Structure et ontogénie des nodules bactériens foliaires deI'Ardisia crenataSims (Myrsinaceae)

Abstract

Resume.-Ardisia crenata Sims vit en symbiose avec des bacteries localisees dans des nodules distribues sous Ia marge foliaire. Les bacteries sont presentes dans l'apex caulinaire don! les ebauches foliaires developpent des poils secreteurs qui emettent une substance enveloppant les bacteries. Dans Ia zone marginale des jeunes feuilles, un meristeme precurseur du nodule est present au voisinage de Ia future nervure marginale. Au-dessus, face adaxiale, un stomate precoce se ditferencie. Mots-cles : symbiose bacterienne foliaire -nodule foliaire -Ardisa crenata - Psychotria bacteriophyllaAbstract.-Ardisia crenata Sims lives in symbiosis with bacteria localized in nodules distributed under the leaf margin. Bacteria are present in the shoot apical region whose leaf sketches develop secretory trichomes which secrete a substance surrounding bacteria. In the marginal zone of the young leaves a meristem precursor of the nodule is present in the neighborhood of the future marginal nervure. Above, adaxial face, a premature stomate differs. Bacteria contained in the secreted substance are going to penetrate by the stomatal pore and to invade meristematic cells. In the functional nodule they are very plentiful towards the adaxial face and the infection cord surmounting them is visible in the grown-up leaf. On the base the marginal vein is neighboring to the nodule and numerous massifs of phloem line it laterally. In comparison with the bacterian leaf symbiosis of Psychotria bacteriophylla Val., Ardisia crenata by the premature implementation of a nodule meristem and an allowing stomate entry of bacteria appears in the specialization of this type of symbiosis a higher structural level.

The genome analysis of Candidatus Burkholderia crenata reveals that secondary metabolism may be a key function of the Ardisia crenata leaf nodule symbiosis

“…This low number reflects the absence of a flagellar apparatus and Type III or Type IV secretion systems. The lack of a flagellar apparatus has already been observed in other leaf nodule symbionts and indicate that transmission of the symbionts and invasion of plant tissues is passive, supporting the conclusions of Lebard and Belin‐Depoux (). Remarkably, Ca .…”

“…In vegetative buds, the formation of new leaves is followed by passive infection of sub-stomatal chambers by bacteria, suspended in mucilage surrounding the primordia. The nodule development process seems tightly controlled by the plant, and the distribution of nodules is pre-determined, as opposed to the almost random scattering of nodules on leaves of P. kirkii (Lebard and Belin-Depoux, 2003). The symbiosis is cyclic and the bacteria are vertically transmitted from one generation to the next, through colonization of flowers and seeds (Miller and Donnelly, 1987).…”

“…Like other leaf nodule symbioses, Ardisia sp. harbour a second, distinct population of the Burkholderia symbionts in the shoot apical and axillary buds (Miller, ; Lebard and Belin‐Depoux, ). In vegetative buds, the formation of new leaves is followed by passive infection of sub‐stomatal chambers by bacteria, suspended in mucilage surrounding the primordia.…”