Mucoromycotina Fungi Possess the Ability to Utilize Plant Sucrose as a Carbon Source: Evidence From Gongronella sp. w5

Gongronella sp. w5 hydrolyzes plant sucrose and releases fructose to recruit phosphate-solubilizing bacteria to provide plants with phosphorus

The mechanisms of how plant-beneficial rhizospheric fungi interact with the soil microbial community to promote plant growth by facilitating their phosphorus acquisition are poorly understood. This work supported that a Mucoromycotina fungus, Gongronella sp. w5 (w5), could promote phosphorus uptake of Medicago truncatula by increasing the available phosphorus (P) in the soil. The abundance of phosphate-solubilizing bacteria (PSB) and the activity of alkaline phosphatase (ALP) in alfalfa rhizosphere soil increased after w5 inoculation. Further analysis showed that w5 donated a portion of ALP activity and also stimulated the PSB to secrete ALP during plant-w5-PSB interaction to help release more available P in the rhizosphere of M. truncatula. Unlike most plant-beneficial rhizospheric fungi that mainly acquire hexoses from plants, w5 gained sucrose directly from the host plant and then recruited PSB to aid P acquisition by hydrolyzing sucrose and releasing mainly fructose to induce PSB to secrete ALP.

IMPORTANCE

This work supported that after absorbing plant sucrose, Gongronella sp. w5 mainly releases sucrose hydrolysis product fructose into the environment. Fructose was used as a carbon source and signaling molecules to induce PSB to co-produce higher alkaline phosphatase activity, releasing soil-available phosphorus and promoting M. truncatula growth. This is the first report that plant-beneficial fungi could directly metabolize sucrose from plants and then recruit PSB to aid P acquisition by providing fructose. Our findings revealed the diversity in pathways of plant-fungi-PSB interactions on soil P acquisition and deepened our understanding of the cooperation of growth-promoting microorganisms in plant rhizosphere.

Direct nitrogen, phosphorus and carbon exchanges between Mucoromycotina 'fine root endophyte' fungi and a flowering plant in novel monoxenic cultures

Most plants form mycorrhizal associations with mutualistic soil fungi. Through these partnerships, resources are exchanged including photosynthetically fixed carbon for fungal-acquired nutrients. Recently, it was shown that the diversity of associated fungi is greater than previously assumed, extending to Mucoromycotina fungi. These Mucoromycotina 'fine root endophytes' (MFRE) are widespread and generally co-colonise plant roots together with Glomeromycotina 'coarse' arbuscular mycorrhizal fungi (AMF). Until now, this co-occurrence has hindered the determination of the direct function of MFRE symbiosis. To overcome this major barrier, we developed new techniques for fungal isolation and culture and established the first monoxenic in vitro cultures of MFRE colonising a flowering plant, clover. Using radio- and stable-isotope tracers in these in vitro systems, we measured the transfer of 33 P, 15 N and 14 C between MFRE hyphae and the host plant. Our results provide the first unequivocal evidence that MFRE fungi are nutritional mutualists with a flowering plant by showing that clover gained both 15 N and 33 P tracers directly from fungus in exchange for plant-fixed C in the absence of other micro-organisms. Our findings and methods pave the way for a new era in mycorrhizal research, firmly establishing MFRE as both mycorrhizal and functionally important in terrestrial ecosystems.

Characterization of the mitogenome of Gongronella sp. w5 reveals substantial variation in Mucoromycota

Gongronella is a genus of fungi in Mucorales (Mucoromycota). Some of its members have important biotechnological applications, but until now, not a single mitogenome has been characterized in Gongronella. Here, we present the complete mitogenome assembly of Gongronella sp. w5, a soil isolate known to interact with plants and several fungi. Its 36,593-bp circular mitogenome encodes the large and small subunit rRNAs, 14 standard mitochondrial proteins, 24 tRNAs, three free-standing ORF proteins, and the RNA subunit of RNase P (rnpB). These genes arrange in an order novel to known fungal mitogenomes. Three group I introns are present in the cob, cox1, and nad5 genes, respectively, and they are probably acquired by horizontal gene transfer. Phylogenetic analysis based on mitochondrion-encoded proteins supports the grouping of Gongronella sp. w5 with Absidia glauca, forming the Cunninghamellaceae clade within Mucoromycota. Gongronella and most other Mucoromycota species are predicted to use the standard genetic code in mitochondrial translation, rather than code 4 assigned by GenBank. A comparison among seven publicly available mitogenomes in Mucoromycota reveals the presence of the same 14 typical protein-coding genes plus rnpB, yet substantial variation in mitogenome size, intron number, gene order, and orientation. In this comparison, the uniqueness of Gongronella is evident from similarly large differences to its closest phylogenetic neighbor, A. glauca. This study promotes our understanding of fungal evolution in Mucoromycota. KEY POINTS: • This study reports the first mitogenome in Gongronella, which presents a novel gene order. • Different Mucoromycota mitogenomes show substantial variation of gene organizations. • Most Mucoromycota species use the standard genetic code to translate mitochondrial genes.

Foliar carbon dot amendment modulates carbohydrate metabolism, rhizospheric properties and drought tolerance in maize seedling

Improving maize drought tolerance is of great importance for scaling up production due to food security and population growth. Carbon dots (CDs) were synthesized by hydrothermal method with citric acid and ethylenediamine as carbon sources. Then, CDs (5 ml, 5 mg‧L^-1) were sprayed on 25th day-old maize (Zea mays L., drought-stress, 35% soil moisture) for seven consecutive days (spraying ultra-pure water as control), after which the physiological parameters and rhizospheric properties of maize under drought were evaluated. Foliar sprayed CDs (5 mg‧L^-1) could increase root exudates (e.g., succinic acid (14.5 folds), pyruvic acid (10.0 folds), and betaine (11.8 folds)), and modify microbial community. Particularly, the relative abundance of Pseudomonas, Sphingomonas, Nitrospira, and Conocybe were significantly increased by 344.4%, 233.3%, 126.2%, and 122.6%, respectively. The altered microbial abundance could improve soil available nitrogen and phosphorus by 33.5% and 16.8%, respectively, and increase plant water uptake by 37.2%. The change of exudate synthesis and microbial abundance could be driven by the significantly increased in net photosynthesis rate by 122.9%, and carbohydrate content by 35.4% in shoots and 113.6% in roots, respectively upon foliar application of CDs. Meanwhile, fresh weight of shoots and roots were increased by 62.1% and 50.6%, and dry weight of shoots and roots were increased by 29.2% and 37.5%, respectively. These results demonstrated that foliar application of CDs could improve the rhizosphere environment to enhance maize drought tolerance and even growth. Therefore, foliar application of CDs would be a promising strategy for sustainable nano-agriculture in response to drought stress.