Phloem small RNAs, nutrient stress responses, and systemic mobility

<jats:title>Abstract</jats:title> <jats:sec> <jats:title>Background</jats:title> <jats:p>Nutrient availabilities and needs have to be tightly coordinated between organs to ensure a balance between uptake and consumption for metabolism, growth, and defense reactions. Since plants often have to grow in environments with sub-optimal nutrient availability, a fine tuning is vital. To achieve this, information has to flow cell-to-cell and over long-distance via xylem and phloem. Recently, specific miRNAs emerged as a new type of regulating molecules during stress and nutrient deficiency responses, and miR399 was suggested to be a phloem-mobile long-distance signal involved in the phosphate starvation response.</jats:p> </jats:sec> <jats:sec> <jats:title>Results</jats:title> <jats:p>We used miRNA microarrays containing all known plant miRNAs and a set of unknown small (s) RNAs earlier cloned from <jats:italic>Brassica</jats:italic> phloem sap [1], to comprehensively analyze the phloem response to nutrient deficiency by removing sulfate, copper or iron, respectively, from the growth medium. We show that phloem sap contains a specific set of sRNAs that is distinct from leaves and roots, and that the phloem also responds specifically to stress. Upon S and Cu deficiencies phloem sap reacts with an increase of the same miRNAs that were earlier characterized in other tissues, while no clear positive response to -Fe was observed. However, -Fe led to a reduction of Cu- and P-responsive miRNAs. We further demonstrate that under nutrient starvation miR399 and miR395 can be translocated through graft unions from wild type scions to rootstocks of the miRNA processing <jats:italic>hen1-1</jats:italic> mutant. In contrast, miR171 was not transported. Translocation of miR395 led to a down-regulation of one of its targets in rootstocks, suggesting that this transport is of functional relevance, and that miR395, in addition to the well characterized miR399, could potentially act as a long-distance information transmitter.</jats:p> </jats:sec> <jats:sec> <jats:title>Conclusions</jats:title> <jats:p>Phloem sap contains a specific set of sRNAs, of which some specifically accumulate in response to nutrient deprivation. From the observation that miR395 and miR399 are phloem-mobile in grafting experiments we conclude that translocatable miRNAs might be candidates for information-transmitting molecules, but that grafting experiments alone are not sufficient to convincingly assign a signaling function.</jats:p> </jats:sec>