Date Published: May 21, 2019
Publisher: Public Library of Science
Author(s): Shuzhen Zhang, Xiaolu Wu, Jie Cui, Fan Zhang, Xueqin Wan, Qinglin Liu, Yu Zhong, Tiantian Lin, Siva Ramamoorthy.
Populus deltoides Marsh has high ornamental value because its leaves remain yellow during the non-dormant period. However, little is known about the regulatory mechanism of leaf coloration in P. deltoides Marsh. Thus, we analyzed the physiological and transcriptional differences of yellow leaves (mutant) and green leaves (wild-type) of P. deltoides Marsh. Physiological experiments showed that the contents of chlorophyll (Chl) and carotenoid were lower in mutant leaves, and the flavonoid content did not differ significantly between mutant and wild-type leaves. Transcriptomic sequencing was further used to identify 153 differentially expressed genes (DEGs). Functional classifications based on Gene Ontology enrichment and Genome enrichment analysis indicated that the DEGs were involved in Chl biosynthesis and flavonoid biosynthesis pathways. Among these, geranylgeranyl diphosphate (CHLP) genes associated with Chl biosynthesis showed down-regulation, while chlorophyllase (CLH) genes associated with Chl degradation were up-regulated in yellow leaves. The expression levels of these genes were further confirmed using quantitative real-time PCR (RT-qPCR). Furthermore, the estimation of the main precursors of Chl confirmed that CHLP is a vital enzyme for the yellow leaf color phenotype. Consequently, the formation of yellow leaf color is due to the disruption of Chl synthesis or catabolism rather than flavonoid synthesis. These results contribute to our understanding of mechanisms and regulation of leaf color variation in poplar at the transcriptional level.
Leaf color is an important feature of ornamental plants, and trees with colored leaves have been widely cultivated in landscape gardens. The main factors that determine foliage color are the pigment types and their relative concentrations. The formation of red leaves is the result of anthocyanin accumulation, which has been extensively studied . In contrast, there are only a few studies focused on the mechanism of yellow leaves. Leaf yellowing is generally considered to be caused by decreased chlorophyll (Chl) content because Chl is the main pigment content of green leaves . Therefore, studies of leaf yellowing have mostly focused on Chl biosynthesis and degradation. In addition, leaf yellowing may be due also to the accumulation of flavonoids, such as flavanol, flavonol, chalcone and aurone [3,4].
P. deltoides Marsh developed yellow leaves during growth and throughout the lifespan by bud mutation. Understanding the regulatory mechanisms underlying the leaf color of P. deltoides Marsh is of significant importance. Changes in ratio of Chl, carotenoids and flavonoids in leaves will change the expression of leaf color. In this study, the compounds affecting the pigment of leaves were determined. The Chl content in the mutant P. deltoides Marshwas significantly lower than that of wild-type. Consistent with the physiological results, transcriptional analysis demonstrated that down-regulation of CHLP and up-regulation of CLH involved in porphyrin and Chl metabolism pathways were crucial genes that resulted in yellow leaves.
In this study, physiological and transcriptome sequence analysis showed that there were distinct differences in coloration between green and yellow mutant leaves of P. deltoides Marsh. Transcriptional sequence analysis identified 5 DEGs that participated in porphyrin and Chl metabolism and flavonoid biosynthesis pathways. Furthermore, RT-qPCR verified that those DEGs were expressed differentially in mutant and wild-type plants. Down-regulation of CHLP and up-regulation of CLH might cause the difference of leaves. These results provide an excellent platform for future studies to uncover the molecular mechanisms underlying the yellowing phenotype in P. deltoides Marsh and other closely related species.