Exploration of mechanism underlying the lipid alterations in the yellowing leaves of 'HAES344' macadamia

Macadamia variety 'HAES344', cultivated widely in tropical and subtropical regions for its premium edible kernels, is prone to generate the yellowing leaves under high temperature, resulting in a decrease of yield. Lipids are the important components of membrane and cuticle in leaves that perform the vital roles in plant growth, development, and responses to environmental fluctuations. However, the profile and the metabolic mechanism of lipids during leaf yellowing of 'HAES344' macadamia remain unclear. In this study, the lipid composition and the transcript of genes related to lipid metabolism were explored, based on an integrative analysis of metabolome and transcriptome. The results showed that a total of 22 differential accumulated lipids, including 9 glycerolipids, 5 free fatty acids (FAAs), 5 lyso-phosphatidylcholines (lysoPC) and 3 lyso-phosphatidylethanolamines (lysoPE), were identified, and all these lipids were significantly down-regulated in the yellowing leaves. Transcriptomic analysis revealed 132 differential expressed genes (DEGs) related to lipid metabolism, among 71 DEGs with a mean FPKM >1.0 were selected and classified into 10 pathways of lipid metabolism, and the largest proportion of genes involved in the cutin, suberine and wax biosynthesis and the alpha-linolenic acid metabolism, while the least genes participated in the sphingolipid metabolism. Of these 71 DEGs, the most were significantly down-regulated in the moderately and/or severely yellowing leaves compared with the green leaves. Correlation analysis showed that a total of 33 DEGs, mainly including 3 HHT1 , 2 DOX1 , 2 CYP86B1 and 2 KCS12 , were significantly correlated with all these 22 lipids except for lysoPE 18:2 (2n isomer) and octadeca-11 E ,13 E ,15 Z - trienoic acid. And a regulatory network of lipid metabolism was established. This result of the differentially expressed genes was positively connected with the reduced lipid metabolites during macadamia leaf yellowing.