Functional Categories of Root Exudate Compounds and their Relevance to AM Fungal Growth

Abstract

It is well established that plants grown under limited phosphorus (Pi) conditions are more readily colonized by arbuscular mycorrhizal (AM) fungi. It is also known that certain components of host root exudates can stimulate hyphal growth and branching of AM fungi and these compounds are elevated when the host is grown under Pi stress. To obtain a more general picture of the types of compounds exuded by host roots that effect the growth of AM fungi, a global analysis was performed on crude exudates of Ri TDNA-transformed carrot roots grown in the presence and absence of Pi. The results show that there is a distinct population of exudate compounds that are elevated in the absence versus the presence of Pi. Putative identifications were made for some of these compounds from data obtained by Fourier Transform Ion Cyclotron Mass Spectrometry (FTMS). The results were then compared to components of biologically active fractions purified by two dimensional thin layer chromatography (2D TLC). The data selection was restricted to compounds that were initially identified to be elevated in the crude – Pi exudate. The categories of compounds, such as plant growth regulators, phenols, flavonoids, and sesquiterpenoids were selected that had the most relevance to AM fungal/host interactions. The previous results with some of these compounds will be discussed in relation to new results obtained from FTMS and their involvement in presymbiotic growth of AM fungi. None of the reported strigolactones which stimulate AM fungi hyphal were found although several sesquiterpene lactones were identified. A number of hydroxy fatty acids were also found, and they were elevated in the – Pi crude exudate and found to be present in 2D TLC fractions. One hydroxy fatty acid, 2-hydroxytetradecanoic acid, stimulated lateral branching of the primary germ tube of Gigaspora gigantea, when applied to Petri dishes in amounts as low as 1–10 ng.