Metabolomic and transcriptomic insights into yellow leaf coloration in Ginkgo biloba

Ginkgo biloba is cultivated worldwide as a landscape tree species for its golden foliage. However, the molecular mechanisms underlying leaf yellowing in ginkgo remain largely unknown. Here, the physiological, metabolomic and transcriptomic analyses were performed on normal green leaves (GL) and yellow-leaf mutants (YL). The value of a* (redness) in YL was lower, and the values of L* (lightness), and b* (yellowness) and a*/b* ratio in YL were higher than those in GL. The concentrations of chlorophyll a, chlorophyll b, carotenoid and flavonoids in YL were lower, and the ratios of chlorophyll a/b and carotenoid/chlorophyll in YL were higher than those in GL. Consistently, a number of differentially abundant metabolites and differentially expressed genes implicated in chlorophyll biosynthesis and degradation and flavonoid biosynthesis were identified in YL vs. GL. For instance, the concentrations of protoporphyin IX, Mg-protoporphyin IX and protochlorophyllide, as late-stage chlorophyll biosynthesis intermediates, in YL were lower when compared with those in GL. Concurrently, the transcript levels of genes associated with chlorophyll degradation, including chlorophyllase (CLH), non-yellowing (NYE) and red chlorophyll catabolite reductase (RCCR), were upregulated in YL vs. GL. Additionally, 85 differentially expressed transcription factors, such as ERFs, bHLHs and MYBs, were identified in YL vs. GL. Overall, these results indicate that chlorophyll concentration and proportion are significant for the leaf yellowing, and the limited chlorophyll biosynthesis and accelerated chlorophyll degradation as well as differentially expressed transcription factors are essential for the lower chlorophyll concentration in yellow-leaf ginkgo mutant.