The strigolactone biosynthesis gene DWARF27 is co-opted in rhizobium symbiosis.
BACKGROUND
Strigolactones are a class of plant hormones whose biosynthesis is activated in response to phosphate starvation. This involves several enzymes, including the carotenoid cleavage dioxygenases 7 (CCD7) and CCD8 and the carotenoid isomerase DWARF27 (D27). D27 expression is known to be responsive to phosphate starvation. In Medicago truncatula and rice (Oryza sativa) this transcriptional response requires the GRAS-type proteins NSP1 and NSP2; both proteins are essential for rhizobium induced root nodule formation in legumes. In line with this, we questioned whether MtNSP1-MtNSP2 dependent MtD27 regulation is co-opted in rhizobium symbiosis.


RESULTS
We provide evidence that MtD27 is involved in strigolactone biosynthesis in M. truncatula roots upon phosphate stress. Spatiotemporal expression studies revealed that this gene is also highly expressed in nodule primordia and subsequently becomes restricted to the meristem and distal infection zone of a mature nodules. A similar expression pattern was found for MtCCD7 and MtCCD8. Rhizobium lipo-chitooligosaccharide (LCO) application experiments revealed that of these genes MtD27 is most responsive in an MtNSP1 and MtNSP2 dependent manner. Symbiotic expression of MtD27 requires components of the symbiosis signaling pathway; including MtDMI1, MtDMI2, MtDMI3/MtCCaMK and in part MtERN1. This in contrast to MtD27 expression upon phosphate starvation, which only requires MtNSP1 and MtNSP2.


CONCLUSIONS
Our data show that the phosphate-starvation responsive strigolactone biosynthesis gene MtD27 is also rapidly induced by rhizobium LCO signals in an MtNSP1 and MtNSP2-dependent manner. Additionally, we show that MtD27 is co-expressed with MtCCD7 and MtCCD8 in nodule primordia and in the infection zone of mature nodules.
Department of Plant Science, Laboratory of Molecular Biology, Wageningen University, Droevendaalsesteeg 1, 6708 PB, Wageningen, The Netherlands. arjan.vanzeijl@wur.nl.
Arjan
A
van Zeijl
Department of Plant Science, Laboratory of Molecular Biology, Wageningen University, Droevendaalsesteeg 1, 6708 PB, Wageningen, The Netherlands. wliu@genetics.ac.cn.|State Key Laboratory of Molecular and Developmental Biology, Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, Beijing, 100101, China. wliu@genetics.ac.cn.
Wei
W
Liu
Department of Plant Science, Laboratory of Molecular Biology, Wageningen University, Droevendaalsesteeg 1, 6708 PB, Wageningen, The Netherlands. tingting.xiao@wur.nl.
Ting Ting
TT
Xiao
Department of Plant Science, Laboratory of Molecular Biology, Wageningen University, Droevendaalsesteeg 1, 6708 PB, Wageningen, The Netherlands. wouter.kohlen@wur.nl.
Wouter
W
Kohlen
State Key Laboratory of Molecular and Developmental Biology, Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, Beijing, 100101, China. wcyang@genetics.ac.cn.
Wei-Cai
WC
Yang
Department of Plant Science, Laboratory of Molecular Biology, Wageningen University, Droevendaalsesteeg 1, 6708 PB, Wageningen, The Netherlands. ton.bisseling@wur.nl.
Ton
T
Bisseling
Department of Plant Science, Laboratory of Molecular Biology, Wageningen University, Droevendaalsesteeg 1, 6708 PB, Wageningen, The Netherlands. rene.geurts@wur.nl.
René
R
Geurts
D001499:Bayes Theorem; D053898:Biosynthetic Pathways / Q000235:genetics*; D002686:Chitin / Q000031:analogs & derivatives / Q000494:pharmacology; D048271:Chitosan; D018506:Gene Expression Regulation, Plant / Q000187:drug effects; D055785:Gene Knockdown Techniques; D017343:Genes, Plant; D007783:Lactones / Q000378:metabolism*; D046913:Medicago truncatula / Q000187:drug effects / Q000235:genetics* / Q000382:microbiology*; D008954:Models, Biological; D009844:Oligosaccharides; D010710:Phosphates / Q000172:deficiency; D010802:Phylogeny; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D012231:Rhizobium / Q000187:drug effects / Q000502:physiology*; D053204:Root Nodules, Plant / Q000187:drug effects / Q000235:genetics; D015398:Signal Transduction / Q000187:drug effects / Q000235:genetics; D013559:Symbiosis / Q000187:drug effects / Q000235:genetics
D007783:Lactones; D009844:Oligosaccharides; D010710:Phosphates; D010940:Plant Proteins; D012333:RNA, Messenger; C493484:oligochitosan; D002686:Chitin; D048271:Chitosan