Fungal Endophyte Alternaria tenuissima Can Affect Growth and Selenium Accumulation in Its Hyperaccumulator Host Astragalus bisulcatus

Abstract

Endophytes can enhance plant stress tolerance by promoting growth and affecting elemental accumulation, which may be useful in phytoremediation. In earlier studies, up to 35% elemental selenium (Se0) was found in Se hyperaccumulator Astragalus bisulcatus. Since Se0 can be produced by microbes, the plant Se0 was hypothesized to be microbe-derived. Here we characterize a fungal endophyte of A. bisulcatus named A2. It is common in seeds from natural seleniferous habitat containing 1,000–10,000 mg kg-1 Se. We identified A2 as Alternaria tenuissima via 18S rRNA sequence analysis and morphological characterization. X-ray microprobe analysis of A. bisulcatus seeds that did or did not harbor Alternaria, showed that both contained >90% organic seleno-compounds with C-Se-C configuration, likely methylselenocysteine and glutamyl-methylselenocysteine. The seed Se was concentrated in the embryo, not the seed coat. X-ray microprobe analysis of A2 in pure culture showed the fungus produced Se0 when supplied with selenite, but accumulated mainly organic C-Se-C compounds when supplied with selenate. A2 was completely resistant to selenate up to 300 mg L-1, moderately resistant to selenite (50% inhibition at ∼50 mg Se L-1), but relatively sensitive to methylselenocysteine and to Se extracted from A. bisulcatus (50% inhibition at 25 mg Se L-1). Four-week old A. bisulcatus seedlings derived from surface-sterilized seeds containing endophytic Alternaria were up to threefold larger than seeds obtained from seeds not showing evidence of fungal colonization. When supplied with Se, the Alternaria-colonized seedlings had lower shoot Se and sulfur levels than seedlings from uncolonized seeds. In conclusion, A. tenuissima may contribute to the Se0 observed earlier in A. bisulcatus, and affect host growth and Se accumulation. A2 is sensitive to the Se levels found in its host’s tissues, but may avoid Se toxicity by occupying low-Se areas (seed coat, apoplast) and converting plant Se to non-toxic Se0. These findings illustrate the potential for hyperaccumulator endophytes to affect plant properties relevant for phytoremediation. Facultative endophytes may also be applicable in bioremediation and biofortification, owing to their capacity to turn toxic inorganic forms of Se into non-toxic or even beneficial, organic forms with anticarcinogenic properties.

The reduction of selenium(IV) by boreal Pseudomonas sp. strain T5-6-I – Effects on selenium(IV) uptake in Brassica oleracea

“…However, further characterization of the proteins is needed, to confirm their role in selenium accumulation. Previously high fractions (up to 35% of all selenium) of elemental reduced Se 0 has been reported in roots of Astragalus bisulcatus (Fabaceae), which was suggested to be produced by microbial endophytes (Lindblom et al 2012, Lindblom et al 2018). Reduced Se 0 was observed especially in the A. bisulcatus nodules and roots inoculated with Se 0 -producing fungi Alternaria astragali or A. tenuissima.…”

The diversity and distribution of endophytes across biomes, plant phylogeny and host tissues: how far have we come and where do we go from here?

“…halophiles and extremophiles). As an example, certain Astragalus taxa can hyperaccumulate selenium, and recent research has suggested that these plants may harbour unusual endophytic taxa that could influence selenium uptake (Sura‐de Jong et al ., ; Lindblom et al ., , ). Following a similar rationale, we also suggest surveying those plant families that are phylogenetically distinctive.…”

Effect of Sodium Selenite on the Metabolite Profile of Epichloë sp. Mycelia from Festuca sinensis in Solid Culture

“…When grown on high Se dosage media, Penicillium expansum, Aureobasidium pullulans, Mortierella humilis, and Phoma glomerata all exhibit a similar phenotype [30,32]. This could be because when certain fungi reduce elemental selenium from inorganic sodium selenite [4,12], the hyphal matrix provides sites for the resulting Se nanoparticles [32]. Selenium is also incorporated into other biological macromolecules, including proteins [33].…”

“…Currently available scientific research on selenium’s effect on fungal properties are focused on growth [ 2 ], morphology [ 4 , 5 ], antioxidation [ 6 – 8 ], selenium metabolomics [ 3 , 9 ], and chemical compounds [ 6 , 10 , 11 ]. In general, selenium tolerance is often associated with the accumulation of organic Se in certain fungi [ 12 ]. Previously, the selenium-tolerance mechanism of microorganisms has been investigated using genomic [ 13 ], proteomics [ 14 ], and metabolomic analysis [ 11 ].…”

“…Plant endophytes grow biotrophically in host grasses in nature, but nearly all may be grown saprotrophically in culture using potato dextrose agar (PDA) or nutrient agar (NA). They can help plants cope with stress by promoting growth and regulating selenium accumulation [ 12 , 16 ]. The asexual Epichloë endophytes are formally referred to as Neotyphodium spp.…”