TCUP: A Novel hAT Transposon Active in Maize Tissue Culture

Alan M Smith,Shawn M Kaeppler,Candice N Hansey

doi:10.3389/fpls.2012.00006

Abstract

Transposable elements (TEs) are capable of inducing heritable de novo genetic variation. The sequences capable of reactivation, and environmental factors that induce mobilization, remain poorly defined even in well-studied genomes such as maize. We treated maize tissue culture with the demethylating agent 5-aza-2-deoxcytidine and examined long-term tissue culture lines to discover silenced TEs that have the potential to induce heritable genetic variation. Through these screens we have identified a novel low copy number hAT transposon, Tissue Culture Up-Regulated (TCUP), which is transcribed at high levels in long-term maize black Mexican sweet (BMS) tissue culture and is transcribed in response to treatment with 5-aza-2-deoxycytidine. Analysis of the TIGR Maize Gene Index revealed that this element is the most frequently represented EST from the BMS cell culture library and is not represented in other tissue libraries, which is the basis for its name. A full-length sequence was assembled in inbred B73 that contains the putative functional motifs required for autonomous movement of a hAT transposon. Transposon display detected novel TCUP insertions in two long-term tissue-cultured cell lines of the genotype Hi-II A × B and BMS. This research implicates TCUP as a transposon that is capable of reactivation and which may also be particularly sensitive to the stress of the tissue culture environment. Our findings are consistent with the hypothesis that epigenetic alterations potentiate genomic responses to stress during clonal propagation of plants.

Highlights

Transposable elements (TEs) are ubiquitous members of all higher eukaryotic genomes and compose the majority of DNA present in the maize genome (Schnable et al, 2009)
Expression of Tissue Culture Up-Regulated (TCUP) in black Mexican sweet (BMS), newly initiated Hi-II callus cultures, seedling leaf, seedling root, and developing endosperm was experimentally evaluated by RT-PCR and TCUP transcription was not detected in seedling, leaf, root, endosperm, and newly initiated untreated callus
These evaluations supported that at least one member of the TCUP family is actively transcribed in BMS callus cultures, and that TCUP is stably quiescent in the other plant tissues as evaluated by EST and RT-PCR

Summary

Introduction

Transposable elements (TEs) are ubiquitous members of all higher eukaryotic genomes and compose the majority of DNA present in the maize genome (Schnable et al, 2009) These mobile DNAs are critical elements in the creation of genetic variation that can drive the reorganization and evolution of genomes. The transcription of TEs near or within genic regions can change the spatial and temporal expression of genes leading to altered phenotypes by both genetic and epigenetic mechanisms (Morgan et al, 1999; Kashkush et al, 2003). They may be mutagenic when mobilized and transpose into or near genes. Somaclonal variation is defined as genetic or phenotypic variation among clonally propagated plants derived from a single donor clone and results in a wide range of phenotypes displayed in the progenies of regenerated plants (Kaeppler et al, 2000)

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