Journal Article

Heterochronic development of the floret meristem determines grain number per spikelet in diploid, tetraploid and hexaploid wheats

Naoki Shitsukawa, Hiroko Kinjo, Shigeo Takumi and Koji Murai

in Annals of Botany

Published on behalf of The Annals of Botany Company

Volume 104, issue 2, pages 243-251
Published in print August 2009 | ISSN: 0305-7364
Published online June 2009 | e-ISSN: 1095-8290 | DOI: http://dx.doi.org/10.1093/aob/mcp129
Heterochronic development of the floret meristem determines grain number per spikelet in diploid, tetraploid and hexaploid wheats

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

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Background and Aims

The inflorescence of grass species such as wheat, rice and maize consists of a unique reproductive structure called the spikelet, which is comprised of one, a few, or several florets (individual flowers). When reproductive growth is initiated, the inflorescence meristem differentiates a spikelet meristem as a lateral branch; the spikelet meristem then produces a floret meristem as a lateral branch. Interestingly, in wheat, the number of fertile florets per spikelet is associated with ploidy level: one or two florets in diploid, two or three in tetraploid, and more than three in hexaploid wheats. The objective of this study was to identify the mechanisms that regulate the architecture of the inflorescence in wheat and its relationship to ploidy level.

Methods

The floral anatomy of diploid (Triticum monococcum), tetraploid (T. turgidum ssp. durum) and hexaploid (T. aestivum) wheat species were investigated by light and scanning electron microscopy to describe floret development and to clarify the timing of the initiation of the floret primordia. In situ hybridization analysis using Wknox1, a wheat knotted1 orthologue, was performed to determine the patterning of meristem formation in the inflorescence.

Key Results

The recessive natural mutation of tetraploid (T. turgidum ssp. turgidum) wheat, branching head (bh), which produces branched inflorescences, was used to demonstrate the utility of Wknox1 as a molecular marker for meristematic tissue. Then an analysis of Wknox1 expression was performed in diploid, tetraploid and hexaploid wheats and heterochronic development of the floret meristems was found among these wheat species.

Conclusions

It is shown that the difference in the number of floret primordia in diploid, tetraploid and hexaploid wheats is caused by the heterochronic initiation of floret meristem development from the spikelet meristem.

Keywords: Triticum; wheat; inflorescence; spikelet; floret; meristem; heterochrony; heterochronic development; knotted1; polyploidy

Journal Article.  5533 words.  Illustrated.

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

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