Chromoplast development plays an important role in flesh color formation in watermelon(Citrullus lanatus)fruit.Here,we identified a watermelon phosphate transporter gene ClPHT4 which is critical for the chromoplast development in red-fleshed fruit.Transcriptome and real-time PCR analysis showed that ClPHT4 transcription level in fruit flesh increased significantly in a red-fleshed and sweet watermelon inbred 97103(Citrullus lanatus subsp.vulgaris East-Asia Ecotype),whereas the level was always low in white-fleshed and non-sweet watermelon accession PI296341FR(Citrullus lanatus subsp.lanatus)fruit.With other 128 accessions including 96 RILs and 32 natural lines,we found the high expression level of ClPHT4 in flesh is correlated with the flesh color and sweetness development.This gene was regarded as phosphate transporter gene based on the protein domain composition,genomic structure,and phylogenetic analysis.The function of CIPHT4 as a Pi transporter was demonstrated by heterologous expression and uptake of radioactive Pi into a yeast mutant defective in Pi uptake.Western blotting analysis and immunogold electron microscopy observation indicated that CIPHT4 is a chromoplast-targeted protein.A key transcriptional factor Cla014572 which lead to the different expression of the ClPHT4 in 97103and PI296341FR were found by yeast one hybrid,and testified by EMSA.In vivo assays of DNA-protein interaction in tobacco leaves showed this TF-Cla014572 can repress ClPHT4 transcript.Cla014572 is a bZIP transcription factor which own conserved SC-uORF.This SC-uORF was proved to lead to sucroseinduced repression of marker gene translation(SIRT).Together,our results demonstrate that the highly express of phosphate transporter CIPHT4 is necessary for chromoplast development,and a sucrose repressed TF-CIa014572 repressed this gene expression.Our results indicate that the increased sucrose activates the chromoplast development,and CIPHT4 together with Cla014572 may link the flesh color formation and sugar accumulation pathways in watermelon fruit development.