Loquat (Eriobotrya japonica (Thunb.) circadian clock gene cloning and heterosis studies of artificial triploid loquat

Abstract

Previous observation in our laboratory found that growth vigor of triploid loquats was greater than in diploids and tetraploids despite the existence of ploidy effects, but it is still lack of systematic studies (i.e., histological, physiological). In addition, the underlying mechanisms of the heterosis are largely unknown due to the fact that the origin and genetic pedigree are unclear. In this study, we sought to further confirm the heterosis of triploid loquat systematically and elucidate the relationship between heterosis and circadian rhythms in triploid loquat by using trees with defined genetic pedigree. Two sets of triploid loquat were generated by crossing a tetraploid loquat with two wild diploid loquats (GC-1 and GC-23 from Guizhou Province, China (GC). Heterosis was systematically evaluated by comparing various histological, physiological and disease resistance metrics to the parental types. The results showed that the two sets of triploid loquat generated here exhibited pronounced degree of heterosis in all three categories and this was consistent with our previous observation. To determine if heterosis was also affected by the circadian clock, three circadian clock genes (EjLHY, EjTOC1 and EjGIGANTEA) were screened out, and the expression level of three circadian clock genes and 7 clock output genes were also analyzed. Phylogenetic analysis of three circadian genes showed that they were highly homologous with any other species in Rosaceae family. QRT-PCR analysis of the three circadian clock genes showed that all of them expressed above-high-parent (AHP) in almost all of the hybrids. Surprisingly, the 7 clock output genes also demonstrated AHP expression pattern in almost all the hybrids, and this was consistent with the results from our histological, physiological and disease resistance analysis. Our results indicated that triploid loquat exhibits pronounced heterosis compared with the diploid and tetraploid loquats, and the triploid loquat heterosis may be related to the altered amplitude of circadian rhythms.