Opening new avenues for plant developmental research
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KeywordsDevelopmental biology Semi-in-vivo Synthetic biology
Over the past three decades, Arabidopsis thaliana (L.) Heynh. has been widely utilized as a model organism for various research fields in plant biology (The history was reviewed by Koornneef and Meinke 2010). Especially, the introduction of genetic analyses dramatically accelerated the progress in our understanding at the molecular level. In the context of developmental research, forward genetics and subsequent reverse genetics have contributed to the identification of various key regulators. The more factors are isolated, the more efforts researchers have made to investigate the relationship among these isolated regulators, consequently enabling the construction of cross-talk models and regulatory networks. However, such network models are becoming gradually complicated, making it difficult to understand what are the essentials for controlling plant growth and development.
Recently, mathematical modeling has often been used to prove our current knowledge. Such kind of “theoretical” approaches can address whether identified factors are enough to account for various developmental phenomena or whether additional factors are required. On the other hand, it is now possible to artificially reconstitute several developmental processes based on our accumulated knowledge. Such kind of “synthetic biological” approaches enables the simple understanding of plant growth and development, leading to uncovering essential components or freely manipulating plant morphology as desired (Nemhauser and Torii 2018).
This special issue of JPR Symposium titled “Semi-in-vivo developmental biology” focuses on recent attempts to analyze developmental processes and biological phenomena with the use of unique assays. Vascular development takes places in the central parts of the plant body, in contrast to root or flower development. Kondo (2018) introduced the utility of in vitro culture system named VISUAL for understanding vascular development. Although phloem development has been much less-studied compared to xylem development, VISUAL can dissect the phloem differentiation process by a reconstitutive approach (Kondo et al. 2016). On the other hand, Blob et al. (2018) reviewed the molecular framework during root phloem differentiation from in vivo aspects (Furuta et al. 2014). Mutual information obtained from in vivo and in vitro phloem differentiation will compensate with each other. Similarly, xylem differentiation can be exposed using in vitro xylogenic culture system harboring the master transcription factors VASCULAR-RELATED NAC DOMAINs (VNDs) (Oda et al. 2010). Oda (2018) reviewed the functions of cortical microtubule-plasma membrane interaction during secondary cell wall deposition. From these model cases, it is suggested that establishments of unique in vitro assay systems enable examination of developmental processes that have been considered difficult to analyze from in vivo contexts.
In the other case, biological phenomena, as well as developmental processes, can be investigated at the semi-in-vivo level, allowing the elucidation of molecular mechanisms behind them. Grafting, which happens in natural and is also utilized as an agricultural technique, can be easily and efficiently imitated with the model plant Arabidopsis (Melnyk et al. 2015). Nanda and Melnyk (2018) reviewed the roles of a variety of plant hormones during regeneration and vascular reconnection associated with the grafting. Moreover, pollen tube growth and guidance during plant fertilization takes place inside the pistil. A device made by CAD-assisted microfabrication enables direct observation of pollen tube chemoattractions (Horade et al. 2013). Together with this semi-in-vivo approach, Kanaoka (2018) reviewed the molecular mechanisms involved in cell-to-cell communication during plant fertilization. Muranaka and Oyama (2018) reviewed the recent progress of investigation of the circadian clock in plants. Although previous studies have mainly focused on genetic components regulating the clock, this review introduced a new observation system that can monitor circadian rhythms at the single cell level by using the emerging model plant species, duckweeds, Lemna gibba (Muranaka and Oyama 2016). These model cases suggest that semi-in-vivo system allows us to look closer at the biological phenomena of interest. Furthermore, the use of microdevices or suitable model plants expands the possibility of semi-in-vivo approaches.
As described above, “Semi-in-vivo developmental biology” makes it possible to isolate essential factors by molecular and cell biological approaches. However, we must pay attention to the differences from in vivo developmental biology. For this purpose, bidirectional understanding from in vivo and in vitro will be more important to validate and expand our knowledge of plant growth and development. Nowadays, the improvement of genome editing technique and sequence technology is leading to diversification of model plants. In future, semi-in-vivo system would freely utilize special organisms that have unique developmental phenomena or enable the easy observation of the developmental processes of interest.
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