BORON, LIGNIFICATION AND THE ORIGIN OF VASCULAR PLANTS‐A UNIFIED HYPOTHESIS

Abstract

It is proposed that the development of an essential role for boron was a prerequisite for the evolution of vascular from prevascular plants since there is a prima jade case that a primary role of boron concerns the biosynthesis of lignin and, in conjunction with auxin, differentiation of xylem. In particular, a potential role for borate in regulating the hydroxylase and oxidase activities of the phenolases involved in the biosynthesis of caffeic and hydroxyferulic acids is suggested. The origin of this role for boron depended on the selection of sucrose in the Chlorophyta as a mobile and storage carbohydrate since, compared with the acyclic sugar alcohols accumulated in other algal groups, sucrose forms only a very weak complex with borate. It is argued that only when borate was not sequestered by complexing with other carbohydrates did the way become clear for it to acquire an essential role. This acquisition catalysed the evolutionary dichotomy between non‐lignified and lignified photosynthetic land‐plants, the bryophytes and tracheophytes. Boron subsequently became involved in two other requirements for success on land ‐ the exploitation of soil for anchorage, water and minerals, and the emancipation of fertilization from external water‐since this element is required for development of adventitious roots (again in conjunction with plant hormones) and, in angiosperms, for germination of pollen. Copyright © 1980, Wiley Blackwell. All rights reserved