SignificanceArbuscular mycorrhizal (AM) fungi promote phosphorus uptake into host plants in exchange for organic carbon. Physiological tracer experiments showed that up to 100% of acquired phosphate can be delivered to plants via the mycorrhizal phosphate uptake pathway (MPU). Previous studies revealed that the CTTC cis-regulatory element (CRE) is required for promoter activation of mycorrhiza-specific phosphate transporter and H+-ATPase genes. However, the precise transcriptional mechanism directly controlling MPU is unknown. Here, we show that CBX1 binds CTTC and AW-box CREs and coregulates mycorrhizal phosphate transporter and H+-ATPase genes. Interestingly, genes involved in lipid biosynthesis are also regulated by CBX1 through binding to AW box, including RAM2. Our work suggests a common regulatory mechanism underlying complex trait control of symbiotic exchange of nutrients.

Phosphorus (P) availability and soil water are two important environmental factors in lowland rice paddies. They limit the ability of rice to form mutualistic associations with arbuscular mycorrhizal fungi (AMF). The dynamics of this symbiotic interaction are intensified by phosphorus deficiency and attenuated by anaerobic conditions. However, the effects of combined phosphorus deficiency and anaerobic conditions on AMF symbiosis in paddy soil were unproven. The main objective of this study is to determine the influence of phosphorus and water availabilities on indigenous AMF colonization and community in Sangyod Muang Phatthalung (SMP) rice. Rice seedlings were grown in pots containing P-deficient organic paddy soil with or without phosphorus fertilization under non-flooded and flooded conditions for 2, 4 and 6 weeks. The application and omission of P soil fertilization influenced phosphate accumulations in rice seedlings, producing conditions of P-sufficiency and P-deficiency, respectively, in the plants. To determine the effects of phosphorus and water availabilities on AMF colonization and community structures, roots were analyzed microscopically and molecularly. Flooding considerably reduced the intensity of indigenous AMF root colonization whereas the nonenrichment of P availability did not. Reduced AMF colonization was concomitant with lower abundances of two major Glomeromycota ASVs in roots under flooding. This result suggested that soil water availability plays the primary role in shaping AMF communities in SMP roots. This study emphasized the primacy of water management when considering the use of AMF in the production of SMP rice in an organic cultivation system.

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The use of arbuscular mycorrhizal fungi (AMF) as biofertilizer in agriculture is a sustainable approach to fertilization. The first step in the production of AMF biofertilizer is inoculation of mycotrophic plants with a composite of soil and native plant roots, containing potentially viable AMF spores from natural habitats, to a trap culture. A single host plant or a consortium of host plants can be used to propagate AMF spores. However, the difference in the comparative efficiency of mono- and co-cultivated host plants used for the production of AMF spores and the maintenance of original AMF community composition has not been well elucidated. Here, we prepared trap culture with nutrient-poor soil from coastal sand dune vegetation collected during the dry season when the AMF spore density and relative abundance of Glomeromycota ITS2 sequences were significantly higher (p = <0.05) than in the wet season. The AMF communities in the soil were mainly composed of Glomus spp. Maize (Zea mays L.) and/or Sorghum (Sorghum bicolor (L.). Moench) were grown in trap cultures in the greenhouse. Our results demonstrated that co-cultivation of the host plants increased the production of AMF spores but, compared to mono-cultivation of host plants, did not better sustain the native AMF community compositions in the coastal sand dune soil. We propose that the co-cultivation of host plants in a trap culture broadens AMF-host plant compatibilities and thus sustains the symbiotic association of the natively diverse AMF. Therefore, the results of this study suggest that further research is needed to confirm whether the co-culturing of more than one host plant is as efficient a strategy as using a monoculture of a single host plant.

Flooding in rainfed lowlands greatly impairs the mutualistic relationship between indigenous arbuscular mycorrhizal fungi (AMF) and rice. In flooded soils, root colonization by AMF is arrested, but some AMF genera, defined as the core AMF, remain present. However, the core AMF in rainfed lowlands and their symbiotic roles remain unknown. Here, we showed that Acaulospora fungi were the core AMF in rice seedling roots of the Sangyod Muang Phatthalung (SMP) landrace rice variety grown in non-flooded and flooded paddy soils. Subsequently, indigenous Acaulospora spores were propagated by trap cultures using maize as the host plants. Therefore, to clarify the roles of cultured Acaulospora spores in a symbiotic partnership, the model japonica rice variety Nipponbare was grown in sterile soil inoculated with Acaulospora spores, and recolonized with a native microbial filtrate from the organic rice paddy soil. Our data demonstrated that the inoculation of Acaulospora spores in well-drained soil under a nutrient-sufficient condition for six weeks enabled 70 percent of the rice roots to be colonized by the fungi, leading to higher phosphate (Pi) accumulation in the mycorrhizal roots. Unexpectedly, the growth of rice seedlings was significantly suppressed by inoculation while photosynthetic parameters such as fractions of incoming light energy and relative chlorophyll content were unaltered. In the soil, the Acaulospora fungi increased soil phosphorus (P) availability by enhancing the secretion of acid phosphatase in the mycorrhizal roots. The findings of this work elucidate the symbiotic roles of the dominant Acaulospora fungi from lowland rice paddies.

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Allelopathy is a phenomenon in which plants exude biochemicals that inhibit the growth of nearby plants. Allelopathic activity and biochemical production are induced by environmental stresses. This study investigated the effect of phosphorus (P) deficiency on growth and allelopathic activities in two landrace rice cultivars of southern Thailand, Nang loy and Niaw look pueng. Rice seedlings were hydroponically grown for 14 days in Yoshida Solution with normal (HP) and low (LP) P concentrations. Growth and phosphate (Pi) contents in rice seedlings were determined to evaluate the difference in internal P status between plants grown under the HP and LP conditions. Water extracts were prepared from shoots and roots of rice seedlings to test allelopathic activity against lettuce seedlings. The results showed that rice seedlings responded to P deficiency by reducing shoot and root Pi content and increasing leaf greenness. Changes in seedling growth were scarcely detected in the young rice seedlings. In a germination assay, water extracts of P-deficient seedlings from both rice cultivars lowered germination indices and inhibited lettuce seedling growth. Furthermore, shoot extracts of the same rice cultivar more strongly inhibited the development of lettuce seedlings than root extracts. The greater allelopathic activity of the shoot extracts might be a result of their higher phenolic contents, which could be induced by P deficiency. Therefore, an allelopathic trait of rice seedlings is an early response to P deficiency prior to growth inhibition under the low P condition.

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