De novo assembly of plant body plan: a step ahead of Deadpool

Abstract

While in the movie Deadpool it is possible for a human to recreate an arm from scratch, in reality plants can even surpass that. Not only can they regenerate lost parts, but also the whole plant body can be reborn from a few existing cells. Despite the decades old realization that plant cells possess the ability to regenerate a complete shoot and root system, it is only now that the underlying mechanisms are being unraveled. De novo plant regeneration involves the initiation of regenerative mass, acquisition of the pluripotent state, reconstitution of stem cells and assembly of regulatory interactions. Recent studies have furthered our understanding on the making of a complete plant system in the absence of embryonic positional cues. We review the recent studies probing the molecular mechanisms of de novo plant regeneration in response to external inductive cues and our current knowledge of direct reprogramming of root to shoot and vice versa. We further discuss how de novo regeneration can be exploited to meet the demands of green culture industries and to serve as a general model to address the fundamental questions of regeneration across the plant kingdom.

Keywords: auxin; callus; cytokinin; de novo organogenesis; pluripotency; regeneration; stem cells; transdifferentiation.

Figure 1

Cartoon showing various modes of regeneration in plants. (A) Regeneration potential of various plant organs. (B) Indirect regeneration of root and shoot from callus on suitable inductive medium. (C) Natural regeneration of shoot from root. (D) Direct regeneration of root from wounded leaf explants on hormone‐free medium. (E) Direct shoot regeneration from root explants on suitable inductive medium

Figure 2

Schematic representation of the two stages involved in callus mediated de novo regeneration

Figure 3

Schematic representing regeneration of shoot progenitors and assembly of regulatory interactions in the callus. Dome shaped structures on the left mark regenerating shoot progenitors at different developmental stages. The structure on the top right denotes shoot promeristem and the one on the bottom right denotes functional shoot meristem with organ primordia

Figure 4

Intermediate developmental phases of de novo shoot regeneration. (A) Initial shoot progenitor cells are labeled with membrane localized PIN1‐GFP (green). Non‐progenitor cells are marked with DR5‐VENUS (yellow). (B) Developing shoot promeristem showing the confined expression of pWUS‐CFP (green). (C) Functional SAM bearing developing organ primordia marked with PIN1‐GFP and DR5‐VENUS. (D)−(H) Dynamic expression pattern of PIN1‐GFP and DR5‐VENUS during LRP to shoot conversion. (I)−(M) Expression pattern of pWUS‐erCFP during the various stages of LRP to shoot conversion. The scale bar represents 50 μm. (Images are reprinted from Kareem et al. (2015) with permission from Cell Press. License number: 3884940031971.)

Figure 5

Larger regulatory network involved in two‐step mechanism of shoot regeneration. A detailed description of the regulatory interactions is given in the text. Auxin requires the activity of ALF4 to induce the callus (Sugimoto et al., 2010). WUS activates the expression of TOPLESS (TPL) (Busch et al., 2010) and TPL represses PLT1 and PLT2 (Smith & Long, 2010 during embryogenesis). MP (Ckurshumova et al., 2014) and CUC (Daimon et al., 2003) induce STM expression during regeneration. STM in turn activate CUC during in‐planta shoot development (Spinelli, Martin, Viola, Gonzalez & Palatnik, 2011).