Partial mycoheterotrophy in green plants forming Paris-type arbuscular mycorrhiza requires a thorough investigation

Japonolirion osense, a close relative of the mycoheterotrophic genus Petrosavia, exhibits complete autotrophic capabilities

The plant kingdom exhibits a diversity of nutritional strategies, extending beyond complete autotrophy. In addition to full mycoheterotrophs and holoparasites, it is now recognized that a greater number of green plants than previously assumed use partly of fungal carbon. These are termed partial mycoheterotrophs or mixotrophs. Notably, some species exhibit a dependency on fungi exclusively during early ontogenetic stages, referred to as initial mycoheterotrophy. Japonolirion osense, a rare plant thriving in serpentinite soils, emerges as a potential candidate for initial mycoheterotrophy or mixotrophy. Several factors support this hypothesis, including its diminutive sizes of shoot and and seeds, the establishment of Paris-type arbuscular mycorrhizal associations, its placement within the Petrosaviales-largely composed of fully mycoheterotrophic species-and its ability to face the challenging conditions of its environment. To explore these possibilities, our study adopts a multidisciplinary approach, encompassing stable isotope abundance analyses, in vitro experiments, anatomical analyses, and comparative plastome analyses. Our study aims to (1) determine whether J. osense relies on fungal carbon during germination, indicating initial mycoheterotrophy, (2) determine if it employs a dual carbon acquisition strategy as an adult, and (3) investigate potential genomic reductions in photosynthetic capabilities. Contrary to expectations, our comprehensive findings strongly indicate that J. osense maintains complete autotrophy throughout its life cycle. This underscores the contrasting nutritional strategies evolved by species within the Petrosaviales.

Ubiquitous arbuscular mycorrhizal fungi in the roots of herbaceous understory plants with hyphal degeneration in Colchicaceae and Gentianaceae

Due to the loss of photosynthetic ability during evolution, some plant species rely on mycorrhizal fungi for their carbon source, and this nutritional strategy is known as mycoheterotrophy. Mycoheterotrophic plants forming Paris-type arbuscular mycorrhizas (AM) exhibit two distinctive mycorrhizal features: degeneration of fungal materials and specialization towards particular fungal lineages. To explore the possibility that some understory AM plants show partial mycoheterotrophy, i.e., both photosynthetic and mycoheterotrophic nutritional strategies, we investigated 13 green herbaceous plant species collected from five Japanese temperate forests. Following microscopic observation, degenerated hyphal coils were observed in four species: two Colchicaceae species, Disporum sessile and Disporum smilacinum, and two Gentianaceae species, Gentiana scabra and Swertia japonica. Through amplicon sequencing, however, we found that all examined plant species exhibited no specificity toward AM fungi. Several AM fungi were consistently found across most sites and all plant species studied. Because previous studies reported the detection of these AM fungi from various tree species in Japanese temperate forests, our findings suggest the presence of ubiquitous AM fungi in forest ecosystems. If the understory plants showing fungal degeneration exhibit partial mycoheterotrophy, they may obtain carbon compounds indirectly from a wide range of surrounding plants utilizing such ubiquitous AM fungi.

At the core of the endomycorrhizal symbioses: intracellular fungal structures in orchid and arbuscular mycorrhiza

Arbuscular (AM) and orchid (OrM) mycorrhiza are the most widespread mycorrhizal symbioses among flowering plants, formed by distinct fungal and plant species. They are both endosymbioses because the fungal hyphae can enter inside the plant cell to develop intracellular fungal structures that are surrounded by the plant membrane. The symbiotic plant-fungus interface is considered to be the major site of nutrient transfer to the host plant. We summarize recent data on nutrient transfer in OrM and compare the development and function of the arbuscules formed in AM and the pelotons formed in OrM in order to outline differences and conserved traits. We further describe the unexpected similarities in the form and function of the intracellular mycorrhizal fungal structures observed in orchids and in the roots of mycoheterotrophic plants forming AM. We speculate that these similarities may be the result of convergent evolution of mycorrhizal types in mycoheterotrophic plants and highlight knowledge gaps and new research directions to explore this scenario.

Mycobiont identity and light conditions affect belowground morphology and physiology of a mixotrophic orchid Cremastra variabilis (Orchidaceae)

We have investigated whether mycobiont identity and environmental conditions affect morphology and physiology of the chlorophyllous orchid: Cremastra variabilis. This species grows in a broad range of environmental conditions and associates with saprotrophic rhizoctonias including Tulasnellaceae and saprotrophic non-rhizoctonian fungi from the family Psathyrellaceae. We cultured the orchid from seeds under aseptic culture conditions and subsequently inoculated the individuals with either a Tulasnellaceae or a Psathyrellaceae isolate. We observed underground organ development of the inoculated C. variabilis plants and estimated their nutritional dependency on fungi using stable isotope abundance. Coralloid rhizome development was observed in all individuals inoculated with the Psathyrellaceae isolate, and 1-5 shoots per seedling grew from the tip of the coralloid rhizome. In contrast, individuals associated with the Tulasnellaceae isolate did not develop coralloid rhizomes, and only one shoot emerged per plantlet. In darkness, δ13C enrichment was significantly higher with both fungal isolates, whereas δ15N values were only significantly higher in plants associated with the Psathyrellaceae isolate. We conclude that C. variabilis changes its nutritional dependency on fungal symbionts depending on light availability and secondly that the identity of fungal symbiont influences the morphology of underground organs.

Impact of mycoheterotrophy on the growth of Gentiana zollingeri (Gentianaceae), as suggested by size variation, morphology, and 13C abundance of flowering shoots

Gentiana zollingeri is an annual photosynthetic plant that employs a mycoheterotrophic growth strategy during its underground seedling stage (initial mycoheterotrophy). Notably, the morphological characteristics of its flowering shoots, such as shoot size, leaf size, and leaf color, are highly variable, and it was hypothesized that these variations may be linked to nutritional mode. The morphological characteristics of G. zollingeri individuals were thus investigated alongside environmental factors, 13C abundance, and diversity of colonizing arbuscular mycorrhizal (AM) fungi. The majority of G. zollingeri flowering individuals were found to exhibit a high affinity for the specific AM fungi that exclusively colonize roots of the mycoheterotrophic seedlings, while other phylogenetically diverse AM fungi could also be detected. The leaves to shoot dry weight ratio (leaf ratio) was negatively correlated with the canopy openness in the habitat, suggesting that leaf development is impeded in sunny conditions. Furthermore, the shoot weight of G. zollingeri was positively correlated with leaf 13C abundance. Given that 13C enrichment can provide indirect evidence of mycoheterotrophy in AM plants, the results suggest that the utilization of carbon obtained through mycoheterotrophy, at least during the underground seedling stage, is crucial for G. zollingeri.