Developmental anatomy and auxin response of lateral root formation in Ceratopteris richardii

Abstract

The homosporous fern Ceratopteris richardii exhibits a homorhizic root system where roots originate from the shoot system. These shoot-borne roots form lateral roots (LRs) that arise from the endodermis adjacent to the xylem poles, which is in contrast to flowering plants where LR formation arises from cell division in the pericycle. A detailed study of the fifth shoot-borne root showed that one lateral root mother cell (LRMC) develops in each two out of three successive merophytes. As a result, LRs emerge alternately in two ranks from opposite positions on a parent root. From LRMC initiation to LR emergence, three developmental stages were identified based on anatomical criteria. The addition of auxins (either indole-3-acetic acid or indole-3-butyric acid) to the growth media did not induce additional LR formation, but exogenous applications of both auxins inhibited parent root growth rate. Application of the polar auxin-transport inhibitor N-(1-naphthyl)phthalamic acid (NPA) also inhibited parent root growth without changing the LR initiation pattern. The results suggest that LR formation does not depend on root growth rate per se. The result that exogenous auxins do not promote LR formation in C. richardii is similar to reports for certain species of flowering plants, in which there is an acropetal LR population and the formation of the LRs is insensitive to the application of auxins. It also may indicate that different mechanisms control LR development in non-seed vascular plants compared with angiosperms, taking into consideration the long and independent evolutionary history of the two groups.

Lateral Root Initiation or the Birth of a New Meristem

“…In Marsilea quadrifolia (Lin and Raghavan, 1991) and the fern Ceratopteris richardii (Hou et al, 2004), LRs arise from endodermis cells located opposite the protoxylem poles within the meristematic region of the parent root.…”

Auxin transport in the evolution of branching forms

“…The data suggest that bulk basipetal transport in mosses is PIN ‐mediated, and that the innovation of sporophytic branching forms may have involved changes in PIN function (Harrison, ). Lycophyte (Wochok & Sussex, , ; Sanders & Langdale, ) and monilophyte (Walters & Osborne, ) sporophytes have bulk‐basipetal transport that is PATI ‐sensitive (Hou et al ., ; Sanders & Langdale, ). In a lycophyte, PATI application disrupts bifurcation (Sanders & Langdale, ), but as yet there are no functional data on the nature of auxin transporter involved.…”

PIN proteins and the evolution of plant development

“…These data indicate that auxin transport has conserved (or at least deeply homologous) developmental roles across the tracheophyte clade. Inhibition of auxin transport in the fern Ceratopteris reinhardtii reduced primary root growth, but not lateral root initiation [32], and also led to formation of extra meristems in the free-living gametophytic stage [33]. Control of fern root architecture is not homologous to that in spermatophytes [5] and spermatophytes do not have free-living gametophytes; therefore, little can be currently concluded about the conservation of auxin transport in monilophyte and spermatophyte development.…”