Journal Article

Lobe-generating centres in the simple leaves of <i>Myriophyllum aquaticum</i>: evidence for <i>KN1</i>-like activity

Laura Bourque and Christian Lacroix

in Annals of Botany

Published on behalf of The Annals of Botany Company

Volume 107, issue 4, pages 639-651
Published in print April 2011 | ISSN: 0305-7364
Published online February 2011 | e-ISSN: 1095-8290 | DOI: http://dx.doi.org/10.1093/aob/mcr014
Lobe-generating centres in the simple leaves of Myriophyllum aquaticum: evidence for KN1-like activity

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  • Ecology and Conservation
  • Evolutionary Biology
  • Plant Sciences and Forestry

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Background and Aims

The mature morphology of most plants can usually be said to consist of three mutually exclusive organs: leaves, stems, and roots. The vast majority of mature morphologies may be easily grouped into one of these mutually exclusive categories. However, during very early stages of development and in many instances from inception, the division between organ categories becomes fuzzy due to the overlap in developmental processes that are shared between the aforementioned mutually exclusive categories. One such overlap has been described at the gene level where KNOXI homologues, transcription factors responsible for maintaining indeterminate cell fate, are expressed in the shoot apical meristem and during early stages of compound leaf development. This study characterizes the occurrence and spatial localization of mRNA of a KNOXI homologue, MaKN1, during the early stages of development in the simple leaves of Myriophyllum aquaticum, an aquatic angiosperm from the family Haloragaceae exhibiting pentamerous whorls of finely lobed leaves.

Methods

A 300-bp KNOXI fragment was sequenced from M. aquaticum and used in an RNA localization study to determine the temporal and spatial expression of KNOXI during the early stages of leaf lobe development in M. aquaticum. The developmental sequence of leaves of M. aquaticum was also described using scanning electron microscopy.

Key Results

Lobe development of M. aquaticum occurs in two very distinct regions at the leaf base in an alternating fashion reminiscent of a distichous shoot system. It was discovered that MaKN1 expression is localized to both the shoot apical meristem and early stages of leaf primordia development (P1–P7). Initially, MaKN1 is expressed ubiquitously throughout primordia (P1–P3); however, as lobes develop, MaKN1 becomes localized to recently emerged lobe primordia, and disappears as lobes develop basipetally.

Conclusions

The pattern of gene expression is indicative of shared developmental processes during early development between shoots, compound leaves, highly lobed simple leaves and unifoliate simple leaves which lack KNOXI expression. These findings are supportive of Arber's less rigid ‘partial shoot’ theory, which conceptualizes compound leaves as having shoot-like elements.

Keywords: Simple leaf development; Myriophyllum; KNOX; KN1; partial shoot

Journal Article.  8087 words.  Illustrated.

Subjects: Ecology and Conservation ; Evolutionary Biology ; Plant Sciences and Forestry

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