Dissecting the Concept of the Thin Cell Layer: Theoretical Basis and Practical Application of the Plant Growth Correction Factor to Apple, Cymbidium and Chrysanthemum

Abstract

A thin cell layer (TCL) is a thin layer of plant cells. TCLs have served as a simple, but important biotechnological tool in plant science, with several dozen crop species having had tissue culture regeneration protocols developed using TCLs generated from multiple explant sources. There are two types of TCLs, transverse TCLs, or tTCLs and longitudinal TCLs, or lTCLs. The former is the most common, ranging from 100 μm to 1–2 mm in thickness, usually cutting through several tissue types. In contrast, the latter usually targets a very specific layer of cells or tissues, and may vary in length but is as thick as a tTCL. The developmental question that needs to be addressed will determine the choice between one or the other and its use in plant tissue culture. The often unappreciated beauty of the TCL is not so much in its actual regeneration capacity, but rather in its potential regeneration capacity. Herein, we use data from three model species, a woody temperate fruit tree, Malus sp. (apple; Rosaceae), and two herbaceous ornamentals, Cymbidium (orchid; Orchidaceae) and Dendranthema (chrysanthemum; Asteraceae), to demonstrate the theory and functionality of TCLs. Moreover, using a new concept, the plant growth correction factor, or GCF, the ability to theoretically predicts the organogenic outcome in vitro is presented through mathematical models based on the geometric analysis of explant size and shape. A new factor, the geometric factor, or GF, was also determined for all three plants to compare regeneration from different explant types with different shapes. The GF, which is calculated, is independent of plant species or any in vitro conditions, but depends only on the size and shape of the explant and on tissue that is capable of regeneration. The GF and GCF would, in theory, allow for the direct comparison of plant in vitro studies in different laboratories provided that explant size is known, and to predict the theoretical outcome of a regeneration protocol if different explants were to be used.