In Vanilla Orchids Bacterial Biofilms in Shoot Meristems Fix Nitrogen and Transfer it to Plant Tissues
James F. White, Jr.1, Mónica S. Torres1, Holly Johnson2, Ivelisse Irizarry1, Qiang Chen1, Chaim Frenkel1, Daphna Frenkel1, Faith Belanger1
1 Rutgers University,
New Brunswick, NJ
2Department of Computer Science, Central Washington University
Shoot apex bacterial biofilms in a cultivated hybrid variety of vanilla orchid (Vanilla pompona x V. planifolia) fix atmospheric nitrogen and fixed nitrogen flows into tissues of the shoot apex, from where it is transported into older tissues. Both V. planifolia and V. pompona possess similar vase-like shoot tip arrangements. Isotopic nitrogen (15N2) gas assimilation experiments were conducted on two cultivated varieties of Orchidaceae, including a hybrid vanilla (Vanilla pompona x V. planifolia, obtained in Puerto Rico) and a commercial Oncidium sp. After removal of the substrate or soil roots, both plants were placed in a 8-liter chamber where the air was enriched with approximately 20 ml of high purity 15N2 gas. A set of control plants, treated similarly, was incubated in lab ambient air. After 1-wk plant tissue samples of leaves, aerial roots, and pseudobulbs (Oncidium sp.) were subjected to mass spectroscopic analysis to determine sites of assimilation of 15N. Assimilation of 15N2 gas was detected only in the vanilla plant where assimilation was highest in leaves closest to the shoot meristem and decreased proportionately with distance from the shoot apex. Microscopic examination of sections of the vanilla shoot apex revealed the presence of a bacterial biofilm between layers of leaves surrounding the shoot apical meristem. Isolations of bacteria from shoot tips on nitrogen free agarose media demonstrated the presence of several diazotrophic species. Vanilla orchids appear to be adapted to obtain nutrients from microbiome bacteria through use of a vase-like shoot apex that collects and cultivates diazotrophic bacteria around the meristematic tissues.
Dr. James White is a Professor in the Department of Plant Biology & Pathology at Rutgers University. Dr. White studies the biology of non-pathogenic and beneficial microbes that associate with plants. His work over the past 29 years has focussed on microbes that become endophytic in plants and affect the ecology of the host plant. Over the past 4 years Dr. White has been examining plant microbiomes and their defensive and nutritional functions.