First record of phytomelanin in aerial vegetative organs and its evolutionary implications in Lychnophorinae (Vernonieae: Asteraceae)

Abstract

Background and aimsPhytomelanin is a hard and resistant, brown to black substance found mainly in the reproductive organs of Asparagales and Asteraceae, and may serve to protect organs against external agents. In this study, we report for the first time the occurrence of phytomelanin in the stems and leaves of Asteraceae (Lychnophorinae, Vernonieae) and try to answer the following questions: How is phytomelanin distributed in aerial vegetative organs? What are the patterns of phytomelanin deposition in these organs? How did the evolution of phytomelanin occur in aerial vegetative organs of subtribe Lychnophorinae?. MethodsSamples of leaves and stems of 77 species were analyzed using standard techniques in plant anatomy. Ancestral character state reconstructions were performed using maximum likelihood (ML) assumptions, combining anatomical information with the phylogenetic history of Lychnophorinae. Key ResultsThe deposition of phytomelanin in Lychnophorinae is frequently associated with sclereids and occurs in intercellular spaces. Phytomelanin is mostly found in the secondary phloem of the thickened stem but may occur in all young and thickened stem tissues and in all leaf parts. The species can present different patterns of phytomelanin deposition, related to distinct processes of the formation of sclereids. Ancestral character state reconstructions suggest that the most recent ancestor of Lychnophorinae probably had phytomelanin in the secondary phloem and pith of the thickened stem. ConclusionsNew findings on occurrence, deposition, and evolution of phytomelanin in aerial vegetative organs are important advances in the knowledge of this subject, particularly for Asteraceae. The results indicate that the occurrence of phytomelanin, mainly in vegetative organs, is underestimated and understanding the distribution of this pigment may bring light to new evolutionary perspectives related to this complex plant family. In addition to this evidence, melanins are considered highly protective substances in cells of living organisms, with remarkable functions. Therefore, considering that studies of the chemical nature of this pigment show a basic structure common among melanized organisms, future melanin research in plants may reveal unexpected ecologicaland evolutionary implications.