Abstract
BackgroundStrigolactones 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.ResultsWe 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.ConclusionOur 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.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-015-0651-x) contains supplementary material, which is available to authorized users.
Highlights
Strigolactones are a class of plant hormones whose biosynthesis is activated in response to phosphate starvation
We show that in nodule primordia and mature nodules MtD27 is co-expressed with MtCCD7 and MtCCD8
The gene remains expressed in the dividing cells of the nodule primordium and at subsequent stages its expression becomes confined to the nodule meristem and distal infection zone of the mature nodule
Summary
Strigolactones are a class of plant hormones whose biosynthesis is activated in response to phosphate starvation. We focus on expression of the Nodule formation is initiated upon perception of lipochitooligosaccharide (LCO) signals excreted by compatible rhizobium bacteria [7] These signals mitotically activate root cortical and pericycle cells, resulting in the formation of a nodule primordium [7, 8]. Rhizobium LCOs ( known as Nod factors) are required to initiate an infection process to establish intracellular accommodation of the prokaryotic endosymbiont This infection process starts in curled root hairs where a tube-like structure is formed intracellularly, which guides the rhizobia to the newly formed nodule primordium. There, the rhizobia are released as organelle-like structures (named symbiosomes), which remain surrounded by van Zeijl et al BMC Plant Biology (2015) 15:260 a plant-derived membrane These symbiosomes act as nitrogen-fixing units that provide ammonia to the plant cell in exchange of nutrients [7]
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