Impacts Of Strigolactone On Shoot Branching Under Phosphate Starvation In Chrysanthemum (Dendranthema Grandiflorum Cv. ’Jinba’)

Chrysanthemum (Dendranthema grandiflorum cv. Jinba) shoot branching is determined by bud outgrowth during the vegetative growth stage. The degree of axillary bud outgrowth is highly influenced by environmental conditions, such as nutrient availability. Root is directly attached the nutrient content of the soil. And nutrient uptake, such as nitrogen, phosphate, would influence the axillary bud outgrowth. Long-distance signals participate in environmental stimulation, which connects roots and shoots. As to axillary buds outgrowth, the signal response locally would cause shoot branching. Strigolactone is considered to be a long-distance signal associated with soil phosphate content. Meanwhile in chrysanthemum, SL inhibits bud outgrowth along with competitive auxin source. At the same time, phosphate starvation can affect the sensitivity of auxin in plants. The mechanism among SL, auxin and phosphate starvation is unclear. Here, we demonstrated that:1. A SL biosynthesis gene, DgCCDl, was isolated and characterized as an ortholog of MAX3/DAD3/RMS5/D17. There are two mRNA sequences of DgCCD7 isolated from chrysanthemum. As we only isolated one clone from genomic DNA, the reason for the missing exon in DgCCD7b may due to alternative splicing. With functional complementation of max3-9 mutants, subcellular localization in the plastid stroma and β-carotene cleavage of DgCCD7a supported a conserved function.2. Along with uniformed stem, axillary buds outgrowth is associated with the distance to the top bud. And phosphorus starvation significantly reduces axillary bud outgrowth in chrysanthemum.3. Phosphate starvation can re-locate auxin in chrysanthemum plant. By using ultra-performance liquid chromatography coupled with mass spectrometry (UPLC-MS), three putative SLs were identified and levels of all three SLs showed strong increase under phosphate starvation conditions.4. Determinations of the distribution of SLs and negative regulation of DgCCD7/8 time-course in root indicated systemic response of SL. However, temporal expression patterns of biosynthesis and signaling genes in nodes revealed that Pi starvation causes a local response of SL pathway.5. Treatment of node segments with or without auxin and Pi revealed that in the absence of exogenous auxin, Pi delayed axillary buds outgrowth and up-regulated local SL pathway genes.These data indicated that an auxin-SL regulatory loop responded to Pi starvation for delaying bud outgrowth locally, root biosynthesized SLs were transported acropetally and functioned in shoot branching inhibition under Pi starvation. We proposed that SLs contributed to chrysanthemum shoot branching control in response to Pi-limiting conditions in a systemic way.