Developmental and Structural Section
Christianson, Michael L. .
The signal for determinate growth in Psilotum..
Rats, cats, dogs and hogs mature at characteristic adult sizes. Plants, however, (perennials, at least) add to their primary bodies for as long as they live. While the ultimate basis of the indeterminate growth of plants is the iterative production of determinate units (morphological phytomers, cellular merophytes), there is no direct homology or equivalence between those determinate units and the indeterminate unit (shoot, root). Psilotum, however, grows as a dichotomizing, indeterminate plagiotropic shoot, where some branches switch to growth as dichotomizing, but determinate (and orthotropic) shoots; this morphological equivalence permits experiments on the nature of determinate and indeterminate growth. – Experiments with erect shoots, ablating one of the sister branches as each dichotomy forms, have shown that erect shoots CAN make more segments than they usually do, and suggest that their determinate growth involves signaling within erect shoots. New experiments, comparing the growth of paired sister-branches from the first dichotomies of erect shoots, show that a dichotomizing branch will limit growth of a sister-ablated branch. Indeed, experiments blunt-ablating the central 2 of 4 tips on a young erect shoot, and comparing the growth of the two remaining branches, one sister-ablated, one not, show that 1/4 of an erect shoot system stops growth at the usual time (6-7 rounds of dichotomy), and, as the sister-ablated branch also stops with 6-7 rounds of dichotomy, that the physiologic signal moves in basipetal and acropetal directions and travels significant distances. Other experiments, delaying the time at which ablation of sister-branches begins, show that 8-tipped shoots are not yet committed to determinate growth. – I acknowledge the hospitality of the Lewis Feldman lab and the examples of work with angiosperms by Erin Irish and Carl McDaniel.
1 - University of California, Plant and Microbial Biology, Berkeley, California, 94720
Presentation Type: Paper
Location: Alpine A (Snowbird Center)
Date: Wednesday, August 4th, 2004
Time: 3:45 PM