Researchers William J. Lucas at the University of California, Davis, and Tony J. Lough at AgriGenesis Biosciences, Auckland, New Zealand, and their coauthors, show that small RNA corresponding to authentic siRNAs and miRNAs can enter and move through the phloem of several plant species. Furthermore, these authors identify a novel protein, Cucurbita maxima PHLOEM SMALL RNA-BINDING PROTEIN1 (CmPSRP1), and show that it likely plays a role in trafficking of small RNA through the phloem. The research is significant because small RNAs have not previously been observed in the phloem. In plants, the long-distance transport of protein and RNA through the phloem translocation stream plays a critical role in non-cell-autonomous signaling that contributes significantly to plant development. The systemic spread of RNA silencing via a phloem-transmissible signal is a well-characterized phenomenon that can be readily observed following localized viral infection or with hetero-grafting experiments. Small RNAs have been viewed as likely candidates for the systemic silencing signal, but there has been no direct evidence of their transport through the phloem and the nature of the mobile silencing signal has remained elusive. The extent to which miRNA function requires long-distance transport through the phloem is unknown and has not been explored previously.
In another report, researchers Ramanjulu Sunkar and Jian-Kang Zhu at the University of California, Riverside describe a new library of small RNAs isolated from Arabidopsis seedlings exposed to dehydration, salinity, cold stress, or the plant stress hormone abscisic acid. They identify 15 new miRNA families which include 26 new miRNAs corresponding to 34 loci, and show that some of these miRNAs are expressed in specific tissues and several are up- or down-regulated in response to abiotic stress. This report doubles the number of known miRNA families in Arabidopsis and identifies a number of new potential miRNA target genes, and further suggests that some miRNAs might be associated with regulation of gene expression in response to stress.
Together, these two reports open new avenues of research into small regulatory RNAs, and propel us several steps forward in understanding the biological functions and mechanism of action of this fascinating and important class of regulatory molecule.
The research by William Lucas and colleagues was supported by the National Science Foundation and the Department of Energy, Division of Energy Biosciences.
The research by Ramanjulu Sunkar and Jian-Kang Zhu was supported by National Institutes of Health and the National Science Foundation.
You can read the full report News and Reviews article on this research in The Plant Cell at http://www.plantcell.org/current.shtml#IN_THIS_ISSUE. The Plant Cell (http://www.plantcell.org) is published by the American Society of Plant Biologists. News and Reviews articles are available without subscription. For more information about ASPB, please visit http://www.aspb.org/.
Journal
The Plant Cell