Combined analysis of metabolome and transcriptome provides insights into metabolisms of chlorophylls, carotenoids, and flavonoids in the yellowing leaves of?HAES344? macadamia

Macadamia variety 'HAES344' is widely popularized in macadamia producing areas around the world due to its premium edible kernels. However, 'HAES344' is prone to generate the etiolated leaves under high temperature, but the molecular and metabolic mechanisms of leaf yellowing remain unclear. In this study, an integrative analysis of the metabolome and transcriptome profiles was performed for the 'HAES344' leaves at four different stages. We found that the ratio of carotenoids to chlorophylls increased in the yellowing leaves. A total of 131 differential accumulated metabolites had been obtained, of which 35 were flavonoids, and 27 out of 35 flavo-noids were identified between the green and the etiolated leaves. The major flavonoids including 10 flavonols, 10 flavones, and 5 flavanones were significantly up-regulated in the yellowing leaves. RNA-Seq revealed 10,301 differential expressed genes, of which 5, 4, and 58 genes involved in the metabolism of chlorophylls, carotenoids, and flavonoids, respectively. Transcription factors AP2/ERFs (20), WRKY (17), MYBs (17), bHLH (12), C2H2 (12), NAC (10), HD-ZIP (8), HSF (7), bZIP (4) and C3H (4), with significant difference, were also analyzed. The increasing expression of CHS, CHS1, CHS2, CHS7, CYP75A2, F3H, UGT75L6, UGT78D2, and RhGT1 might play a key role in the flavonoid accumulation, and a regulatory network of flavonoid biosynthesis was established. The AP2/ERFs, WRKY, MYBs, NAC, bHLH, and C2H2 families were commonly involved in chlorophyll, carotenoids, and flavonoid metabolism. This result of the differentially expressed genes and metabolites showed that the metabolome and transcriptome data were positively connected with the leaf yellowing of 'HAES344' macadamia.