignment of MTs had shifted to paral lel to the root axis but MTs of the epidermal cells remained transvers e. In KCl-treated roots MT reorientation appeared to follow a pattern of development similar to that in controls but without elongation. Our data provide additional evidence that MT reorientation is not the cau se but a consequence of growth inhibition.