Multi-scale mechanisms driving root regeneration: From regeneration competence to tissue repatterning.

Plants possess an outstanding capacity to regenerate enabling them to repair damages caused by suboptimal environmental conditions, biotic attacks, or mechanical damages impacting the survival of these sessile organisms. Although the extent of regeneration varies greatly between localized cell damage and whole organ recovery, the process of regeneration can be subdivided into a similar sequence of interlinked regulatory processes. That is, competence to regenerate, cell fate reprogramming, and the repatterning of the tissue. Here, using root tip regeneration as a paradigm system to study plant regeneration, we provide a synthesis of the molecular responses that underlie both regeneration competence and the repatterning of the root stump. Regarding regeneration competence, we discuss the role of wound signaling, hormone responses and synthesis, and rapid changes in gene expression observed in the cells close to the cut. Then, we consider how this rapid response is followed by the tissue repatterning phase, where cells experience cell fate changes in a spatial and temporal order to recreate the lost stem cell niche and columella. Lastly, we argue that a multi-scale modeling approach is fundamental to uncovering the mechanisms underlying root regeneration, as it allows to integrate knowledge of cell-level gene expression, cell-to-cell transport of hormones and transcription factors, and tissue-level growth dynamics to reveal how the bi-directional feedbacks between these processes enable self-organized repatterning of the root apex.

Companion basil plants prime the tomato wound response through volatile signaling in a mixed planting system.

KEY MESSAGE: Volatile compounds released from basil prime the tomato wound response by promoting jasmonic acid, mitogen-activated protein kinase, and reactive oxygen species signaling. Within mixed planting systems, companion plants can promote growth or enhance stress responses in target plants. However, the mechanisms underlying these effects remain poorly understood. To gain insight into the molecular nature of the effects of companion plants, we investigated the effects of basil plants (Ocimum basilicum var. minimum) on the wound response in tomato plants (Solanum lycopersicum cv. 'Micro-Tom') within a mixed planting system under environmentally controlled chamber. The results showed that the expression of Pin2, which specifically responds to mechanical wounding, was induced more rapidly and more strongly in the leaves of tomato plants cultivated with companion basil plants. This wound response priming effect was replicated through the exposure of tomato plants to an essential oil (EO) prepared from basil leaves. Tomato leaves pre-exposed to basil EO showed enhanced expression of genes related to jasmonic acid, mitogen-activated protein kinase (MAPK), and reactive oxygen species (ROS) signaling after wounding stress. Basil EO also enhanced ROS accumulation in wounded tomato leaves. The wound response priming effect of basil EO was confirmed in wounded Arabidopsis plants. Loss-of-function analysis of target genes revealed that MAPK genes play pivotal roles in controlling the observed priming effects. Spodoptera litura larvae-fed tomato leaves pre-exposed to basil EO showed reduced growth compared with larvae-fed control leaves. Thus, mixed planting with basil may enhance defense priming in both tomato and Arabidopsis plants through the activation of volatile signaling.

GLUTAMATE RECEPTOR-like gene OsGLR3.4 is required for plant growth and systemic wound signaling in rice (Oryza sativa).

Recent studies have revealed the physiological roles of glutamate receptor-like channels (GLRs) in Arabidopsis; however, the functions of GLRs in rice remain largely unknown. Here, we show that knockout of OsGLR3.4 in rice leads to brassinosteroid (BR)-regulated growth defects and reduced BR sensitivity. Electrophoretic mobility shift assays and transient transactivation assays indicated that OsGLR3.4 is the downstream target of OsBZR1. Further, agonist profile assays showed that multiple amino acids can trigger transient Ca2+ influx in an OsGLR3.4-dependent manner, indicating that OsGLR3.4 is a Ca2+ -permeable channel. Meanwhile, the study of internode cells demonstrated that OsGLR3.4-mediated Ca2+ flux is required for actin filament organization and vesicle trafficking. Following root injury, the triggering of both slow wave potentials (SWPs) in leaves and the jasmonic acid (JA) response are impaired in osglr3.4 mutants, indicating that OsGLR3.4 is required for root-to-shoot systemic wound signaling in rice. Brassinosteroid treatment enhanced SWPs and OsJAZ8 expression in root-wounded plants, suggesting that BR signaling synergistically regulates the OsGLR3.4-mediated systemic wound response. In summary, this article describes a mechanism of OsGLR3.4-mediated cell elongation and long-distance systemic wound signaling in plants and provides new insights into the contribution of GLRs to plant growth and responses to mechanical wounding.

The subtilisin-like protease SBT3 contributes to insect resistance in tomato.

