transport and localization play a fundamental role in the determination of cell polarity and morphogenesis, asymmetric cell divisions, embryonic patterning, and cell migration [2]. Moreover, by
localizing mRNAs to specific cellular target sites, the RNA localization processes prevent adverse effects that the proteins of localized
mRNAs may have if translated elsewhere in the cell [3–5]. mRNA
localization is evolutionarily conserved across the Eukarya and
transcriptome-wide approaches have demonstrated that mRNA
localization has a rather prominent than exceptional role in the
spatial regulation of genome expression [6, 7]. However, the ability
to target specific mRNA to subcellular sites is not limited to eukaryotes. In contrast to the conventional view of co-transcriptional
translation in bacteria, in vivo studies show that E. coli also has the
capacity to localize RNAs independent of and, thus, prior to
translation [8].
Although the localization of RNAs is a rational solution for
localized synthesis of the encoded proteins, the mechanisms by
which the cell orchestrates the complex distribution of individual
mRNAs to their respective subcellular compartments remain to be
further studied. The development of in vivo RNA tagging methods
and high-resolution fluorescent microscopy techniques have been
fundamental for advancing the understanding of RNA transport. It
is now accepted that mobile RNAs carry cis-acting “zipcode”
sequences that are recognized by trans-acting RBPs, which
together with their cognate target RNA molecules are assembled
into ribonucleoprotein (RNPs) complexes termed “RNA granules.” The heterogeneity of mRNAs and RBPs complicates the
identification of rules for RNA-protein interactions leading to
RNA granule assembly. Nevertheless, we know today that these
granules, which may contain only single mRNA copies [9], are
transported by molecular motors along the elements of the cytoskeleton, and in a translationally repressed state, to their final
destination [5, 10, 11].
In addition to transporting and localizing mRNA to different
subcellular compartments [12–14], plants transport RNAs also
between cells through plasmodesmata (PD), gatable membranous
pores within the cell wall of neighboring cells. The PD provide
cytoplasmic as well as plasma membrane and endoplasmic reticulum (ER) continuity between adjacent cells [15]. Moreover, the
system of PD in leaves is connected to the phloem sieve elements in
the stems and thus creates a cell-to-cell and long-distance communication network [16]. The profiling of heterografted plants led to
the identification of thousands of mRNAs that are transported
constitutively or in response to environmental stresses through
the graft junction into distant tissues [17, 18].
However, little is known about the cellular machinery and the
RNA features involved in cell-to-cell and long-distance RNA transport in plants. Although recent studies imply a role of RNA
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Eduardo Jose ´ Pen ˜ a and Manfred Heinlein
localizing mRNAs to specific cellular target sites, the RNA localization processes prevent adverse effects that the proteins of localized
mRNAs may have if translated elsewhere in the cell [3–5]. mRNA
localization is evolutionarily conserved across the Eukarya and
transcriptome-wide approaches have demonstrated that mRNA
localization has a rather prominent than exceptional role in the
spatial regulation of genome expression [6, 7]. However, the ability
to target specific mRNA to subcellular sites is not limited to eukaryotes. In contrast to the conventional view of co-transcriptional
translation in bacteria, in vivo studies show that E. coli also has the
capacity to localize RNAs independent of and, thus, prior to
translation [8].
Although the localization of RNAs is a rational solution for
localized synthesis of the encoded proteins, the mechanisms by
which the cell orchestrates the complex distribution of individual
mRNAs to their respective subcellular compartments remain to be
further studied. The development of in vivo RNA tagging methods
and high-resolution fluorescent microscopy techniques have been
fundamental for advancing the understanding of RNA transport. It
is now accepted that mobile RNAs carry cis-acting “zipcode”
sequences that are recognized by trans-acting RBPs, which
together with their cognate target RNA molecules are assembled
into ribonucleoprotein (RNPs) complexes termed “RNA granules.” The heterogeneity of mRNAs and RBPs complicates the
identification of rules for RNA-protein interactions leading to
RNA granule assembly. Nevertheless, we know today that these
granules, which may contain only single mRNA copies [9], are
transported by molecular motors along the elements of the cytoskeleton, and in a translationally repressed state, to their final
destination [5, 10, 11].
In addition to transporting and localizing mRNA to different
subcellular compartments [12–14], plants transport RNAs also
between cells through plasmodesmata (PD), gatable membranous
pores within the cell wall of neighboring cells. The PD provide
cytoplasmic as well as plasma membrane and endoplasmic reticulum (ER) continuity between adjacent cells [15]. Moreover, the
system of PD in leaves is connected to the phloem sieve elements in
the stems and thus creates a cell-to-cell and long-distance communication network [16]. The profiling of heterografted plants led to
the identification of thousands of mRNAs that are transported
constitutively or in response to environmental stresses through
the graft junction into distant tissues [17, 18].
However, little is known about the cellular machinery and the
RNA features involved in cell-to-cell and long-distance RNA transport in plants. Although recent studies imply a role of RNA
104
Eduardo Jose ´ Pen ˜ a and Manfred Heinlein