The sweet orange is
the product of a cross between pummelo and mandarin, and then a re-cross of the
resulting hybrid with mandarin.
The
sweet orange’s parents and mechanism for producing vitamin C are revealed in
its draft genome sequence
The sweet orange, Citrus sinensis, has long dominated
fruit production worldwide. Yet attempts to study this fruit’s genetics and
improve its desirable traits have proved difficult because it reproduces
asexually and seedlings are nearly identical to the mother plant. Plant
biologists had even failed to determine with certainty which fruits had been
crossed to produce the sweet orange, over 2,000 years ago in China. An
international research team, including members from the A*STAR Genome Institute
Singapore (GIS), has now broken the deadlock by sequencing the genome of the
sweet orange1. The team has also revealed the fruit’s parentage: pummelo, which
is similar to grapefruit, and mandarin, a small and easy-peeling orange.
Xiaoan Ruan of GIS along with Qiang Xu and Ling-Ling
Chen of Huazhong Agricultural University, China, and their co-workers compared
the orange's genome with those of pummelo, C. grandis, and mandarin, C.
reticulata, using simple sequence repeat and single-nucleotide polymorphism
markers — two types of short and highly variable DNA sequence data.
One-quarter of the sweet orange’s markers matched
pummelo, and three-quarters matched mandarin. The researchers also knew that
the sweet orange’s chloroplast — the organelle that performs photosynthesis —
originated in pummelo, indicating that this fruit was the maternal parent.
Plants inherit DNA only from their ‘fathers’, whereas they inherit DNA,
chloroplasts and mitochondria from their ‘mothers’. Ruan and his co-workers
therefore inferred that the original breeders first crossed a female pummelo
with a male mandarin, and then crossed the resulting hybrid with a male
mandarin, resulting in the asexual sweet orange.
The research team also mined the sequence data to
uncover the genetic underpinnings of one of orange's most important traits:
production of vitamin C, a powerful antioxidant essential for connective tissue
building and wound-healing. They searched for genes similar to GalUR, which
produces a key enzyme in the vitamin C production pathway and found 18 copies.
Other vitamin-C-rich crops, such as papaya and apple, contain between 13 and 17
copies only. From studies of when and where genes are expressed during
development, the team observed that the GalUR genes are highly expressed in
orange fruits. “GalUR may be the most important contributor to vitamin C
accumulation in orange fruit,” says Ruan.
Availability of the sweet orange genome will
facilitate the study of many other important traits, including disease
resistance, flavor, sugar content and fruit color, the team notes. “The
findings provide new tools and approaches for future plant breeding using
genetic modification or engineering for high-yield vitamin C production,” says
Ruan.
The A*STAR-affiliated researchers contributing to this
research are from the Genome
Institute of Singapore
References
- Xu, Q., Chen, L.-L., Ruan, X., Chen, D., Zhu, A. et al. The draft genome of sweet orange (Citrus sinensis).Nature
Genetics 45, 59–66 (2013). | article
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