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식물의 관다발 시스템 | A new paradigm is emerging in which plants utilize proteins and RNA as non-cell-autonomously acting signaling macromolecules to mediate local and long-distance regulation over physiological and developmental processes. The cell-to-cell pathway for the trafficking of these non-cell-autonomous proteins (NCAPs) and RNA, in the form of ribonucleoprotein (RNP) complexes, is established by plasmodesmata (PD), the intercellular organelles unique to plants. The interconnection of local tissues to the phloem sieve tube system, through PD, establishes an integrated supracellular organism. Regulation of these local and long-distance macromolecular trafficking networks is essential for the coordinated exchange of information between distantly located plant organs to orchestrate events at the whole plant level. In angiosperms, the sieve tube system is comprised of two main cell types, the sieve elements (SEs) and their associated companion cells (CCs). At maturity, the enucleate SEs are highly modified to a low-resistance pathway, the sieve tube, for the translocation of photoassimilates; CCs function in the maintenance of the associated SEs. Mass spectrometry-based analysis of the phloem sap has established that the translocation stream contains a complex set of proteins (~ 2000 in number). Detailed analyses of many of these phloem proteins have demonstrated their capacity for cell-to-cell movement through PD: thus, the entry and exit of these phloem proteins appears to be regulated by the CC-SE PD. A number of these phloem proteins can bind to RNA and these RBPs mediate the cell-to-cell and long-distance translocation of RNA. These studies provide support for the concept that the CC-SE complex has the machinery necessary to mediate long-distance delivery of NCAPs and RNPs. This notion is consistent with the discovery that the phloem translocation stream contains a specific population of RNA molecules (>1,500 mRNA and many 1000s of si/miRNA species). Grafting experiments showed that many of these mRNAs are translocated within the phloem and delivery of these RNA has been correlated with development of specific phenotypes within the shoot apex. Systemic spread of RNA interference (RNAi), a sequence-specific RNA degradation process, is also consistent with the concept that RNA can be delivered to distant organs, through the phloem. Thus, PD and the sieve tube system act, in concert, to establish an information superhighway in plants. To advance our understanding of the roles played by PD and the phloem, in the trafficking of NCAPs and RNPs, we are currently developing proteomic-based strategies to (a) elucidate the supramolecular structure of PD, (b) establish the function of the phloem-mobile proteins, and (c) develop a phloem transcriptome for the RNA molecules (both large and small) that function within the context of the enucleate SEs. The impact of these findings on studies in applied plant biology will be discussed in terms of engineering novel agronomic traits through the trafficking of information macromolecules to targeted plant organs. |
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