Response of Meristems to Indoleacetic Acid and Colchicine: Differences between Primordia and Root Meristems

A 24-hr treatment with IAA (3 x 10-4 M) induces severe abnormalities or kills primary and lateral roots of Vicia faba. If roots are treated with 0.025% colchicine for 1-3 hr 1 day before treatment with IAA, the roots are less severely affected; for example, pretreatment with colchicine reduces the frequency of laterals killed by IAA from 86% to 26%. The lateral-root primordia present in the root systems at the time of treatment are apparently less sensitive to IAA than fully formed roots; they grow out at the time when other meristems are collapsing. The primordia that reveal this level of insensitivity to IAA are also relatively insensitive to colchicine. However, though their growth may not be prevented by IAA, it is reduced by exposure to 3 x 10-4 M IAA. The results suggest that (a) following a colchicine treatment the endogenous levels of auxins in fully formed meristems fall, resulting in a lower sensitivity to exogenous auxin, and (b) primordia are less sensitive to exogenous IAA than other roots because their endogenous levels are lower. This appears to represent a step in the development of a primordium into a lateral root, and it may be related to other known differences between primordia and lateral-root meristems-in mitotic index and duration of a mitotic cycle.