The red flesh trait gives red pitayas more healthful components and a higher price, while the genetic mechanism behind this trait is unknown. In this manuscript, transcriptome analysis was employed to discover the genetic differences between white and red flesh in pitayas. A total of 27.99 Gb clean data were obtained for four samples. Unigenes, 79,049 in number, were generated with an average length of 1333 bp, and 52,618 Unigenes were annotated. Compared with white flesh, the expression of 10,215 Unigenes was up-regulated, and 4853 Unigenes were down-regulated in red flesh. The metabolic pathways accounted for 64.6% of all differentially expressed Unigenes in KEGG pathways. The group with high betalain content in red flesh and all structural genes, related to betalain biosynthesis, had a higher expression in red flesh than white flesh. The expression of the key gene, tyrosine hydroxylase CYP76AD1, was up-regulated 245.08 times, while 4,5-DOPA dioxygenase DODA was up-regulated 6.46 times. Moreover, the special isomers CYP76AD1α and DODAα were only expressed in red flesh. The competitive anthocyanin biosynthesis pathway had a lower expression in red flesh. Two MYB transcription factors were of the same branch as BvMYB1, regulating betalain biosynthesis in beet, and those transcription factors had expression differences in two kinds of pitayas, which indicated that they should be candidate genes controlling betalain accumulation in red pitayas. This research would benefit from identifying the major gene controlling red flesh trait and breed new cultivars with the red flesh trait. Future research should aim to prove the role of each candidate gene in betalain biosynthesis in red pitayas.
Background Overexpression of MYB transcription factors can induce the expression of structural genes for anthocyanin biosynthesis and increase the anthocyanin content of plant tissues. However, it remains unclear whether MYB transcription factor overexpression effects the activation of other genes and the concomitant accumulation of chemical compounds. Results Overexpression of LrAN2 promoted anthocyanin accumulation in a variety of tissues in tobacco cultivar Samsun. Only 185 unigenes, from total of 160,965, were expressed differently in leaves and 241 chemical compounds exhibited differences in accumulation. Four anthocyanins, including apigeninidin chloride, cyanidin 3-O-malonylhexoside, pelargonidin 3-O-beta-D-glucoside, and cyanidin 3,5-O-diglucoside were detected only in transgenic lines, which could explain the purple leaf phenotype. Beside anthocyanins, the phenylpropanoids, polyphenols (catechins), flavonoids, flavones, and flavonols were also upregulated. Overexpression of LrAN2 activated the basic helix-loop-helix transcription factor AN1b, and the MYB transcription factor MYB3. Additionally, structural genes associated with the phenylpropanoid biosynthetic pathway were activated, which lead to the upregulated accumulation of phenylpropanoid, polyphenol (catechin), flavonoid, flavone, flavonol, and anthocyanin. The MYB transcription factor CPC, a negative regulator of anthocyanin biosynthesis, was also expressed at increased levels in transgenic lines, which implie that a negative regulation mechanism existed in the anthocyanin biosynthesis pathway. The relative contents of all 19 differently accumulated amino groups and derivatives were decreased in transgenic lines, which meant that the phenylalanine biosynthesis pathway used other amino acids as substrates. Interestingly, the expression of acetylalkylglycerol acetylhydrolase was suppressed in transgenic lines, which caused the accumulation of 19 lyso-phosphatidylcholine derivatives and a decrease in production of eight octodecane derivatives. Conclusions Overexpression of LrAN2 activates the pathway of anthocyanin synthesis and metabolism in tobacco. Four anthocyanins lead to the purple leaf phenotype The main pathways of flavonoid biosynthesis were up-regulated. This research provides more information about the function of MYB transcription factors in anthocyanin biosynthesis and the production of other chemical compounds. This work will help breeders to obtain new plant cultivars with high anthocyanin contents using biotechnology.
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