Cell-to-cell movement of plastids in plants

Abstract

Our objective was to test whether or not plastids and mitochondria, the two DNA-containing organelles, move between cells in plants.As our experimental approach, we grafted two different species of tobacco, Nicotiana tabacum and Nicotiana sylvestris. Grafting triggers formation of new cell-to-cell contacts, creating an opportunity to detect cell-to-cell organelle movement between the genetically distinct plants. We initiated tissue culture from sliced graft junctions and selected for clonal lines in which gentamycin resistance encoded in the N. tabacum nucleus was combined with spectinomycin resistance encoded in N. sylvestris plastids. Here, we present evidence for cell-to-cell movement of the entire 161-kb plastid genome in these plants, most likely in intact plastids. We also found that the related mitochondria were absent, suggesting independent movement of the two DNA-containing organelles. Acquisition of plastids from neighboring cells provides a mechanism by which cells may be repopulated with functioning organelles. Our finding supports the universality of intercellular organelle trafficking and may enable development of future biotechnological applications.plasmodesmata | horizontal gene transfer | plastid transformation | chloroplast biotechnology P lant cells have three DNA-containing cellular compartments: the nucleus, plastids, and mitochondria. The plastid and mitochondrial genomes (ptDNA and mtDNA) in all plant species have been massively reduced relative to their prokaryotic ancestors through the evolutionary process of intracellular gene transfer. The 155-kb plastid and 430-kb mitochondrial genomes of Nicotiana tabacum encode only 112 (1) and 60 (2) genes, respectively. Experimental reconstruction of this evolutionary process in the laboratory revealed that plastid-to-nucleus gene transfer occurs at a surprisingly high frequency (3). The recent demonstration of the exchange of genetic material between cells in plant tissue grafts reconstructed the evolutionary process of intercellular gene transfer (4).Intercellular movement of mitochondria in mammalian cells was found to be a basic biological process and involved in tissue repair (5). In coculture, donor cells extend cytoplasmic projections toward target cells and mitochondria stream from cell to cell (6-8). This transfer was shown to result in replacement of diseased mitochondrial genomes with mtDNA from the donor cells (5).However, there is no report yet on the intercellular movement of DNA-containing organelles, plastids, and mitochondria, between plant cells. In contrast to animal cells, plant cells have a rigid cell wall. However, plant cells are connected by sophisticated intercellular channels (plasmodesmata), which actively and passively regulate cell-to-cell movement of nutrients, hormones, and information macromolecules, including transcription factors, phloem proteins, mRNA, and sRNAs (9, 10).Our objective was to determine whether chloroplasts or mitochondria could move from cell to cell in plants. To test this hypothesis, we grafted...

Cell-to-cell movement of mitochondria in plants

“…Thus, far when cell-to-cell movement of plastids was observed, the entire selected plastid genome was recovered, rather than a ptDNA fragment comprising the selected marker. Absence of recombination between the plastid genomes suggested movement of intact organelles rather than naked, fragmented DNA (11,12). Indeed, chloroplasts rarely fuse (22), precluding recombination that enables new combinations of chloroplast genes.…”

“…The flower phenotype of plants carrying orf293 in mitochondria depends on the nuclear background: in N. tabacum the anthers are petaloid, or petaloidstigmatoid (Fig. 1D); in N. sylvestris, the anthers are converted into stigmatoid structures (11). Based on the flower phenotype, we tentatively named orf293 "homeotic conversion of anthers" (hca) gene.…”

“…Grafting is a versatile approach that enables recovery of plants with: (i) chloroplasts of the graft partner without the transfer of mitochondria and nuclei (11,12); (ii) the transfer of both chloroplasts and mitochondria in the absence of any nuclear DNA input (this paper); and (iii) the formation of nuclear hybrids with input of all organelles (33). Table S1.…”

“…The first experimental evidence in support of grafting as a potential mechanism of HGT came from demonstrating exchange of plant DNA in tobacco tissue grafts (10). Movement of entire chloroplast genomes was subsequently demonstrated through tobacco graft junctions, interpreted as evidence for cell-to-cell movement of the organelles (11,12). However, evidence for cellto-cell movement of mitochondrial DNA is missing in plants despite the fact that the majority of horizontal gene transfer events involve mitochondrial sequences.…”

New insights into cytomixis: specific cellular features and prevalence in higher plants

“…The cytomictic migration of plastids attracts a special attention, since it has been repeatedly demonstrated that the plastids carrying selective traits can migrate between cells of vegetative tissues at the zone of transplant fusion (Stegemann and Bock 2009;Thyssen et al 2012). Presumably, CCs also play a key role in this case; however, the mechanisms of their formation in somatic tissues require a separate study.…”