Why are oranges sour or sweet?

Why are oranges sour or sweet?

Why are oranges sour or sweet? Why are oranges sour or sweet?

The phenotype of the sweet orange bud mutation (somatic mutation) population photo courtesy Xu Qiang

The phenotype of the sweet orange bud mutation (somatic mutation) population photo courtesy Xu Qiang

Bingtang orange, brocade orange, sweet orange, blood orange, summer orange.
.
.
These oranges with different sweetness and sourness are all sweet oranges in plant taxonomy
.

The genomic background of sweet oranges in the world is exactly the same, and varieties with diverse flavors are mainly derived from somatic mutations

Recently, "Nature-Plants" published online a high-quality sweet orange reference genome constructed by the research group of Professor Xu Qiang of the College of Horticulture and Forestry of Huazhong Agricultural University
.

They clarified the genomic basis of somatic mutation of sweet orange for the first time, provided theoretical support for the bud mutation mechanism, and also provided genetic resources for fruit flavor breeding

Fruit quality is the main factor in the selection and breeding of sweet oranges

Fruit quality is the main factor in the selection and breeding of sweet oranges

According to Xu Qiang, the corresponding author of the paper, China is one of the origin and evolution centers of citrus plants in the world and has abundant germplasm resources
.

Most of the low-acid sprout materials are distributed in Asia

According to reports, the Chinese sweet oranges originally introduced by merchants from Persia (now Iran) and the Mediterranean are high-acid types.
Such high-acid fruits are easy to store and transport for long distances but have a bad flavor
.

"Perhaps with the fruit quality is closely related

"People are particularly concerned about fruit quality.
Fruit flavor is an important factor in the market value of fresh fruit strains
.

Wild citrus resources or raw materials are mostly high-acid materials

Wang Lun said that after long-term cultivation and breeding, citrus has rich variety resources and variation types, and new citrus varieties are also emerging
.

For example, most of the citrus varieties bred in the Mediterranean region are varieties with medium acidity

Studies have found that the genetic factors in the genome that cause the decrease of citric acid in the fruit of sweet orange buds are mainly caused by transposons
.

"Through large-scale research on the association between somatic budding strains and low acidity, it can provide new ideas for cultivating low acid citrus varieties

Somatic Variations Affecting the Flavor of Sweet Orange

Somatic Variations Affecting the Flavor of Sweet Orange

Xu Qiang introduced that most fruit trees reproduce asexually, and vegetative reproduction can fix the hybrid advantage and rapidly expand the population.
Therefore, bud mutation breeding based on somatic mutation has become an important breeding method for fruit trees
.

According to statistics, 60% of the citrus family to which sweet orange belongs are derived from bud mutation breeding, and 80% of sweet orange varieties are derived from bud mutation breeding

Sweet oranges are cultivated in 114 countries around the world, and most of the varieties are derived from somatic mutations
.
At the same time, "Because of the characteristics of apomixis and strict asexual reproduction under natural conditions, sweet oranges have become the model species for somatic mutation research
.
" Xu Qiang said
.

Wang Lun, the first author of the paper and the College of Horticulture and Forestry, Huazhong Agricultural University, explained that every time a eukaryotic cell divides, there is a certain probability that mutations will occur during the replication and repair of the genetic material DNA
.
In addition to germ cells, the number of somatic cells is huge, and somatic cell mutations can also occur
.
In human diseases, the accumulation of somatic mutations is closely related to the occurrence of cancer
.
In plants, somatic mutations can be passed on to the next generation, but people are less aware of somatic mutations in plants
.
Nevertheless, in fruit tree breeding, somatic mutation is widely used in bud mutation selection
.

Wang Lun told the Chinese Journal of Science that somatic mutations have shown a rich variety of phenotypes in fruit morphology, color, acidity, maturity, floral organ fertility, changes in flowering period, and tree structure
.

Citric acid is a key factor in the flavor of citrus fruits.
The team's previous research found that the sugar-acid ratio is the determinant of citrus fruits
.

The first author of the paper and doctoral student Liu Ziang introduced that the citric acid content of the 114 sprout materials of sweet oranges has changed significantly.
For example, the natural high-sour sweet oranges collected in southern Hunan of China have a fruit acid content close to that of lemons.
The citric acid content of sour orange is close to zero
.

"Finding somatic mutation is an important way to study the budding mechanism
.
" Xu Qiang said, but high-precision somatic mutation detection is very difficult, which creates difficulties for studying the population evolution of asexual reproduction populations
.

Xu Qiang believes that for plants, genome-wide somatic mutation detection and precise location prediction are the "golden keys" to open bud mutation research, which can be used to design molecular markers for later bud mutation materials and reveal the "true features" of various important bud mutations.

.

How to improve the prediction accuracy of mutation sites

How to improve the prediction accuracy of mutation sites

"Sweet orange is a hybrid offspring of orange and pomelo, and the genome is highly heterozygous
.
A high-quality genome can facilitate the detection of high-quality somatic mutations
.
" Xu Qiang told the China Science Journal, in order to analyze the genetic basis of somatic mutations in sweet oranges and dig out the impact on the fruit The key gene of sourness, they adopted the strategy of bud mutation population combined with genomics to improve the prediction accuracy of somatic mutation sites
.

First, the team established a high-quality reference genome of sweet orange, and mounted the assembled sequence on 9 chromosomes of sweet orange.
On average, each chromosome contained only 3 deletions
.
They identified 29,875 genes in total
.

"At present, this genome is currently the best genome in the citrus genus, and its quality is also among the best in all fruit tree genomes
.
" said Huang Yue, the first author of the paper
.

At the same time, they also completed the genomes of 5 sweet orange bud variant lines.
Through genome comparison, they found that there were large fragments of structural variation among the bud variant lines, as many as 2,321
.
877 transposon skipping events were further identified
.
These transposons were inserted into related genes in low-acid and acid-free mutants, which affected the pH and citric acid content of the fruit
.

Secondly, they collected 114 sweet orange bud change groups, "rich in number, covering various bud change resources at home and abroad, and are sufficiently representative for bud change research
.
" Jiang Dong, the author of the paper and a researcher at the Citrus Research Institute of Southwest University, said that these varieties include local varieties with a long history of cultivation in southern China, and cultivated varieties from the Mediterranean coast and the Americas
.

Through analysis and identification, the team determined that these budding materials are strictly clonal materials
.
"The background similarity is as high as 99.
99%, there is no hybridization event, no chromosomal recombination, and the differences between genomes are only derived from somatic mutations
.
" Wang Lun said
.

Then, they developed a big data analysis program suitable for the sweet orange buds
.
Wang Lun introduced that they randomly divided 114 sweet orange materials into 4 groups
.
By constructing a population comparison model, the signals of somatic mutations in the genome are carefully selected, and high-quality candidate mutations are screened based on the significance of statistical analysis to narrow the range of mutation signals
.

Finally, based on the above results, a PCR experiment was designed to verify the polymorphism of transposons in budding varieties
.
"The results show that the verification rate is more than 87%, and the predicted results are turned into actual usable data step by step
.
" Xu Qiang said that these data have become important basic data for the study of sweet orange populations for the study of the clonal evolutionary law and budding mechanism of sweet oranges.
, Asexual reproduction of crop breeding, etc.
provide reliable database support
.

At the same time, their research conclusions also support the historical spread of sweet oranges, and more somatic mutations have been detected in Mediterranean and American varieties
.

Related paper information: https://doi.
org/10.
1038/s41477-021-00941-x