Transcriptome in Combination Proteome Unveils the Phenylpropane Pathway Involved in Garlic (Allium sativum) Greening

Garlic (Allium sativum) is an important vegetable crop that is widely used in cooking and medicine. The greening phenomenon of garlic severely decreases the quality of garlic and hinders garlic processing. To study the mechanism of garlic greening, comprehensive full-length transcript sets were constructed. We detected the differences in greening between Pizhou (PZ) garlic and Laiwu (LW) garlic that were both stored at −2.5°C and protected from light at the same time. The results showed that 60,087 unigenes were respectively annotated to the NR, KEGG, GO, Pfam, eggNOG and Swiss Prot databases, and a total of 30,082 unigenes were annotated. The analysis of differential genes and differential proteins showed that PZ garlic and LW garlic had 923 differentially expressed genes (DEGs), of which 529 genes were up regulated and 394 genes were downregulated. Through KEGG and GO enrichment analysis, it was found that the most significant way of enriching DEGs was the phenylpropane metabolic pathway. Proteomics analysis found that there were 188 differentially expressed proteins (DAPs), 162 up-regulated proteins, and 26 down-regulated proteins between PZ garlic and LW garlic. The content of 10 proteins related to phenylpropanoid biosynthesis in PZ garlic was significantly higher than that of LW garlic. This study explored the mechanisms of garlic greening at a molecular level and further discovered that the formation of garlic green pigment was affected significantly by the phenylpropanoid metabolic pathway. This work provided a theoretical basis for the maintenance of garlic quality during garlic processing and the future development of the garlic processing industries.

Garlic greening: Pigments biosynthesis and control strategies

Greening is a major problem for garlic’s quality. This phenomenon leads to discoloration of the product and is directly related to the alliinase-catalyzed conversion of isoalliin into 1-propenyl-containing thiosulfates. Garlic crushing, refrigeration, and storage in normal atmosphere, as well as in the presence of monocarboxylic acids, are established the main factors that promote its greening. In last decades, the study of biochemical pathway of this phenomenon has allowed to effectively understand the main steps and key enzymes involved, and to identify optimum conditions for chemical and enzymatic reactions leading to discoloration. These findings have, in some cases, determined the development of new tools for the control of garlic greening on large scale. After providing an updated description of the biochemistry of green pigments produced in garlic, this review reports an overview on the strategies for controlling discoloration of garlic at industrial level.

Analysis of Regulation Networks in the Formation of Green Pigment in Garlic

Background: Garlic (Allium sativum L.) has high medical and edible value, but is prone to producing green pigment during storage, which seriously affects product quality. Herein, we found multiple metabolic pathways involved in green pigment formation by analyzing the transcriptome and proteome changes of garlic stored at 4 °C for 0 day (DS0), 10 days (DS1), 15 days (DS2) and 40 days (DS3). Results: In this work, we found that porphobilinogen is a precursor of garlic green pigment and the citrate cycle (TCA cycle) can promote the formation of garlic green pigment by increasing the content of porphobilinogen. The salicylic acid (SA) inhibits garlic greening, whereas the lignin synthesis pathway positively correlates with greening. In addition to the above metabolic pathways, other pathways may be associated with the formation of garlic’s green pigment, such as glycolysis/gluconeogenesis, pyruvate metabolism and fatty acid metabolism. It has been reported that pyruvate and amino acids play a role in garlic greening, which were also demonstrated in this study. Conclusion: our results have important implications for understanding the mechanism of green pigment formation in garlic.