Pars Pro Toto: Every Single Cell Matters

Abstract

Compared to other species, plants stand out by their unparalleled self-repair capacities. Being the loss of a single cell or an entire tissue, most plant species are able to efficiently repair the inflicted damage. Although this self-repair process is commonly referred to as "regeneration," depending on the type of damage and organ being affected, subtle to dramatic differences in the modus operandi can be observed. Recent publications have focused on these different types of tissue damage and their associated response in initiating the regeneration process. Here, we review the regeneration response following loss of a single cell to a complete organ, emphasizing key molecular players and hormonal cues involved in the model species Arabidopsis thaliana. In addition, we highlight the agricultural applications and techniques that make use of these regenerative responses in different crop and tree species.

Keywords: callus; crops; regeneration; single cell; wounding.

FIGURE 1

Modes of regeneration in the Arabidopsis root apical meristem following (A) single cell death, (B) vascular stem cell death, or (C) root tip excision. The left sides of the roots show the type of damage inflicted, the right sides illustrate the regenerative response, and the schemes below the roots indicate the molecular response. (A) Upon single cell damage, activated JA signaling results in the induction of the ERF115 transcription factor (green dot) in cells neighboring the dead cell, via the action of MYC2/ERF109. In parallel, a local auxin response is activated in these cells (light blue). The co-occurrence of ERF115 and activated auxin signaling stimulates these cells to engage into regenerative divisions in order to replace the outward located cell. (B) Vascular stem cell death results in the transcriptional activation of ERF115 in the surrounding cells, including endodermal cells and QC cells. Due to the stem cell death, the perturbed auxin flow results in the establishment of a new auxin maximum in the neighboring endodermal cells, triggering regenerative cell divisions in cells expressing ERF115 and downstream target gene MP, thereby replacing the inward located cells. (C) Loss of the root tip, including stem cells, results in activated JA signaling that, via the action of MYC2/ERF109, triggers ERF115 expression in the vascular and endodermal cells in close contact to the wound site. YUCCA9-mediated auxin biosynthesis at the wound site ensures ample auxin levels required for enforcing ERF115-mediated regenerative divisions. WIND1, directly downstream of ERF115, facilitates root tip regeneration. QC, quiescent center; En, endodermis; Co, cortex; Pe, pericycle; Vasc, vasculature.

FIGURE 2

Transcriptional response following (A) partial incision in stem tissue and (B) complete incision and following tissue reuniting. (A) Upon partial incision of the inflorescence stem, a different response in the top versus bottom side of the cut can be observed. In the top, auxin accumulation activates expression of ANAC071, that in its turn induces the XTH19 and XTH20 cell wall modifying genes, that, in combination with cytokinin-dependent RAP2.6L expression in the bottom side, ensure vascular tissue reconnection. (B) Complete incision results in an induction of auxin-related, cell division and wound healing genes, being AXR1, CYCB1;2 and ANAC071, respectively, in both scion and rootstock. Following grafting, symmetric and asymmetric responsive genes, being HCA2 and TMO6, and WOX4, respectively, facilitate tissue reconnection in the hypocotyl.

FIGURE 3

Regeneration response following (A) leaf blade excision, (B) stem cuttings and (C) root pruning, and the corresponding molecular cascades involved. (A) Upon leaf blade excision, YUCCA4-mediated auxin biosynthesis results in the induction of WOX11 and WOX12, resulting in callus generation from local cambial cells. Following callus induction, WOX11 and WOX12 activate the expression of WOX5 and WOX7 that will initiate root primordia. (B) In cuttings from stems, auxin triggers callus formation and the expression of ARF6 and ARF8, being positive regulators of the rooting process, as well as ARF17, a negative regulator of de novo rooting. (C) Following cutting of the main root system, YUCCA9-dependent auxin biosynthesis activates the expression of WOX11 and WOX12, that in turn regulate LBD16 expression, being needed to induce rooting.

FIGURE 4

In vitro tissue culturing using (A) various tissue explants to (B) generate callus and subsequent (C) shoot regeneration or (D) somatic embryogenesis. (A,B) Different tissue explant material can be used for in vitro callus generation, including leaf, hypocotyl and root explants, immature maize embryos and single cultured cells. (C) Shoot regeneration occurs in a two-step mechanism, where callus is first generated by the action of the PLT3, PLT5, and PLT7 transcription factors, whereas in a second step, expression of CUC2 is required for shoot induction. In parallel, a new shoot organizing center is established by the action of ARRs that facilitate cytokinin production, that in turn activates WUS expression. In addition, wounding-induced expression of WIND1 allows in turn induction of ESR1 that contributes to regeneration of shoots. (D) Somatic embryogenesis from callus occurs by the expression of a network of transcription factors, composed of BBM, LEC2, FUSCA, ABI3, and AGL15.