Symbiotic Protocorm Development

Orchid mycorrhizal symbiosis is a complex and intricate interaction between orchids and mycorrhizal fungi, crucial for orchid seed germination and subsequent plant growth. The infection site of fungal hyphae into germinating orchid seeds and developing protocorms, whether through suspensor or rhizoids, may influence the success of symbiotic association. However, the specific chemicals that attract compatible mycorrhizal fungi to orchid seeds and protocorms remain elusive. The fungal hyphae penetrate the cortical cells and form coils of hyphal loops called a peloton. Peloton formation and breakdown enable nutrient exchange, with mechanisms such as tolypophagy and ptyophagy observed in different orchid-mycorrhiza associations. Orchid-mycorrhiza interactions can vary notably depending on the specific orchid species involved and the fungal partners they interact with. Moreover, the tools utilized can influence the interpretation of these interactions, leading to varying conclusions or even controversies between studies. The formation of an interfacial matrix between orchid cells and fungal hyphae resembles structures seen in arbuscular mycorrhiza (AM), aiding in nutrients and signal exchange. Orchids acquire carbon and nutrients through various transporters across fungal and plant membranes of the interfacial matrix. While extensive research exists on phytohormones in AM symbiosis, studies on the role of phytohormones regulating orchid-mycorrhizal interactions are limited. Recent research on Dendrobium and Bletilla explored the possible role of abscisic acid (ABA), gibberellic acids (GAs), and strigolactones (SLs) in mycorrhizal symbiosis. Current research continues to unravel the intricacies of the orchid–mycorrhiza relationship, shedding light on its ecological significance and potential applications.