Translationally Controlled Tumor Protein (TCTP) is an almost ubiquitous protein found in eukaryotes, fundamental for the regulation of development and general growth. The multiple functions of TCTP have been inferred from its involvement in several cell pathways, but the specific function of TCTP is still not known in detail. On the other hand, TCTP seems to respond to a plethora of external signals, and appears to be regulated at the transcriptional and/or translational levels by mechanisms yet to be determined. In the present work, we analyzed the capacity of AtTCTP2 gene products (mRNA and protein) to translocate long distance through tobacco heterografts (transgenic/WT and WT/transgenic). The results indicate that both AtTCTP2 mRNA and protein are capable of moving long distance in both directions (stock-scion and scion-stock) with a tendency for movement from source to sink tissue (stock to scion). Interestingly, aerial roots emerged only in heterografts where the protein was detected in both stock and scion, suggesting a correlation between the presence of AtTCTP2 and aerial root appearance. More detailed analysis showed that these aerial roots harbored the transgene and expressed both transcript and protein. In addition, the protein localization pattern in transgenic aerial and primary roots was basically the same, indicating specific nuclear destination in roots, but also in leaves. These findings provide an approach to understand the role of long-distance movement in the function of plant TCTPs, supporting the notion that some of these act in a non-cell autonomous manner, as the human counterpart, the Histamine Releasing Factor (HRF).
The phloem is the conduit through which photoassimilates are distributed from autotrophic to heterotrophic tissues and is involved in the distribution of signaling molecules that coordinate plant growth and responses to the environment. Phloem function depends on the coordinate expression of a large array of genes. We have previously identified conserved motifs in upstream regions of the Arabidopsis genes, encoding the homologs of pumpkin phloem sap mRNAs, displaying expression in vascular tissues. This tissue-specific expression in Arabidopsis is predicted by the overrepresentation of GA/CT-rich motifs in gene promoters. In this work we have searched for common motifs in upstream regions of the homologous genes from plants considered to possess a “primitive” vascular tissue (a lycophyte), as well as from others that lack a true vascular tissue (a bryophyte), and finally from chlorophytes. Both lycophyte and bryophyte display motifs similar to those found in Arabidopsis with a significantly low E-value, while the chlorophytes showed either a different conserved motif or no conserved motif at all. These results suggest that these same genes are expressed coordinately in non-vascular plants; this coordinate expression may have been one of the prerequisites for the development of conducting tissues in plants. We have also analyzed the phylogeny of conserved proteins that may be involved in phloem function and development. The presence of CmPP16, APL, FT, and YDA in chlorophytes suggests the recruitment of ancient regulatory networks for the development of the vascular tissue during evolution while OPS is a novel protein specific to vascular plants.
The Translationally Controlled Tumor Proteins, or TCTP, is a superfamily of exclusively eukaryotic proteins essential in the regulation of proliferation and general growth. However, it is clear that these are multifunctional proteins given (1) the pleiotropic effects of its mutations, and (2), the multiple processes in which this protein is involved. TCTP function in general is conserved, since Arabidopsis AtTCTP1 can rescue a Drosophila mutant, and vice versa. It has become clear, however, that these proteins may have “taxon-specific” functions. In the case of plants, mRNA and/or proteins have been found in the phloem translocation stream of different species, suggesting a role in long-distance signaling. We have found that a second Arabidopsis TCTP gene, AtTCTP2, codes for a protein that moves long-distance through a graft union in tobacco. Interestingly, the mRNA is also transported long-distance. Both mRNA and protein move long-distance; interestingly, the movement, while more efficient from source to sink tissues, also occurs in the opposite direction. The protein reaches the nuclei of parenchyma cells and adventitious roots. Furthermore, it is clear that the long-distance delivery of AtTCTP2 protein and mRNA is required for the induction of adventitious roots. A model is presented that accounts for these observations.
Plants, as most eukaryotic organisms, harbor several genes encoding a reverse transcriptase domain. The majority of them are part of transposable elements (TEs) and/or retroviral genomes that have been inserted into their genomes. However, there are some examples of RT domain-containing genes that have been endogenized during plant evolution; these genes appear to display functions other than “selfish” maintenance and replication of TEs, and subjected to host gene regulation. In the present work we have analyzed a subset of genes in Arabidopsis with an RT domain (RVT) containing a zinc finger motif (Znf), termed RVT-Znf domain, with structural characteristics of endogenous genes i.e., contain potential upstream regions as well as 5’UTR, and 3’UTR, and are not flanked by retroelement features. Phylogenetic analysis of these genes, based on the RVT-Znf domain, indicates that there are three clades, the members of which having additional domains. When compared to additional sequences, RVT-Znf formed a cluster that is more closely related to non-LTR retrotransposons and group II introns. Extant data from microarray databases indicate that several Arabidopsis genes are expressed. These data indicate that these RTs may have been endogenized. Possible roles for these genes are discussed.
The vascular tissue has been essential for the evolutionary radiation of land plants, and has allowed them to colonise a wide range of habitats. The phloem is involved in the distribution of fixed carbon throughout the whole plant. The concept of this tissue as a central distributor of signals that regulate plant development and responses to environmental cues has emerged recently. Indeed, several phenomena, such as post‐transcriptional gene silencing, flowering and tuber induction, as well as systemic acquired resistance to pathogens rely on signals, originating in source leaves, and transported via the phloem to systemic tissues. Considering that the phloem translocation stream contains a large array of macromolecules. Given that the functional phloem consists of interconnected enucleate cells, the sieve elements (SEs), the majority of proteins and ribonucleic acids (RNAs) found in phloem sap exudates must originate from the adjoining companion cell (CC). While several proteins play a role in the maintenance of the SEs, it is possible that several species of proteins and RNAs function in long‐distance signalling. In this article the current state of the art of this subject is described, as well as perspectives and possible applications for crop improvement. Key Concepts The vascular tissue consists of xylem and phloem. The xylem is composed of vessels that are dead cells upon maturity that transport mineral nutrients from roots to shoots. The phloem is composed of vessels formed from enucleated living cells involved in the transport of fixed carbon from photosynthetic to heterotrophic organs. The phloem is involved in the delivery of signals that control development as well as responses to external stimuli, such as defence response, post‐transcriptional gene silencing and response to phosphate starvation. Viruses and viroids are transported through the phloem during systemic infection. These probably take advantage of a system enabling long‐distance transport of endogenous proteins and RNAs. The phloem translocation stream contains a wide variety of proteins and RNAs, some of which appear to have a signalling function. RNA and protein exchange can occur between plant pathogenic fungi and parasitic plants and their hosts. Knowledge of this RNA and protein phloem transport system can have applications for crop improvement.
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