THE INFLUENCE OF SALICYLIC ACID ON PEARL MILLET (PENNISETUM GLAUCUM: POACEAE) EPIGENOME DURING HEADMINER (HELIOCHEILUS ALBIPUNCTELLA: LEPIDOPTERA) INFESTATION AND ALUMINUM TOXICITY STRESSES

Abstract

Millets are in the family of cereals grown globally with differential importance across African countries. They are grown in arid and semi-arid areas with a production area of 18.50 million ha by 28 countries covering 30% of the continent. With nine species, pearl millet remains the most important millet crop in term of production and harvested areas in Africa. Pearl millet presents many advantages in responding to the needs and welfare of the poor including food security, nu-trition and health, poverty alleviation, potential markets and dry environment enhancement. How-ever, the production faces many constraints in a context of soil degradation and acidity and increase of pest pressure causing yield losses. These include aluminum toxicity and the millet headminer (Heliocheilus albipunctella). Aluminum is widely distributed in tropical and subtropical regions, constituting approximately 30% of the total area of the planet and 50% of the arable land in the world. Aluminum stress is one of abiotic stresses occurring in acidic soils limiting therefore the yield. The main symptom of aluminum toxicity is the inhibition of root growth through plasma membrane damages in the root apex including root cap, meristem, and elongation zone. The millet headminer is one of the most important pest of pearl millet causing important losses about 60% in high infestation during the panicle stage with 73% of incidence. In both stresses, plants respond via cascade of metabolic reactions from signaling to defense compound synthesis through resistance and tolerance genes controlled by epigenetic regulations. These epigenetic regulations include DNA methylation that is a mechanism of controlling the ex-pression of gene with adding and removal of methyl groups mostly on the fifth cytosine bases through DNA methyltransferases. In plant, “De novo” is the formation of newly methylated pat-terns, and “maintenance” DNA methyltransferases in which the preexisting methylation patterns are maintained after DNA replication. These patterns can be changed by external cues including stresses, influencing therefore the resistance gene expression. The external cues may be from biotic release e.g. oral secretion, abiotic elements e.g. Al3+ or from internal signals e.g. salicylic acid. In this study, the aim was to determine the changes of methylation patterns following millet headminer feeding, salicylic acid and aluminum exposure during seedling and panicle develop-ment using mainly screening tests and MSAP epigenotyping. The results showed that stresses in-cluding H. albipunctella and aluminum toxicity increased the DNA methylation level, while ap-plication of salicylic acid had reverse effects by decreasing the level. The DNA methylation and demethylation processes occurred mostly at the external cytosine. Moreover, application of sali-cylic acid inhibited the root growth during seedling development whereas its foliar treatment sig-nificantly reduced the larval density during panicle development. These results demonstrated the importance of salicylic acid in plants. Salicylic acid acts as an elicitor of plant defense mechanism through DNA demethylation pathways during aluminum and millet headminer stresses. That DNA methylation reverse effects caused by salicylic acid during stress could be from the external cytosine of CCGG sites. This may be the key element regulating the defense-related genes during stress. Moreover, increase of DNA methylation following feeding of millet headminer revealed an important strategy of the pest. It seems the oral secretion released by the larvae during feeding could contain suppressors of plant defense mechanisms. This offered new insights on salicylic acid in the epigenetic defense of plants. In a time of climate change, pest pressure increasing and degradation of soils, the use of epigenetic tools like bio-epidesigners hav-ing the biological ability to modulate gene expression through the epigenome, could be an im-portant step in overcoming these issues. Additionally, research should be carried out on the secre-tome of millet headminer that indicates a presence of an unknown bio-epidesigner.