Yaron Sitrit
The Jacob Blaustein Institutes for Desert Research, Israel
Title: Mycorrhizal association between the desert truffle Terfezia boudieri Chatin and Helianthemum sessiliflorum is mediated by fungal auxin and alters plant morphology, physiology and fitness to arid conditions
Biography
Biography: Yaron Sitrit
Abstract
The ectomycorrhizal (ECM) desert truffle Terfezia boudieri Chatin produces edible fruit bodies and forms symbiosis with its host plant Helianthemum sessiliflorum (Cistaceae) in the Negev desert of Israel. The symbiosis is vital for both partners' survival under desert conditions. Diurnal measurements revealed that mycorrhizal (M) plants had higher rates of photosynthesis (35%), transpiration (18%), and night respiration (49%) than non-mycorrhizal (NM) plants. Consequently, M plants exhibited higher biomass accumulation, higher shoot-to-root ratios, and improved water use efficiency compared to NM plants. Total chlorophyll content was higher in M plants, and the ratio between chlorophyll a to chlorophyll b was altered in M plants. Calculation of the photosynthetic activation energy indicated lower energy requirements for CO2 assimilation in M plants than in NM plants (48.62 kJ mol-1 and 61.56 kJ mol-1, respectively). Continuous measurements provided a complete picture of the daily physiological differences brought on by the ectomycorrhizal relationships. To secure a successful encounter, in the course of evolution, both partners have responded by evolving special signals exchange that facilitates recognition. Study of pre mycorrhizal signal exchange revealed previously unrecognized root-fungus interaction mediated by the fungal auxin. The secreted fungal auxin induced negative taproot gravitropism, attenuated taproot growth rate and inhibited initial host development. Auxin also induced expression of Arabidopsis carriers AUX1 and PIN1, both involved in the gravitropic response. Exogenous application of auxin fully mimicked that one induced by the ectomycorrhizal fungus. Co-cultivation of Arabidopsis auxin receptor mutants tir1-1, tir1-1 afb2-3, tir1-1 afb1-3 afb2-3 and tir1-1 afb2-3 afb3-4 with Terfezia confirmed that auxin is the signaling molecule, which induces the root phenotype. In a model proposed here, the fungal auxin induces horizontal root development, coordinates growth rates between partners, and lateral root induction that increases the availability of accessible sites for colonization at the soil plane of fungal spore abundance.