| Sweetpotato [Ipomoea batatas(L.)Lam.] is an important food,feed,vegetable,industrial raw material and bioenergy crop,and its storage roots are rich in carbohydrates,starch,protein,carotenoids,anthocyanins,minerals,vitamins and other nutrients,which are the main objects of sweetpotato production and utilization.Browning of sweetpotato storage roots due to trauma exists during harvest,transportation,storage and processing,which affects the commerciality of fresh sweetpotato and the quality of processed products.At present,most of the reports on sweetpotato browning phenomenon are focused on the screening of anti-browning inhibitors,and there are few reports on the screening of anti-browning germplasm resources and the molecular mechanisms of browning effects.Based on above,this paper carried out the following two aspects of research:(1)Based on the study of browning degree values of 87 new lines(including a few control varieties)in our sweetpotato breeding process,21 sweetpotato breeding lines with orange and red flesh storage roots were selected as test materials for further research accoeding to the context of present industrial processing needs,about thier differences in browning degree of storage roots and browning-related enzyme activities;(2)Based on the above experimental data,one sweetpotato germplasm accession prone to browning and one germplasm accession resistant to browning were selected for fresh-cut inducing treatment to determine the changes in root slice appearance quality and browing physiological indexes during shelf life,and the two accessions browning molecular mechanism were investigated by combining RNA-Seq sequencing technology to obtain the relevant pathways and genes that may affect the browning of sweetpotato storage roots.The results are expected to lay the foundation for breeding germplasm screening against browning and clarifying the mechanism of sweetpotato storage roots browning.The research results of this paper are as follows:1.The browning degree of sweetpotato storage root was highly significant different among genotypes,and the browning was more likely to occur and aggravate as the flesh color of the storage root deepened or the dry matter content increased.The clustering analysis of the flesh color values L,a*,b*,C and h°of storage roots flesh color and the grouping analysis of storage root dry matter content data divided the 87 test materials into six flesh color groups(white,yellowish,yellow,orange,orange-red and purple)and nine dry matter content groups.The results of multiple comparisons showed that the browning degree not only differed significantly among genotypes,but also among different flesh color groups,and as the flesh color of the storage roots deepened,the browning was more likely to occur and the browning degree value(BD)was more larger,characterized by the smallest browning degree in the white flesh color group and the larger browning degree in the orange and purple flesh color groups.The bias correlation analysis showed that the BD was significantly negatively correlated with the L value of flesh color and significantly positively correlated with the a*value of color,and the bias correlation analysis showed that the BD significantly positively correlated with the dry matter content when the color value was numerical similarity.This provides a reference for the selection and elimination of lines in the breeding process of sweetpotato against browning.2.Based on the cluster analysis of browning and its related enzyme activities of 21 sweetpotato genotypes with orange and red flesh storage root(ORF),two germplasm resources not prone to browning were screened.Based on the actual development and needs of sweetpotato industry,21 ORF were screened from 87 test materials to further study the browning degree and browning-related enzymes(PAL,PPO,POD)activities of storage roots.The results showed that there were significant differences in BD and related enzyme activities among 21 ORF,with a 3.72-fold difference between the maximum and minimum of BD,and 8.29-fold,2.36-fold and 3.21-fold differences between the maximum and minimum of PAL,PPO and POD activities,respectively,among 21 ORF.Cluster analysis of BD,PAL,PPO and POD activity indexes classified the 21 ORF into four groups,of which group I with heavy browning contained 1 accession,group II with moderate browning contained 7 accessions,group III with light browning contained 11 accessions,and group IV with not easily browning contained 2 accessions.It shows that most of the sweetpotato accessions are in the middle level of browning.Group I with heavy browning line 161828 can be further used as a material to investigate the browning mechanism,and group IV with browning resistant lines171404 and 173307 can be used as excellent resources for sweetpotato storage root browning resistance breeding.3.Significant differences exist in appearance phenotypes and physiological indicators of sweetpotato storage root slices with different degrees of browning after storage root fresh-cutting induction during shelf life.After fresh cutting treatment on storage root of group I heavy browning accession 161828 and group IV less browning prone accession 173307,three methods,namely image observation method,colorimeter measurement method and spectrophotometric method,were applied to evaluate the changes in appearance phenotype of slices during shelf life.It was found that browning not only caused significant increase of brown spots on the 161828 slices surface,but also caused damage to its tissue microstructure;the brightness L value of 161828 slice gradually decreased during shelf life and was significantly lower than that of 173307 slices,and the chromaticity values a* and b* of the two accessions did not differ significantly in the early storing period.The browning-related enzyme activity,phenolic content and malondialdehyde content of 161828 slices during shelf life were significantly higher than those of 173307 slices.Correlation analysis showed that BD was significantly and positively correlated with browning-related enzyme activity,phenolic substance content and malondialdehyde content,indicating that the degree of these physiological changes was more pronounced and browning was more likely to occur in the browning-prone accession due to the increase in phenolic substance content,related enzyme activity and cell membrane damage in the slices tissues induced by mechanical trauma by fresh-cutting.4.Transcriptome sequencing-based analysis obtained relevant pathways and genes that may affect sweetpotato storage roots browning.Transcriptome sequencing analysis of two sweetpotato accessions,161828,which is susceptible to browning,and 173307,which is not susceptible to browning,yielded a total of 517284668 high-quality reads,of which about 83% were compared to the reference genome,while 38979 differential genes were identified,and enrichment analysis indicated that the benzopropane biosynthesis pathway is closely related to sweetpotato browning,with PAL as the representative of the expression of the genes related to benzene propane biosynthesis was consistently higher in 161828 than in 173307,mainly involving 58 differential genes of the three major gene families(i.e.PAL,PPO,POD)and 20 genes significantly up-regulated in the benzene propane biosynthesis pathway,which may be involved in the browning process of sweetpotato.Meanwhile,the transcriptome sequencing results were verified by fluorescence quantitative PCR to be reliable.The preliminary exploration of these browning-related genes will lay a theoretical foundation for the selection and breeding of browning-resistant varieties in the future,and help provide a scientific basis for the browning genetic improvement of sweetpotato varieties by the molecular method.In summary: present study of the BD of 87 new lines in our sweetpotato breeding process showed that the storage roots were susceptible to browning as the root flesh color deepened and the dry matter content accumulated.Two sweetpotato germplasm resources 173307 and 171404 were screened among 21 ORF genotypes,and the browning of sweetpotato storage roots was significantly positively correlated with PAL,PPO,POD enzyme activities,phenolic content,and malondialdehyde content.RNA-Seq sequencing analysis indicated that 78 genes involved in phenylpropane biosynthesis represented by PAL may be involved in the browning process. |
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