By Shi Yeung
Plants have a remarkable regeneration capacity that allows them to heal their wounds or even generate new individuals. The regeneration of whole-body plants is exploited widely in plant propagation of certain species by cutting the leaves, cane, or roots, and growing a fully functional plant from this. In this article, we will be looking at plant signaling, plant damage, and the control of cellular pluripotency for wound healing.
Before any healing can happen, the plant needs to ‘sense’ its wound. Plants know they are injured when they perceive damaged-self signals, such as the signaling cascade induced by glutamate and jasmonic acid (JA) signaling. When a caterpillar feeds on leaves and injures the plant, the plant can respond in minutes by producing defense hormone, such as jasmonic acid, and accumulate compounds like toxins and repellents to prevent future attacks. Interestingly, plant response mechanisms use glutamate and calcium ions, the same signaling molecules that are key players in our nervous system, even though plants themselves lack nervous systems. A research studying long-distance defense signaling in Arabidopsis found that there is an increase in glutamate concentration at wound sites, which activate ion channels leading to an influx of calcium ions into the cytoplasm. The calcium signal is then propagated and transmitted to distant parts of the plants through the phloem and intercellular channels, such that a distant, undamaged leaf can also respond to the damage (Toyota et al., 2018). Another experiment looking at the transcriptomic patterns of damaged plants showed that plants can distinguish different levels of damage and generate specific responses accordingly. Result shows that flame wounding and the application of leaf extract or JA will have similar transcriptomic patterns, but mere mechanical wounding will cause a different response in terms of genes activated (Heil et al., 2012). The evolution of multiple damaged-self recognition mechanisms in plants is significant, allowing the plant to have a faster and more intensive response when they encounter enemies rather than only having a single, general response.
Unlike multicellular animals which can use cell migration for tissue patterning and wound healing, plants cells are fixed in their locations by the rigid cell wall. Therefore, their wound healing relies strongly on controlled cell division and cell differentiation to fill in the wound with daughter cells to restore the original cell types. The mechanism of wound healing can be studied by using a UV laser to eliminate individual cells and introduce local wounding. It is observed that harmed plant tissue will activate cell division in adjacent cells instead of the injured cell and re-orientate the position plane, ultimately causing the regeneration of the wounded tissue. The whole process is termed “restorative cell division”. However, plant cells do not have the same ability to initiate restorative cell division in cells located further away from the stem cell niches, meristems, which generally have a lower competence. PLETHORA (PLT) is a transcription factor that regulates stem cell activities, and is believed that the regenerative competence depends on and correlates with PLT expression (Marhava et al., 2019).
If plants encounter a more severe damage, de novo organogenesis (generating completely new organs) is needed to replace lost body structures, and different developmental programs will be activated. There are multiple tissues and types of cells in an organ and only stem cells can differentiate into these different cell types. Therefore, organ regeneration in plants involves the re-activation of stem cells pathway. Cell dedifferentiation is the process where cells return to their pluripotent, stem cell-like state. It is common in plants after wounding, such that cells can proliferate to form calluses, and differentiate into specific cell types depending on the location of the wound. Wound induced dedifferentiation 1 (WIND1) and its homologs WIND2-4, are transcription factors found to be the one of the regulators for cell dedifferentiation in Arabidopsis. WIND1 will activates enhancer of shoot regeneration (ESR1) which promotes callus formation and shoot regeneration from root explants. A group of researchers removed callus cells from a plant which overexpressed WIND1 and transferred it into a medium without auxin and cytokinin, plant hormones required for dedifferentiation. They found that the cells will still proliferate rapidly despite the absence of auxin and cytokinin (Iwase et al., 2011). Other transcription factor like PLETHORA mentioned above, are also key regulators for root meristem formation and gaining of pluripotency for callus cells (Ikeuchi, Rymen & Sugimoto, 2020).
Understanding the mechanism behind wound healing is useful in agriculture and biotechnology. Possible application of this knowledge might be manipulating the regenerative mechanism to propagate plants that are difficult to germinate from seeds to seedlings. Despite these wonderful mechanisms that plants have to heal themselves, it is impossible to conserve all plant species if we continue our human activities that will permanently change or damage their habitats.
Toyota, M., Spencer, D., Sawai-Toyota, S., Jiaqi, W., Zhang, T., Koo, A.J., Howe, G.A. & Gilroy, S. (2018) Glutamate triggers long-distance, calcium-based plant defence signaling. Science. 361(6407), 1112–1115. Available from: doi:10.1126/science.aat7744.
Heil, M., Ibarra-Laclette, E., Adame-Álvarez, R. M., Martínez, O., Ramirez-Chávez, E., Molina-Torres, J. & Herrera-Estrella, L. (2012) How Plants Sense Wounds: Damaged-Self Recognition Is Based on Plant-Derived Elicitors and Induces Octadecanoid Signaling. PLoS ONE. 7(2), 30537. Available from: doi:10.1371/journal.pone.0030537.
Marhava, P., Hoermayer, L., Yoshida, S., Marhavý, P., Benková, E. & Friml, J. (2019) Re-activation of Stem Cell Pathways for Pattern Restoration in Plant Wound Healing. Cell. 177(4), 957-969. Available from: doi:10.1016/j.cell.2019.04.015.
Iwase, A., Mitsuda, N., Koyama, T., Hiratsu, K., Kojima, M., Arai, T., Inoue, Y., Seki, M., Sakakibara, H., Sugimoto, K. & Ohme-Takagi, M. (2011) The AP2/ERF Transcription Factor WIND1 Controls Cell Dedifferentiation in Arabidopsis. Current Biology. 21(6), 508–514. Available from: doi:10.1016/j.cub.2011.02.020.
Ikeuchi, M., Rymen, B. and Sugimoto, K. (2020) How do plants transduce wound signals to induce tissue repair and organ regeneration? Current Opinion in Plant Biology. 57, 72–77. Available from: doi:10.1016/j.pbi.2020.06.007.