A SRN1-Tre6P-SnRK1 Feed-forward Loop Regulates Source To Sink Carbon Partitioning In Rice

The current trajectory for crop yields is insufficient to meet the global demand for food in the next few decades,owing to the booming population and deteriorating environments.The formation of crop yield is essentially a process of source-sink interaction.Breeding or engineering plants in source-sink relationship of plant could be a potential solution for high yield.Greater and more efforts have been done on source production,carbon flow and sink utilization to genetically improve crop yield.However,improvement on one of the three issues usually triggers the trade-off circuits on other issues,which may result in no improved or even lower yield.Hence,it is believed that optimizing the source-sink interaction from a whole picture view may be a more feasible strategy to achieve higher crop yields.Recently,trehalose-6-phosphate(Tre6P)has been demonstrated to be a source-sink optimizer with promising potential in crop yield improvement.Tre6 P responds to the fluctuations of sugar levels,suppresses the transcription and enzyme activity of carbon modulator SnRK1 in plant.A Tre6P-SnRK1 nexus can sense sucrose status and regulate plant growth and development.This research focused a sugar-inducible rice transcription factor SRN1,which facilitates carbon partitioning from source to sink organs by a feed-forward regulatory loop of SRN1-Tre6P-SnRK1.The research results are as follows:1.We identified a sugar-responsive NAC type transcription factor SRN1 which the m RNA and protein abundances of SRN1 showed strong positive correlations to the endogenous sugar level among tissues and rhythm.Moreover,as indicated by the LUC,immunoblot assays and in vivo GFP fluorescence observation.2.In comparison to the wild type,crsrn1 and a T-DNA insertional mutant srn1 showed reduced plant height,less tillers,later flowering,and significantly smaller size of sink organs such as panicles and seeds,leaves of source organ with lower gross photosynthetic rate and higher NSC content,leading to over 30% loss of yield per plant.The two over-expression lines Ox SRN1-1 and Ox SRN1-2 were opposite of crsrn1 and srn1,except in flowering time and plant height.In comparison to NIP,Ox SRN1 had over 16% higher yield per plant.Prolonged darkness-induced carbon starvation significantly suppressed the NIP growth,which mimicked the crsrn1 seedlings under normal carbon condition.Surprisingly,carbon starvation promoted,instead of suppressed,the crsrn1 seedling growth.Similarly,application of sucrose partially recovered the carbon starved NIP plants to normal growth,but showed more severe suppression on crsrn1.The above results indicated that crsrn1 likely has over amount of endogenous sugar which intoxicated the plant with retarded growth.3.SnRK1 a has been reported as a key modulator in plant energy and carbon availability-related processes in plants.Our results showed SRN1 physically binds to SnRK1 a.K inase assay and Cell-free degradation assay suggesting that His-SnRK1 a,likely through mediating phosphorylation on GST-SRN1,compromise GST-SRN1 protein stability.Up-regulation of SRN1 and similar phenotypes to Ox SRN1 in crsnrk1 a lines,while SnRK1 a transcripts were obviously elevated in crsrn1 and reduced in Ox SRN1 lines,indicating an antagonistic relationship between SnRK1 a and SRN1 indicated an antagonistic relationship between SnRK1 a and SRN1.4.The Tre6 P levels were reduced in crsrn1 and elevated in Ox SRN1,while trehalose contents were the opposite.SRN1 can directly repressed the transcription of TPP1,a trehalose-6-phosphate phosphatase gene.TPP1 displayed opposite expression patterns to SRN1 in tissues,rhythm and carbon starvation response.Moreover,crtpp1 mutants mimicked Ox SRN1 in agronomic traits and trehalose and Tre6 P levels,and knock-out of TPP1 in the srn1 background could partially recovered the srn1 phenotypes.These results indicated that SRN1 regulates rice growth by directly repressing TPP1.5.Exogenous trehalose treatment significantly enhanced the temporal starch,soluble sugar accumulation and NSC level in the flag leaves of both NIP and Ox SRN1 s at grain-filling stage.Meanwhile,seed weight and grain filling rate of rice developing seed were significantly reduced by exogenous trehalose.And most sugar transportation-related genes were down-regulated in trehalose-treated NIP and crsrn1 which had higher endogenous trehalose level,but up-regulated in in Ox SRN1 with lower trehalose level,which demonstrated that that SRN1 promotes carbon partitioning from source to sink organs by directly repressing the quantity of trehalose whose role is to retain sugars in the source.6.Over-expression of SRN1 in NIP,Zhongshui01 and Nangeng46 cultivar backgrounds consistently increased rice yield by over 11%.Ox SRN1,and exhibited a comprehensive upgrade in source-sink relationships in Hangzhou and Lianyungang.Our data demonstrated that SRN1 has great potential in rice genetic improvement in yield.The above results showed that a sugar-inducible rice transcription factor SRN1 directly represses the transcription of TPP1 encoding a trehalose-6-phosphate phosphatase to simultaneously promote Tre6 P and repress trehalose level,which facilitates carbon partitioning from source to sink organs by activating sugar transporter genes.Meanwhile,Tre6 P suppresses the transcription and enzyme activity of SnRK1 a,a low carbon sensor and an antagonist of SRN1,to prevent the SnRK1a-mediated phosphorylation and degradation on SRN1.“SRN1-Tre6P-SnRK1”forms a feed-forward loop to sense sugar and maintain sugar homeostasis through transporting sugars to the sink organs.Importantly,by optimizing the source-sink interaction,over-expression of SRN1 showing great potential in crop genetic improvement.