Subtilisin-like proteases (SBTs) constitute a large family of extracellular plant proteases, the function of which is still largely unknown. In tomato plants, the expression of SBT3 was found to be induced in response to wounding and insect attack in injured leaves but not in healthy systemic tissues. The time course of SBT3 induction resembled that of proteinase inhibitor II and other late wound response genes suggesting a role for SBT3 in herbivore defense. Consistent with such a role, larvae of the specialist herbivore Manduca sexta performed better on transgenic plants silenced for SBT3 expression (SBT3-SI). Supporting a contribution of SBT3 to systemic wound signaling, systemic induction of late wound response genes was attenuated in SBT3-SI plants. The partial loss of insect resistance may thus be explained by a reduction in systemic defense gene expression. Alternatively, SBT3 may play a post-ingestive role in plant defense. Similar to other anti-nutritive proteins, SBT3 was found to be stable and active in the insect's digestive system, where it may act on unidentified proteins of insect or plant origin. Finally, a reduction in the level of pectin methylesterification that was observed in transgenic plants with altered levels of SBT3 expression suggested an involvement of SBT3 in the regulation of pectin methylesterases (PMEs). While such a role has been described in other systems, PME activity and the degree of pectin methylesterification did not correlate with the level of insect resistance in SBT3-SI and SBT3 overexpressing plants and are thus unrelated to the observed resistance phenotype.

Dual localization of the carboxy-terminal tail of GLR3.3 in sieve element-companion cell complex.

Glutamate receptor-like (GLR) 3.3 and 3.6 proteins are required for mediating wound-induced leaf-to-leaf electrical signaling. In the previous study, we found that the carboxy-terminal tail of GLR3.3 contains key residues that are indispensable for its action in electrical signaling. In the present work, we generated plants that expressed the truncated C-tail fraction of GLR3.3. To our expectation, the truncated C-tail itself was not functional in propagating leaf-to-leaf signals. However, we identified that the C-tail-mVENUS fusion proteins had dual localization patterns in sieve elements and companion cells. In companion cells, the fusion proteins overlapped largely with the nucleus. We speculated that a possible nuclear localization signal is present in the C-tail of GLR3.3, paralleling the C-tails of the ionotropic glutamate receptors in animal cells. Our further findings on the C-tail of GLR3.3 open up new possibilities for the regulatory roles of the C-tails to GLR proteins.

How do plants reprogramme the fate of differentiated cells?

Being able to change cell fate after differentiation highlights the remarkable developmental plasticity of plant cells. Recent studies show that phytohormones, such as auxin and cytokinin, promote cell cycle reactivation, a critical first step to reprogramme mitotically inactive, differentiated cells into organogenic stem cells. Accumulating evidence suggests that wounding provides an additional cue to convert the identity of differentiated cells by promoting the loss of existing cell fate and/or acquisition of new cell fate. Differentiated cells can also alter cell fate without undergoing cell division and in this case, wounding and phytohormones induce master regulators that can directly assign new cell fate.

Wound signaling: The missing link in plant regeneration.

Wounding is the first event that occurs in plant regeneration. However, wound signaling in plant regeneration is barely understood. Using a simple system of de novo root organogenesis from Arabidopsis thaliana leaf explants, we analyzed the genes downstream of wound signaling. Leaf explants may produce at least two kinds of wound signals to trigger short-term and long-term wound signaling. Short-term wound signaling is primarily involved in controlling auxin behavior and the fate transition of regeneration-competent cells, while long-term wound signaling mainly modulates the cellular environment at the wound site and maintains the auxin level in regeneration-competent cells. YUCCA (YUC) genes, which are involved in auxin biogenesis, are targets of short-term wound signaling in mesophyll cells and of long-term wound signaling in regeneration-competent cells. The expression patterns of YUCs provide important information about the molecular basis of wound signaling in plant regeneration.

Tissue absence initiates regeneration through follistatin-mediated inhibition of activin signaling.

Regeneration is widespread, but mechanisms that activate regeneration remain mysterious. Planarians are capable of whole-body regeneration and mount distinct molecular responses to wounds that result in tissue absence and those that do not. A major question is how these distinct responses are activated. We describe a follistatin homolog (Smed-follistatin) required for planarian regeneration. Smed-follistatin inhibition blocks responses to tissue absence but does not prevent normal tissue turnover. Two activin homologs (Smed-activin-1 and Smed-activin-2) are required for the Smed-follistatin phenotype. Finally, Smed-follistatin is wound-induced and expressed at higher levels following injuries that cause tissue absence. These data suggest that Smed-follistatin inhibits Smed-Activin proteins to trigger regeneration specifically following injuries involving tissue absence and identify a mechanism critical for regeneration initiation, a process important across the animal kingdom. DOI:http://dx.doi.org/10.7554/eLife.00247.001.