Wound response in plants

Plants, like animals, regenerate cells and tissues after damage such as wounds and injury have occurred. In plants, there are specific cells that are located at the site of such wounds that retain the ability to return to a pluripotent state. This process is called dedifferentiation. Dedifferentiation is the ability of plants to regain and develop into specific cell types required for regeneration.[1]

As plants become wounded, there are different methods of healing. Depending on whether it is superficial, the site of injury will dry up and die. Beneath the wound, deposits of suberin allow for a protective layer to form over the shallow wound sight. Laticifers that produce latex also perform the same properties and are contained within different plants.[2]

When plants have deeper wounds, a gel-like substance called callus is formed. Callus is developed across and underneath the damaged stem or root. Cambium donates callus. As callus begins to form, cambium cells excrete large amounts of parenchyma.[3]

Within the centuries up until the 16th, the common belief that ‘non-living’ beings could not produce the same molecules of life as the living existed. As plants began to be explored for medicinal purposes, plant theologies were developed. In the early 16thcentury period, the first-hand observations of Otto Brunfels, Hieronymus Bock and Leonhart Fuchs along with an age of exploration developed a theory for the medicinal properties of plants. Brunfels focused his findings on the structure of organs within plants, using a logical division technique.[4]

New technological advances such as the invention of the light microscope contributed to the rise of chemistry, allowing for the analysis of secondary metabolites.

Henri-Louis Duhamel, who was one of the first scientists to study plant healing

Henri-Louis Duhamel, an 18th century French botanist, reviewed the development of the theory of plant regeneration after wounding. His interest into biology began when he studied at the Academie des Sciences to study the disease that had been plaguing the saffron crocus in and around the areas. His success can be marked when it was him who found out that it in fact was fungus that had been responsible for the disease. Duhamel was the first to describe callus formation that he had observed growing over the wound of an elm tree.

Structure of a plant cell

Within plant cells, there are many structures not like those of other eukaryotes. Chloroplasts allows for the plant to absorb energy captured from the sun into energy- rich molecules. Chloroplasts are enclosed and have two membrane layers.[5] The outer membrane is penetrable by small molecules, whilst the inner membrane is thicker with transport proteins scattered across.

Wounding of plants allows nutrients to pathogens and exposes their entry to infection. The constitutive defenses are the physical barriers of the plant; including the cuticle or even the metabolites that act toxic and deter herbivores.[6] Plants maintain an ability to sense when they have an injured area and induce a defensive response. Within wounded tissues, endogenous molecules become released and become Damage Associated Molecular Patterns (DAMPs), inducing a defensive response. Such responses to wounds are found at the site of the wound and also systemically. These are mediated by hormones.

As a plant senses a wound, it immediately sends a signal for innate immunity.[7] These signals are controlled by hormones such as jasmonic acid, ethylene and abscisic acid. Jasmonic acid induces the prosystemin gene along with other defense related genes such as abscisic acid, and ethylene, contributing to a rapid induction of defense responses. Other physical factors also play a vital role in wound signalling, which include hydraulic pressure and electrical pulses. Most of these that are involved within wound signalling also function in signalling other defense responses. Cross-talk events regulate the activation of different roles.

Callus cells formation through the induction process

As plant tissue begins to grow, sucrose is used as fuel in order to sustain photomixotrophic metabolism. Sucrose is an energy source which ensures that the development of cells are at a high standard. Many plant cultures cannot photosynthesize optimally, leading to underdeveloped tissues, lack of chlorophyll and inadequate gas exchange in vessels. Sucrose is also seen to support the conservation of water within cells.

Plants adapt to abiotic and biotic stresses using their plasticity. The defence response of the production of callus allow for plants to heal their wounds.[8] An opening of the wound will begin to be formed over by callus that arises from cells of the cambium.  As callus forms, many of its assisting properties become spread across the plant, giving rise to stems, leaves and the roots.[9]

Plant callus is a group of parenchyma cells which can grow when needed.[10] Parenchyma cells make up the majority of the ground tissue of non-woody structures such as the leaves, flowers and fruits. This does not include the epidermis or veins within these structures. Many callus cells are able to regenerate the whole plant body. Under specific conditions, callus can move through embryogenesis; which involves embryos that become generated from adult somatic cells (Steward et al 1958). The word “callus” can include cells with a diverse amount of differentiation.

In a gel-like formation, callus culture consists of agar and a mixture of macro and micronutrients for the specific cell within the plant. Callus contains many nutrients that are used to help the plant regenerate.[11] Callus cells only contain small vacuoles and little to no chloroplasts for photosynthesis. If maintained under the optimal growth environment, callus cultures can differentiate into whole plants.

Within deep abrasions such as tree wounds, the unharmed living cells become stimulated to divide underneath. As the living cells become divided, callus is formed and overgrows the wound. Not to be mistaken with tree sap, where the sap gives nutrients to the tree and insects such as sugar.

Phellogen is the meristematic layer that allows for the development inside the cork cambium. Phellogen appears beside the callus to divide in order to make cork wood. This process appears as a ‘swelling’ over the surface of the covered wound. This is known as a knot. The production of secondary xylem continues in the surrounding tissue by the cambium. As time goes on, tree rings will begin to cover the surface of the scarred wound. as more of the rings form, the base of the wound becomes buried deeply. The cambium layer continuously covers over the scar until the wound becomes completely covered by the secondary xylem.[2]

• Embryo rescue
• Somatic embryogenesis
• Wound healing