Phloem small RNAs, nutrient stress responses, and systemic mobility.
BACKGROUND
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.


RESULTS
We used miRNA microarrays containing all known plant miRNAs and a set of unknown small (s) RNAs earlier cloned from Brassica 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 hen1-1 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.


CONCLUSIONS
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.
Buhtz
Anja
A
Centro de Biotecnología y Genómica de Plantas (UPM-INIA), Campus de Montegancedo, M40 (km38), 28223 Pozuelo de Alarcón/Madrid, Spain.
Pieritz
Janin
J
Springer
Franziska
F
Kehr
Julia
J
D017360:Arabidopsis / Q000235:genetics / Q000254:growth & development / Q000378:metabolism; D029688:Brassica napus / Q000235:genetics / Q000254:growth & development / Q000378:metabolism; D003300:Copper / Q000172:deficiency; D015536:Down-Regulation / Q000235:genetics; D035264:Flowers / Q000235:genetics / Q000378:metabolism; D005502:Food; D018506:Gene Expression Regulation, Plant; D017343:Genes, Plant / Q000235:genetics; D000090463:Iron Deficiencies; D035683:MicroRNAs / Q000235:genetics / Q000378:metabolism*; D009928:Organ Specificity / Q000235:genetics; D052585:Phloem / Q000235:genetics*; D018515:Plant Leaves / Q000235:genetics / Q000378:metabolism; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism; D018517:Plant Roots / Q000235:genetics / Q000378:metabolism; D018547:Plant Stems / Q000235:genetics / Q000378:metabolism; D034443:RNA Transport; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D013312:Stress, Physiological / Q000235:genetics*
D035683:MicroRNAs; D010940:Plant Proteins; D012333:RNA, Messenger; D003300:Copper
Plant Methods. 2007 Oct 12;3:12
17931426
Plant Cell. 2008 Dec;20(12):3186-90
19074682
Plant Physiol. 1997 Oct;115(2):317-9
9342857
Plant Cell. 2005 Jun;17(6):1658-73
15849273
Mol Cell. 2004 Jun 18;14(6):787-99
15200956
Plant Physiol. 2005 Aug;138(4):2145-54
16040653
Planta. 2009 Sep;230(4):671-85
19585144
Development. 2002 Mar;129(5):1085-94
11874905
Nucleic Acids Res. 2005 Jan 31;33(2):e17
15684409
Genes Dev. 2006 Dec 15;20(24):3407-25
17182867
Plant Cell. 2004 Aug;16(8):1979-2000
15258266
Plant J. 2008 Jun;54(5):876-87
18298674
Curr Protoc Microbiol. 2006 Jan;Chapter 16:Unit 16H.2
18770585
Plant Cell. 2009 Jan;21(1):347-61
19122104
Plant J. 2002 Oct;32(2):255-62
12383090
Plant Physiol. 2006 Jul;141(3):1000-11
16679417
Biotechniques. 2005 Oct;39(4):519-25
16235564
Annu Rev Plant Biol. 2006;57:19-53
16669754
Curr Biol. 2005 Nov 22;15(22):2038-43
16303564
Plant Cell. 2006 Feb;18(2):412-21
16387831
Curr Opin Plant Biol. 2006 Jun;9(3):256-63
16616609
Annu Rev Plant Physiol Plant Mol Biol. 2000 Jun;51:323-347
15012195
Amino Acids. 2006 Mar;30(2):163-71
16525754
Plant Cell. 2006 May;18(5):1134-51
16603651
Genes Dev. 2004 Sep 15;18(18):2237-42
15371337
Nat Genet. 2007 Jun;39(6):792-6
17496893
Proteomics. 2006 Feb;6(3):896-909
16400686
Trends Plant Sci. 2007 Jul;12(7):301-9
17573231
Plant Physiol. 2008 Jun;147(2):732-46
18390805
Plant J. 2008 Mar;53(5):739-49
18005229
Nat Genet. 2007 Jul;39(7):848-56
17558406
Plant Physiol. 2006 Jul;141(3):988-99
16679424
Plant J. 2008 Mar;53(5):731-8
17988220
Plant J. 2009 Jan;57(2):313-21
18801012
PLoS Genet. 2009 Jan;5(1):e1000320
19119413
Genome Biol. 2007;8(6):R96
17543110
Plant J. 2006 Jul;47(1):140-51
16824182
Plant Cell. 2005 Aug;17(8):2204-16
16006581
Planta. 2009 Sep;230(4):659-69
19585143
Eukaryot Cell. 2002 Oct;1(5):736-57
12455693
Dev Cell. 2005 Apr;8(4):517-27
15809034
Plant Physiol. 2009 Jul;150(3):1541-55
19465578
Curr Biol. 2002 Sep 3;12(17):1484-95
12225663
J Biol Chem. 2007 Jun 1;282(22):16369-78
17405879
Plant Cell Environ. 2007 Mar;30(3):271-290
17263774
J Biol Chem. 2008 Jun 6;283(23):15932-45
18408011
Trends Plant Sci. 2009 May;14(5):280-5
19375375
Plant Physiol. 2008 Jul;147(3):1181-91
18467463