Journal Article

Ontogeny of embryogenic callus in <i>Medicago truncatula</i>: the fate of the pluripotent and totipotent stem cells

Xin-Ding Wang, Kim E. Nolan, Rina R. Irwanto, Michael B. Sheahan and Ray J. Rose

in Annals of Botany

Published on behalf of The Annals of Botany Company

Volume 107, issue 4, pages 599-609
Published in print April 2011 | ISSN: 0305-7364
Published online January 2011 | e-ISSN: 1095-8290 | DOI: http://dx.doi.org/10.1093/aob/mcq269
Ontogeny of embryogenic callus in Medicago truncatula: the fate of the pluripotent and totipotent stem cells

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  • Evolutionary Biology
  • Plant Sciences and Forestry

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Background and Aims

Understanding the fate and dynamics of cells during callus formation is essential to understanding totipotency and the mechanisms of somatic embryogenesis. Here, the fate of leaf explant cells during the development of embryogenic callus was investigated in the model legume Medicago truncatula.

Methods

Callus development was examined from cultured leaf explants of the highly regenerable genotype Jemalong 2HA (2HA) and from mesophyll protoplasts of 2HA and wild-type Jemalong. Callus development was studied by histology, manipulation of the culture system, detection of early production of reactive oxygen species and visualization of SERK1 (SOMATIC EMBRYO RECEPTOR KINASE1) gene expression.

Key Results

Callus formation in leaf explants initiates at the cut surface and within veins of the explant. The ontogeny of callus development is dominated by the division and differentiation of cells derived from pluripotent procambial cells and from dedifferentiated mesophyll cells. Procambium-derived cells differentiated into vascular tissue and rarely formed somatic embryos, whereas dedifferentiated mesophyll cells were competent to form somatic embryos. Interestingly, explants incubated adaxial-side down had substantially less cell proliferation associated with veins yet produced similar numbers of somatic embryos to explants incubated abaxial-side down. Somatic embryos mostly formed on the explant surface originally in contact with the medium, while in protoplast microcalli, somatic embryos only fully developed once at the surface of the callus. Mesophyll protoplasts of 2HA formed embryogenic callus while Jemalong mesophyll protoplasts produced callus rich in vasculature.

Conclusions

The ontogeny of embryogenic callus in M. truncatula relates to explant orientation and is driven by the dynamics of pluripotent procambial cells, which proliferate and differentiate into vasculature. The ontogeny is also related to de-differentiated mesophyll cells that acquire totipotency and form the majority of embryos. This contrasts with other species where totipotent embryo-forming initials mostly originate from procambial cells.

Keywords: Callus; dedifferentiation; leaf veins; Medicago truncatula; pluripotency; procambium; protoplasts; reactive oxygen species; SERK; somatic embryogenesis; stem cells; totipotency

Journal Article.  6590 words.  Illustrated.

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

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