Mechanisms underlying the formation of reddish orange anthers and pollens in Asian lotus (Nelumbo nucifera Gaertn.)

Abstract

The coloration of anther and pollen significantly influences the success of sexual reproduction in angiosperms, which, particularly in pollen, is accompanied with the accumulation of various pigments and other secondary metabolites, such as carotenoids, flavonoids, and anthocyanins. In recent years, the mechanisms underlying the formation of sporopollenin and flavonoids on pollen surface have been extensively investigated. However, research on carotenoid formation remains limited due to the absence of relevant mutants. In this study, we discovered a unique mutant with reddish orange anther and pollen for the first time in Asian lotus (Nelumbo nucifera Gaertn.) among over 2000 cultivars. The color transitioned from white to yellow, ultimately to reddish orange along with the anther development and the appearance was attributed to the elevated levels of lycopene, β-carotene and β-apocarotenal. The transmission electron microscopy (TEM) revealed that the carotenoids were synthesized in both pollen and tapetal cells, and especially the formation of carotenoids in young microspores preceded that in the tapetum. This was the first time to disclose that the carotenoids could exist inside the young microspores. The accumulation of carotenoids on the surface of mature pollens might be the combined contribution of microspores and tapetum. The key enzyme-encoding genes related to carotenoid metabolism were all highly-expressed specifically in reddish orange anthers, among which the high enzymatic activity of CCD4 led to the formation of apocarotenoid. These findings were of great fundamental significance for promoting the regulation mechanism of carotenoid formation on pollen surface and for breeding new cultivars with colorful anthers in the future.