Relevance of Class I α-Mannosidases to Cassava Postharvest Physiological Deterioration

Comparative Transcriptome Profiling of Cassava Tuberous Roots in Response to Postharvest Physiological Deterioration

Cassava is one of the most versatile tuberous-root crops on Earth. However, the postharvest storage properties of cassava tuberous root mean that it is perishable through a process known as postharvest physiological deterioration (PPD), which seriously affects its starch quality. Therefore, a comprehensive understanding of the transcriptional regulatory activity of cassava against the PPD response is necessary in order to extract key molecular mechanisms related to PPD tolerance. In this study, we found that RYG1 tuberous roots showed delayed PPD compared to those of SC8. In addition, RYG1 roots maintained a more stable cell wall structure after storage than those of SC8. The transcriptome changes in tuberous roots were analyzed for both RYG1 and SC8 after 21 days of storage (SR and SS) compared to fresh (FR and FS) by the RNA-Seq method. The total number of differentially expressed genes (DEGs) in the various comparisons of these four samples ranged from 68 to 3847. Of these, a total of 2008 co-DEGs in SR vs. SS were shared by either SR vs. FR or SS vs. FS. GO and KEGG enrichment analysis revealed that upregulated co-DEGs in SR vs. SS were mainly enriched in photosynthesis, protein processing, hormone and cutin, suberine and wax biosynthesis. By contrast, the downregulated co-DEGs were mainly related to cell wall organization, starch and sucrose metabolism, galactose metabolism, phenylpropanoid biosynthesis, diterpenoid biosynthesis, cysteine and methionine metabolism and flavonoid biosynthesis. The protein-protein interaction (PPI) networks of the co-DEGs showed a complex interaction of genes in different pathways, and 16 hub genes were characterized to have a degree in excess of 15, among which eight genes were associated with photosynthesis. These results provide new information for the study of cassava resistance to PPD and lay a foundation for the further molecular breeding of storage-tolerant cassava varieties.

Systematic Analysis of bHLH Transcription Factors in Cassava Uncovers Their Roles in Postharvest Physiological Deterioration and Cyanogenic Glycosides Biosynthesis

The basic helix-loop-helix (bHLH) proteins are a large superfamily of transcription factors, and play a central role in a wide range of metabolic, physiological, and developmental processes in higher organisms. However, systematic investigation of bHLH gene family in cassava (Manihot esculenta Crantz) has not been reported. In the present study, we performed a genome-wide survey and identified 148 MebHLHs genes were unevenly harbored in 18 chromosomes. Through phylogenetic analyses along with Arabidopsis counterparts, these MebHLHs genes were divided into 19 groups, and each gene contains a similar structure and conserved motifs. Moreover, many cis-acting regulatory elements related to various defense and stress responses showed in MebHLH genes. Interestingly, transcriptome data analyses unveiled 117 MebHLH genes during postharvest physiological deterioration (PPD) process of cassava tuberous roots, while 65 MebHLH genes showed significantly change. Meanwhile, the relative quantitative analysis of 15 MebHLH genes demonstrated that they were sensitive to PPD, suggesting they may involve in PPD process regulation. Cyanogenic glucosides (CGs) biosynthesis during PPD process was increased, silencing of MebHLH72 and MebHLH114 showed that linamarin content was significantly decreased in the leaves. To summarize, the genome-wide identification and expression profiling of MebHLH candidates pave a new avenue for uderstanding their function in PPD and CGs biosynthesis, which will accelerate the improvement of PPD tolerance and decrease CGs content in cassava tuberous roots.