Abstract
The process through induction, proliferation and regeneration of protocorm-like bodies (PLBs) is one of the most advantageous methods for mass propagation of orchids which applied to the world floricultural market. In addition, this method has been used as a tool to identify genes of interest associated with the production of PLBs, and also in breeding techniques that use biotechnology to produce new cultivars, such as to obtain transgenic plants. Most of the molecular studies developed have used model plants as species of Phalaenopsis, and interestingly, despite similarities to somatic embryogenesis, some molecular differences do not yet allow to characterize that PLB induction is in fact a type of somatic embryogenesis. Despite the importance of species for conservation and collection purposes, the flower market is supported by hybrid cultivars, usually polyploid, which makes more detailed molecular evaluations difficult. Studies on the effect of plant growth regulators on induction, proliferation, and regeneration of PLBs are the most numerous. However, studies of other factors and new technologies affecting PLB production such as the use of temporary immersion bioreactors and the use of lighting-emitting diodes have emerged as new tools for advancing the technique with increasing PLB production efficiency. In addition, recent studies on Phalaenopsis equestris genome sequencing have enabled more detailed molecular studies and the molecular characterization of plantlets obtained from this technique currently allow the technique to be evaluated in a more comprehensive way regarding its real applications and main limitations aiming at mass propagation, such as somaclonal variation.
Highlights
Orchids (Family Orchidaceae) represent one of the two largest plant families, including from 736 [1] to 899 genera and 27,800 accepted species names [2] and over 100,000 hybrids produced by artificial pollination [3]
Induction it use not resulted in absence of somaclonal variation in orchids [139]. These observations with other cytokinins plant growth regulators (PGRs) diminish the importance of TDZ as the unique or main factor for VS inducing in orchid IPR–protocorm-like bodies (PLBs), and include other causes, such as the differential susceptibility of genotype and the number of subcultures under proliferation stage of PLB production
Ref. [157] isolated two most expressed transposable elements and identified a new Instability Factor (PIF)-like, one of which, called PePIF1 was identified by similarity to the Phalaenopsis equestris genome sequence, and which was transposed in the somaclonal variants of cultivars of Phalaenopsis from micropropagation, which resulted in the insertion of new genes identified and sequenced by the authors
Summary
Orchids (Family Orchidaceae) represent one of the two largest plant families, including from 736 [1] to 899 genera and 27,800 accepted species names [2] and over 100,000 hybrids produced by artificial pollination [3]. The development of the embryogenic program in orchids, which follows a very specific pattern and different from the conventional embryogenic program occurring in species of other families, such as the absence of endosperm development and gene expression for establishing symbiotic relationships during seed germination process [15] Due to these still-present doubts regarding comparisons of zygotic embryogenesis with induction of PLBs in orchids, we have adopted the term IPR–PLB (induction, proliferation, and regeneration of PLBs) as the standard to describe this technique in this paper. Despite a significant amount of studies with IPR-PLB in different orchid species and hybrids, such as Coelogyne cristata and C. flaccida [33,34], Cyrtopodium paludicolum [35], Grammatophyllum speciosum [36] among others, this review has as its main objective to compile the recent studies and advances found in the induction, proliferation and regeneration of PLBs from the two most important genera in the world flower market, especially Phalaenopsis and Oncidium hybrid groups
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