Microtubule orientation in pea stem cells: a change in orientation follows the initiation of growth rate decline.

The orientation of microtubules in cells of redlight-grown pea plants (Pisum sativum L.) was examined by means of immunofluorescence. Microtubules (MTs) in rapidly elongating, subepidermal cells commonly form multiple, parallel strands that run transverse to the cell's axis of elongation. By contrast, MTs in nonelongating subepidermal cells form steeply pitched helical arrays; MTs in non-elongating epidermal cells are oriented parallel to the axis of elongation. This change in orientation occurs during the time interval in which growth rate is declining. The transition is abrupt rather than gradual and occurs in both epidermal and subepidermal cells at the same time. Plants irradiated for 2 h with a growth-inhibiting fluence of blue light did not undergo the same transition, indicating that factors other than changing elongation rates must be responsible for triggering the reorganization of MT arrays.