Ectopic Transplastomic Expression of a Synthetic MatK Gene Leads to Cotyledon-Specific Leaf Variegation.

Yujiao Qu,Jose M Muino,Oren Ostersetzer,Giovanna Ranzini,Julia Legen,Andreas Weihe,Uwe Ohler,Stephanie Henkel,Christopher Thieme,Gert Weber,Jürgen Arndt,Galina Hoppe,Christian Schmitz-Linneweber

doi:10.3389/fpls.2018.01453

Abstract

Chloroplasts (and other plastids) harbor their own genetic material, with a bacterial-like gene-expression systems. Chloroplast RNA metabolism is complex and is predominantly mediated by nuclear-encoded RNA-binding proteins. In addition to these nuclear factors, the chloroplast-encoded intron maturase MatK has been suggested to perform as a splicing factor for a subset of chloroplast introns. MatK is essential for plant cell survival in tobacco, and thus null mutants have not yet been isolated. We therefore attempted to over-express MatK from a neutral site in the chloroplast, placing it under the control of a theophylline-inducible riboswitch. This ectopic insertion of MatK lead to a variegated cotyledons phenotype. The addition of the inducer theophylline exacerbated the phenotype in a concentration-dependent manner. The extent of variegation was further modulated by light, sucrose and spectinomycin, suggesting that the function of MatK is intertwined with photosynthesis and plastid translation. Inhibiting translation in the transplastomic lines has a profound effect on the accumulation of several chloroplast mRNAs, including the accumulation of an RNA antisense to rpl33, a gene coding for an essential chloroplast ribosomal protein. Our study further supports the idea that MatK expression needs to be tightly regulated to prevent detrimental effects and establishes another link between leaf variegation and chloroplast translation.

Highlights

Land plant chloroplast RNAs are heavily processed post-transcriptionally
Leaf variegation is based on the occurrence of tissue sectors that either contain fully developed chloroplasts or pale, aberrant plastids (Kirk and Tilney-Bassett, 1978; Sakamoto, 2003; Aluru et al, 2006)
Plastome-based variegation can be caused by segregation of two genetically distinct plastids, one of which does not develop into fully green chloroplasts (Baur, 1909; Correns, 1909; Börner and Sears, 1986; Greiner and Bock, 2013)

Summary

Introduction

Land plant chloroplast RNAs are heavily processed post-transcriptionally. RNA editing, a prominent post-transcriptional processing step is RNA splicing of group II introns. Group II intron splicing is mediated predominantly by nuclear-encoded RNA binding proteins, of which about a dozen have been described in maize or Arabidopsis (de Longevialle et al, 2010; Stern et al, 2010) In addition to this nuclear complement of splicing factors, there is a single chloroplast-encoded splicing factor named intron maturase K, short MatK. The MatK protein is related to prokaryotic intron maturases (Neuhaus and Link, 1987). Leaf Variegation in MatK Overexpressors encoded within group II introns, i.e., their “home introns.”. The home intron of MatK is located in the trnK gene Leaf Variegation in MatK Overexpressors encoded within group II introns, i.e., their “home introns.” The home intron of MatK is located in the trnK gene

Objectives

Methods

Results

Conclusion