Abstract
Evidence is emerging that phytohormones represent key inter-kingdom signalling compounds supporting chemical communication between plants, fungi and bacteria. The roles of phytohormones for the lichen symbiosis are poorly understood, particularly in the process of lichenization, i.e. the key events which lead free-living microalgae and fungi to recognize each other, make physical contact and start developing a lichen thallus. Here, we studied cellular and extracellularly released phytohormones in three lichen mycobionts, Cladonia grayi, Xanthoria parietina and Tephromela atra, grown on solid medium, and the effects of indole-3-acetic acid (IAA) on their respective photobionts, Asterochloris glomerata, Trebouxia decolorans, Trebouxia sp. Using ultra-high-performance liquid chromatography coupled with tandem mass spectrometry (UHPLC-MS/MS) we found that mycobionts produced IAA, salicylic acid (SA) and jasmonic acid (JA). IAA represented the most abundant phytohormone produced and released by all mycobionts, whereas SA was released by X. parietina and T. atra, and JA was released by C. grayi only. With a half-life of 5.2 days, IAA degraded exponentially in solid BBM in dim light. When IAA was exogenously offered to the mycobionts’ compatible photobionts at “physiological” concentrations (as released by their respective mycobionts and accumulated in the medium over seven days), the photobionts’ water contents increased up to 4.4%. Treatment with IAA had no effects on the maximum quantum yield of photosystem II, dry mass, and the contents of photosynthetic pigments and α-tocopherol of the photobionts. The data presented may be useful for designing studies aimed at elucidating the roles of phytohormones in lichens.
Highlights
Of the phytohormones included in the UHPLC-MS/MS assay, indole-3-acetic acid (IAA), salicylic acid (SA) and jasmonic acid (JA) were detected in mycobionts and/ or extracellular exudates that were allowed to accumulate in the agar medium over seven days before extraction
Low amounts of IAA in T. atra (Fig. 3g), and of IAA and SA in X. parietina (Fig. 3d, e) were found cellularly, but considerable concentrations of these hormones were found to be released extracellularly by both species; normalized to fungal dry mass (DM), 42 and six times, respectively, higher amounts of IAA were found in exudates of T. atra (Fig. 3g) and X. parietina (Fig. 3d) than cellularly, and around 21 times higher amounts of SA (p value
The mycobionts isolated from the two foliose lichens, C. grayi and X. parietina, produced between two to three times more biomass compared to the mycobiont of the crustose lichen T. atra (Fig. 2), in agreement with the generally faster growth reported for foliose lichens compared to crustose ones (Armstrong and Bradwell 2010, 2011)
Summary
Phytohormones, including auxins, abscisic acid (ABA), jasmonates (JAs), salicylic acid (SA), brassinosteroids, cytokinins (CKs), ethylene, gibberellins (GAs) and strigolactones, are important signalling molecules in higher plants (Santner and Estelle 2009) with pivotal roles for plant development (Santner et al 2009), and they may play key roles in inter-kingdom signalling (Xu et al 2015; Hughes and Sperandio 2008) Other organisms such as green algae, cyanobacteria (Lu and Xu 2015), bacteria and fungi are capable of producing phytohormones and/or responding to these metabolites (Costacurta and Vanderleyden 1995; Tsavkelova et al 2006; Chanclud and Mortel 2016). The term phytohormone is somewhat misleading, but widely used and accepted, and we will use it in the paper with reference to fungal “phytohormones”
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