Abstract
Multiple functions of glomalin released by arbuscular mycorrhizal fungi are well-recognized, whereas the role of exogenous glomalins including easily extractable glomalin-related soil protein (EE-GRSP) and difficultly extractable glomalin-related soil protein (DE-GRSP) is unexplored for plant responses. Our study was carried out to assess the effects of exogenous EE-GRSP and DE-GRSP at varying strengths on plant growth and chlorophyll concentration of trifoliate orange (Poncirus trifoliata) seedlings, along with changes in root nutrient acquisition, auxin content, auxin-related enzyme and transporter protein gene expression, and element contents of purified GRSP. Sixteen weeks later, exogenous GRSP displayed differential effects on plant growth (height, stem diameter, leaf number, and biomass production): the increase by EE-GRSP and the decrease by DE-GRSP. The best positive effect on plant growth occurred at exogenous EE-GRSP at ½ strength. Similarly, the GRSP application also differently affected total chlorophyll content, root morphology (total length, surface area, and volume), and root N, P, and K content: positive effect by EE-GRSP and negative effect by DE-GRSP. Exogenous EE-GRSP accumulated more indoleacetic acid (IAA) in roots, which was associated with the upregulated expression of root auxin synthetic enzyme genes (PtTAA1, PtYUC3, and PtYUC4) and auxin influx transporter protein genes (PtLAX1, PtLAX2, and PtLAX3). On the other hand, exogenous DE-GRSP inhibited root IAA and indolebutyric acid (IBA) content, associated with the downregulated expression of root PtTAA1, PtLAX1, and PtLAX3. Root IAA positively correlated with root PtTAA1, PtYUC3, PtYUC4, PtLAX1, and PtLAX3 expression. Purified EE-GRSP and DE-GRSP showed similar element composition but varied in part element (C, O, P, Ca, Cu, Mn, Zn, Fe, and Mo) concentration. It concluded that exogenous GRSP triggered differential effects on growth response, and the effect was associated with the element content of pure GRSP and the change in auxins and root morphology. EE-GRSP displays a promise as a plant growth biostimulant in citriculture.
Highlights
Arbuscular mycorrhizal fungi (AMF) in the soil are extensively reported to colonize the roots of roughly 80% of terrestrial plants, forming arbuscular mycorrhizas (AMs)
The results of our study showed that the application of exogenous extractable glomalin-related soil protein (EE-glomalin-related soil protein (GRSP)) with quarter and half strength upregulated the expression of PtLAX1 and PtLAX2 and did not affect the expression of PtAUX1, suggesting LAX genes could be induced by EE-GRSP
The change in plant growth under GRSP application conditions was associated with the element content of pure GRSP, the auxin content of the plant, and root morphology
Summary
Arbuscular mycorrhizal fungi (AMF) in the soil are extensively reported to colonize the roots of roughly 80% of terrestrial plants, forming arbuscular mycorrhizas (AMs). AMs help the host plant to absorb water and nutrients, whereas the host delivers the photoassimilates to the mycorrhizal fungi in the root for their growth (Huang et al, 2021a). Such mycorrhizal symbiosis represents important functions on promoting plant growth, improving stress resistance, enriching rhizospheric microbial diversity, and stabilizing ecosystems (Zhao et al, 2015; Wang et al, 2018). The direct contribution of GRSP on WSA stability was much stronger than mycorrhizal extraradical hyphae or root mycorrhizal colonization (Rillig et al, 2001; Wu et al, 2014)
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