-SDS-PAGE
III. Caractérisation fonctionnelle de la protéine LFY FL
6. Quelle relation existe-il entre LFY et l’apparition des plantes à fleurs ?
A mon avis, les Angiospermes ou plantes à fleurs constituent à n’en pas douter la plus belle invention de la nature ! L’apparition de la fleur constitue un avantage sélectif sans égal, succès à l’origine d’une véritable invasion des Angiospermes qui en l’espace de 120 millions d’années ont développé pas moins de 250000 espèces. Selon l’arbre phylogénétique des plantes terrestres, les plantes à fleur sont monophylétiques et ne seraient donc apparues qu’au niveau d’un seul foyer de localisation inconnue. C’est cette origine unique qui expliquerait le fossé morphologique existant aujourd’hui entre les Angiospermes et leurs prédécesseurs, les Gymnospermes. Malgré les efforts entrepris par les botanistes et biologistes-théoriciens de l’évolution, les mécanismes mis en place par
153 les plantes pour expliquer une telle apparition n’ont pas été élucidés. Cette recherche est d’autant plus difficile qu’on ne connaît que très peu de choses sur l’ancêtre des Angiospermes et l’ancêtre commun des Angiospermes et Gymnospermes et que les fossiles connus apportent peu de renseignements quant à leurs caractéristiques morphologiques. Le seul recours à la compréhension de l’apparition des fleurs sur terre semble donc une étude fonctionnelle évolutive des acteurs importants à cette transition.
Les gènes ABCE responsables de l’identité des organes floraux sont une cible de choix, et LFY en tant que régulateur de l’expression de ces gènes en est une autre.
Mon travail de thèse apporte des éléments de réflexion utiles à la compréhension des mécanismes ayant conduit à l’évolution des fonctions de LFY. Cependant expliquer comment LFY a évolué exigera un investissement de plusieurs années, voire décennies. En plus de l’étude sur le domaine conservé C-terminal, il nécessitera l’étude de régions nettement moins caractérisées comme la région conservée N-terminale et les régions non conservées. La difficulté sera d’autant plus accrue que cette évolution peut impliquer la coévolution de résidus qui indépendamment n’ont pas apporté de transition fonctionnelle majeure.
Pour soulager en partie ces difficultés, l’approche de synthèse fonctionnelle (‘functional
synthesis’; Dean and Thornton, 2007) alimente de grands espoirs sur le principe de résurrection de
protéines éteintes. Il est pour cela nécessaire de prédire à partir d’un alignement de séquences entre homologues de LFY issus des principaux groupes végétaux quelle était la séquence des protéines LFY présentent chez l’ancêtre des Angiospermes et l’ancêtre commun des Angiospermes et Gymnospermes. Parvenir à produire ces protéines ‘ancestrales’ et à analyser leurs propriétés par approche biochimique pourrait apporter des éléments de compréhension sur l’évolution du mode de fonctionnement de LFY, ce qui constituerait de précieuses indications sur le rôle évolutif joué par cette protéine. L’‘abominable mystère’ de Charles Darwin trouvera peut-être un jour une solution !
155
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