Plants are very valuable for the existence of life in this universe. We love them and want to protect them from various damages caused by insects, pests and diseases. For this purpose, we are using various poisons. These poisons not only harm them and their users but also pollute the environment. Then question arises, how can we protect them from harm? Recently, scientists have claimed to protect the plants from these damages without poisoning them by using a safe natural element called silicon.
Plants are very valuable for the existence of life in this universe. We love them and want to protect them from various damages caused by insects, pests and diseases. For this purpose, we are using various poisons. These poisons not only harm them and their users but also pollute the environment. Then question arises, how can we protect them from harm? Recently, scientists have claimed to protect the plants from these damages without poisoning them by using a safe natural element called silicon.
Silicon is the second most abundant mineral element in soil after oxygen and comprises approximately 28% of the earth’s crust. Plants absorb silicon exclusively as monosilicic acid by diffusion and also by the influence of transpiration-induced root absorption known as mass flow. Following uptake by the roots, silicon is transported to the shoot via xylem. In shoot, monosilicic acid concentrated through the loss of water and is polymerized. The process of polymerization converts the monosilicic acid to colloidal silicic acid and finally to silica gel when the concentration of monosilicic acid exceeds 2 mM. Silicon is deposited as a 2.5 µm layer in the space immediately beneath the thin (1.0 µm) cuticle layer forming a cuticle-silicon double layer.
Silicon is known to increase rice resistance to leaf and neck blast, sheath blight, brown spot and stem rot. Silicon also decreases the incidence of powdery mildew in cucumber, barley and wheat, rust and ring spot in sugarcane, rust in cowpea and leaf spot in Bermuda grass. The different mechanisms of Silicon enhanced resistance to diseases are reported. One is that silicon acts as a physical barrier, as soluble silicon taken up by plants tends to accumulate in the apoplast, particularly in the epidermal cell walls. This observation has led many investigators to hypothesize that silicon inhibits fungal disease by physically inhibiting fungal germ tube penetration of the epidermis. Another mechanism proposed recently is that soluble silicon acts as modulator of host resistance to pathogens. Several studies in monocots and dicots have shown that plants supplied with silicon can produce phenolics and polyalexins in response to fungal infection such as those causing rice blast and powdery mildew. Silicon amended plants have also been shown to have a significantly higher percentage of infected cells which accumulate phenolics. These antifungal compounds (phenolics and polyalexins) played a crucial role in rice defense response against infection by fungus. Many investigators have shown that fungal hyphae penetrating the phenolic-laden cells of silicon amended plants were found to be seriously damaged by the accumulated phenolics. These phenolics were also conclusively shown to be antifungal.
Silicon is also able to activate some defense mechanisms. For example, in roots of cucumber plants being infected and colonized by fungi, silicon enhanced the activity of chitinases, peroxidises and polyphenoloxidase. In rice, differential accumulation of glucanase, peroxidase and PR-1 transcripts were associated with limited colonization by the fungus in epidermal cells of susceptible rice cultivar supplied with silicon. These biochemical responses are only induced by soluble silicon, suggesting that silicon might play an active role in enhancing host resistance to diseases by stimulating some mechanisms of the defense reaction. Silicon might form complexes with organic compounds in the cell walls of epidermal cells, therefore increasing their resistance to degradation by enzymes released by fungi. Indeed, silicon can be associated with lignin-carbohydrate complexes in the cell wall of epidermal cells. It has been found that in certain areas of heavy silicon deposition delayed fungal ingress and colonization provides the rice plants enough time for antifungal compounds (momilactones), synthesized in response to infection by fungus, to accumulate to considerable levels and express their fungi toxicity within the zone of infection site.
Silicon enhances plant resistance to insects, pests such as stem borer and plant hopper. When insects and pests start their attack on plants such as sugarcane do so by feeding on epidermal tissue of the sheath, leaves and developing internodes in the immature tops of the plants. The presence of silicon crystal in this tissue provides mechanical barrier and hinders the feeding of the insects, which in this phase has rather fragile mandibles. Plants like sugarcane and rice with high silicon contents seem to interfere in the feeding of leave, damaging their mandibles. The high silicon levels in sodium silicate treated plants may have served as deterrent to the borers. A significant negative relation was observed between leaf silicon content and shoot borer incidence. Plant species with higher number of silicon cells per unit area in the leaf sheath portion of 5 to 7 cm from the base more found resistant to shoot borer. The silicon-enhanced effect is attributed to silicon deposition in the plant tissue which acts as physical barrier against boring and chewing by insects.
Many studies involving insects and pests have proved that silicon enhances plant immunity mechanisms by depositing in the plant cell walls and bolstering structures like cystolith hairs. In reviewing the literature about silicates conferring insect resistance, the scientists mention that silicates damage the chewing mouth-parts of insects. Larvae of the stem borer Chilo supressalis fed on plants with high silicon content had their mandibles damaged. Rice nurseries fertilized with silicon, using ashes of rice husk as the source of this element, showed lower number of plants with “dead heart” diseases caused by stem borer. In feeding preference tests conducted with the green aphid S. graminum on silicon-treated and non-treated sorghum plants, almost twice the number of aphids on leaf sections, which did not receive silicon treatment. So, it was concluded that non-preference of aphids for silicon-treated leaves was due to a mechanical barrier provided by the deposition of this element in the cell wall. Silicon, once absorbed by the xylem veins, is deposited in the wall of the plant tissues, forming a mechanical barrier which could have turned difficult or prevented penetration of the stylets of the aphids, impairing their feeding behavior. At molecular level, when damage occurs, soluble silicon in the cell walls is released to stimulate the immune-boosting response that produces volatile repellent phenolics at the site of chewing pest attack.
Dr. Muhammad Ashraf, Dr. Muhammad Afzal and M. Imtiaz
University College of Agriculture, University of Sargodha, Pakistan
