title: The velamen protects photosynthetic orchid roots against UV-B damage, and a large dated phylogeny implies multiple gains and losses of this function during the Cenozoic.
abstract: UV-B radiation damage in leaves is prevented by epidermal UV-screening compounds that can be modulated throughout ontogeny. In epiphytic orchids, roots need to be protected against UV-B because they photosynthesize, sometimes even replacing the leaves. How orchid roots, which are covered by a dead tissue called velamen, avoid UV-B radiation is currently unknown. We tested for a UV-B protective function of the velamen using gene expression analyses, mass spectrometry, histochemistry, and chlorophyll fluorescence in Phalaenopsis × hybrida roots. We also investigated its evolution using comparative phylogenetic methods. Our data show that two paralogues of the chalcone synthase (CHS) gene family are UV-B-induced in orchid root tips, triggering the accumulation of two UV-B-absorbing flavonoids and resulting in effective protection of the photosynthetic root cortex. Phylogenetic and dating analyses imply that the two CHS lineages duplicated c. 100 million yr before the rise of epiphytic orchids. These findings indicate an additional role for the epiphytic orchid velamen previously thought to function solely in absorbing water and nutrients. This new function, which fundamentally differs from the mechanism of UV-B avoidance in leaves, arose following an ancient duplication of CHS, and has probably contributed to the family's expansion into the canopy during the Cenozoic.
authors.affiliation: Systematic Botany and Mycology, Department of Biology, University of Munich (LMU), Munich, 80638, Germany.
authors.forename: Guillaume
authors.initials: G
authors.lastname: Chomicki
authors.affiliation: INRA, UMR AGAP, Montpellier, France.
authors.forename: Luc P R
authors.initials: LPR
authors.lastname: Bidel
authors.affiliation: State Key Laboratory of Genetic Engineering, Institute of Genetics, Shanghai, 200433, China.|Institute of Plant Biology, School of Life Science, Fudan University, 220 Handan Road, Shanghai, 200433, China.
authors.forename: Feng
authors.initials: F
authors.lastname: Ming
authors.affiliation: Institute of Agricultural Sciences, Plant Biochemistry, ETH Zurich, 8092, Zurich, Switzerland.
authors.forename: Mario
authors.identifier: http://orcid.org/0000-0002-0113-0320
authors.initials: M
authors.lastname: Coiro
authors.affiliation: State Key Laboratory of Genetic Engineering, Institute of Genetics, Shanghai, 200433, China.|Institute of Plant Biology, School of Life Science, Fudan University, 220 Handan Road, Shanghai, 200433, China.
authors.forename: Xuan
authors.initials: X
authors.lastname: Zhang
authors.affiliation: State Key Laboratory of Genetic Engineering, Institute of Genetics, Shanghai, 200433, China.|Institute of Plant Biology, School of Life Science, Fudan University, 220 Handan Road, Shanghai, 200433, China.
authors.forename: Yaofeng
authors.initials: Y
authors.lastname: Wang
authors.affiliation: UMR DIADE (UM2/IRD), SMART Team, University of Montpellier 2, Place Eugene Bataillon, Montpellier, F-34 095, France.
authors.forename: Yves
authors.initials: Y
authors.lastname: Baissac
authors.affiliation: UMR DIADE (UM2/IRD), SMART Team, University of Montpellier 2, Place Eugene Bataillon, Montpellier, F-34 095, France.
authors.forename: Christian
authors.initials: C
authors.lastname: Jay-Allemand
authors.affiliation: Systematic Botany and Mycology, Department of Biology, University of Munich (LMU), Munich, 80638, Germany.
authors.forename: Susanne S
authors.initials: SS
authors.lastname: Renner
mesh_terms: D000217:Acyltransferases / Q000235:genetics / Q000378:metabolism; D047310:Apigenin / Q000378:metabolism; D003433:Crosses, Genetic; D005419:Flavonoids / Q000378:metabolism; D020440:Gene Duplication; D018506:Gene Expression Regulation, Plant / Q000528:radiation effects; D005960:Glucosides / Q000378:metabolism; D029595:Orchidaceae / Q000235:genetics / Q000502:physiology* / Q000528:radiation effects*; D010788:Photosynthesis; D010802:Phylogeny; D018517:Plant Roots / Q000033:anatomy & histology* / Q000528:radiation effects*; D013312:Stress, Physiological / Q000528:radiation effects; D013997:Time Factors; D014466:Ultraviolet Rays
chemical_list: D005419:Flavonoids; D005960:Glucosides; C457771:saponarin; D047310:Apigenin; D000217:Acyltransferases; C020141:flavanone synthetase; C049772:isovitexin
keywords: UV-B; chalcone synthase; dated phylogeny; epiphytes; flavonoids; gene duplication; orchids; velamen