The evolution of in vitro tracheary element systems from annual to perennial plant species

Abstract

AbstractTracheary elements (TEs), including vessels and tracheids, occur as a product of xylogenesis and are highly adapted for the transportation of water and solutes. Xylogenesis or wood formation encompasses various stages of cellular development, which requires stringent temporal and spatial regulation. To further complicate matters, TEs are polymorphous and associated with other complex tissues. These complexities have necessitated the development of in vitro culture systems that are capable of synchronously inducing TEs on demand. In this review, we cover the challenges associated with inducing TEs in vitro and how this has been overcome using mesophyll and callus culture systems in herbaceous plants, yielding transdifferentiation efficiencies of up to 76% and 90%, respectively. We postulate that when equipped with such information, a great opportunity exists to optimise these culture systems in commercially valuable woody genera that currently display lower efficiencies in the range of 15.8–65%. Although both the mesophyll and callus induction cultures have proven essential for uncovering the fundamental processes associated with secondary growth, the mesophyll-based systems have recently become much less prominent (2.8x) in the literature compared to the callus-based systems. This is largely due to ease of application of the callus system to other plant species, paving the way for applications ranging from fundamental research in economically valuable woody genera to the 3D-printing of biomaterial products in vitro.