Arbuscular mycorrhizal fungi (AMF) are obligate biotrophs that supply mineral nutrients to the host plant in exchange for carbon derived from photosynthesis. Sucrose is the end-product of photosynthesis and the main compound used by plants to translocate photosynthates to non-photosynthetic tissues. AMF alter carbon distribution in plants by modifying the expression and activity of key enzymes of sucrose biosynthesis, transport, and/or catabolism. Since sucrose is essential for the maintenance of all metabolic and physiological processes, the modifications addressed by AMF can significantly affect plant development and stress responses. AMF also modulate plant lipid biosynthesis to acquire storage reserves, generate biomass, and fulfill its life cycle. In this review we address the most relevant aspects of the influence of AMF on sucrose and lipid metabolism in plants, including its effects on sucrose biosynthesis both in photosynthetic and heterotrophic tissues, and the influence of sucrose on lipid biosynthesis in the context of the symbiosis. We present a hypothetical model of carbon partitioning between plants and AMF in which the coordinated action of sucrose biosynthesis, transport, and catabolism plays a role in the generation of hexose gradients to supply carbon to AMF, and to control the amount of carbon assigned to the fungus.

Las orquídeas son plantas cautivadoras, tanto por su belleza como por su diversidad. Con alrededor de 30,000 a 35,000 especies distribuidas globalmente, estas plantas son conocidas por su delicadeza y necesidades específicas para crecer. Una de las claves para su reproducción es la simbiosis con hongos microscópicos, los cuales juegan un papel crucial en la germinación de sus semillas. Este proceso simbiótico no sólo beneficia a la orquídea, sino también al hongo, y se ha convertido en una herramienta fundamental para la conservación y el cultivo de orquídeas. En este artículo, exploramos cómo se lleva a cabo la germinación simbiótica, los beneficios de esta relación y su potencial para mejorar las estrategias de conservación y propagación de estas fascinantes plantas.

Background. Seeds of the Orchidaceae family lack an endosperm with sufficient reserves to meet the carbon demands required during germination and the pre-autotrophic developmental stage. This limitation is compensated through symbiotic relationships with orchid mycorrhizal fungi (OMF). The roots of adult orchids also establish symbiotic relationships with OMF, some of which can extend their hyphae to colonize new seeds. Epidendrum radicans is a hemiepiphytic lithophytic, or terrestrial orchid native to Mexico, widely used as an ornamental species; and its fruits produce thousands of seeds. Objective. To isolate fungi that promote germination and early development of E. radicans, using roots from adult plants as the source. Methodology. The presence of OMF structures in E. radicans roots was verified using trypan blue staining. An in vitro co-culture system was designed to induce germination by pairing seeds and OMF-colonized roots. Hyphae associated with germinated orchids were isolated and cultured. One representative isolate from each identified genus was selected and evaluated for its colonization capacity and ability to promote germination, using stained protocorms obtained from co-culture. Results. Sixteen isolates were obtained and morphologically characterized, belonging to the genera Fusarium (14), Rhizoctonia (1), and Ceratobasidium (1). The selected Fusarium sp. isolate did not promote seed germination. In contrast, Ceratobasidium sp. and Rhizoctonia sp. facilitated both germination and early plant development. The number of leaf-bearing plantlets generated by Ceratobasidium sp. was 5.7 times higher than that obtained with Rhizoctonia sp. Seedlings and protocorms germinated in co-culture with both genera exhibited characteristic orchid mycorrhizal colonization structures. Implications. The co-culture protocols used for isolating germination-promoting OMF, along with the modified staining protocol for roots and protocorms, offer valuable tools for enhancing sexual propagation of orchids in breeding programs or conservation efforts targeting species at risk of genetic erosion. Conclusion: The co-culture protocol developed in this study enabled the isolation of two OMF species capable of promoting germination and early development of E. radicans.

Background. Biomass biosynthesis and partitioning in plants significantly impact growth and depend on the relationship established between the plant’s photosynthetic (source) tissues and heterotrophic (sink) tissues. Several factors influence the source-sink relationship, including the establishment of interactions with soil microorganisms, such as arbuscular mycorrhizal fungi (AMF). During the interaction with AMF, a bidirectional flow of nutrients is established, whereby AMF receive carbon (C) from the plant in exchange for mineral nutrients, thereby altering the plant’s source-sink relationship and influencing plant growth. Objective. To analyse the mechanism that regulate and influence C partitioning in plants colonized by HMA as well as to expose the current theories that explain how the change of C partitioning in plants influenced by arbuscular mycorrhizal symbiosis alters plant growth. Methodology. Through a literature review, we explored the physiology of growth and C partitioning and allocation in plants, as well as studies concerning the growth responses exhibited by plants in relation to mycorrhizal symbiosis. Results. Colonization by AMF causes profound modifications in carbohydrate anabolism and catabolism, altering the source-sink relationship and potentially eliciting positive growth responses in plants. However, the alteration of the source-sink relationship can also be negative or neutral, indicating that AMF interaction may limit plant growth under certain conditions. Implications. Understanding the conditions and molecular mechanisms that favour the manifestation of positive mycorrhizal growth responses in plants will facilitate the agricultural management of this important association. Conclusion. Mycorrhizal growth response is determined by the alterations that AMF root colonization causes in C partitioning, allocation, and metabolism, which play a pivotal role in regulating plant growth.