Systematized biosynthesis and catabolism regulate citrulline accumulation in watermelon

Area of Science:

• Plant Biochemistry
• Molecular Biology
• Fruit Development

Background:

• Citrulline is a non-protein amino acid abundant in watermelon fruits.
• Its accumulation patterns and genetic variations within cultivars are not fully understood.
• Understanding citrulline biosynthesis regulation is key to optimizing its content.

Purpose of the Study:

• To investigate the regulation of citrulline accumulation in watermelon fruit tissues.
• To analyze the expression of genes involved in citrulline biosynthesis and catabolism during fruit development.
• To correlate gene expression with citrulline content in flesh and rind.

Main Methods:

• Amino acid profiling of watermelon tissues during development.
• Analysis of gene expression for key enzymes in citrulline metabolism (e.g., OTC, N-AOA, AOD-3, N-AOGA, CPS-1, CPS-2, ASS, ASL, ODC, ADC).
• Correlation analysis between citrulline content, related amino acids, and gene expression levels.

Main Results:

• Citrulline accumulates progressively in watermelon flesh and rind, correlating with precursor (ornithine) and by-product (arginine) amino acids.
• Expression of N-acetylornithine aminotransferase (N-AOA) and N-acetylornithine deacetylase (AOD-3) coincided with citrulline accumulation.
• Genes for citrulline breakdown (ASS, ASL, ODC, ADC) were consistently downregulated during fruit development.

Conclusions:

• N-AOA and AOD-3 play significant roles in citrulline synthesis during watermelon fruit development.
• The non-cyclic pathway and localized carbamoyl phosphate synthesis appear less critical for citrulline accumulation.
• Downregulation of catabolic genes facilitates citrulline buildup in ripening watermelon fruit.