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Genetic study of topping-induced cotton (Gossypium hirsutum L.) plant defense reactions, combining: Bioinformatics, VOC capture and genic expression

Villamar Torres R.. 2018. Montpellier : Université de Montpellier, 214 p.. Thèse de doctorat -- Biologie, interactions, diversité adaptative des plantes, Thèse de doctorat -- Biologie, interactions, diversité adaptative des plantes.

Gossypium hirsutum, the Upland cotton, represents more than 95% of the cotton fiber annually produced worldwide and is grown in about 40 countries. The protection of cotton fields against the attack of herbivorous arthropods needs important quantities of synthetic insecticides, around 18% of the world consumption for the year 2000, although this decreased very much thanks to Bt cotton and eradication programs for some pests. Volatile organic compounds (VOC) naturally emitted by crop plants can reduce insect attacks through the influence of VOC on the behaviors of herbivorous arthropods and auxiliary arthropods. Scientific research about plant VOC has been increasing much since around three decades. ¿Topping", that is, cutting the head of the cotton plants during the useful floriferous period, stimulates the emission of defense VOC. It is an environmentally friendly method and it limits health hazards due to the use of synthetic insecticides for the farmers and the surrounding human populations. The stimulation of VOC emissions by cultivated cotton plants is now recommended by entomologists of CIRAD as a component of the cotton fields protection strategy. The objective of the thesis was to improve our knowledge about the genetic bases of VOCs emissions after topping. The combination of several disciplines such as bioinformatics, chemical ecology and molecular genetics allowed to: 1) characterize genomically the genes of the terpene and terpenoid biosynthesis pathways (terpene synthases or ¿TPS¿) and transcription factors (TF) related to VOC emissions, using the genome databases of three cotton species, G. raimondii, G. arboreum (both diploid cottons) and G. hirsutum (tetraploid cotton), 2) study the VOC emissions by cotton leaves of G. hirsutum plants in response to topping, by capturing these molecules and then characterizing their kinetic profiles by means of gas-chromatography mass-spectrometry (GC-MS), with quantitative measurements, and, 3) study the modifications of the RNA expression of G. hirsutum cotton plants after topping, for genes involved in VOC biosynthesis, through quantitative PCR measurements on 26 targeted genes and an RNA-seq aiming at a whole-transcriptome comparison. The results from the three different fields, bioinformatics, chemical analysis and gene expression, could be interrelated in our research thesis: e.g., two of the genes initially identified by bioinformatics, corresponding to two enzymes, EC: 4.2.3.106 - (E) -beta-ocimene synthase and EC: 4.2.3.111 - alpha-terpineol synthase, were shown to increase their expression after topping, while VOC capture and GC-MS analysis showed increased emissions of the corresponding VOC. These compounds had been characterized in other organisms in reaction to attacks by herbivorous insects. This thesis work is a first exploration of the genetic bases of defensive VOC emission by G. hirsutum cultivated cottons. The variability of genic expression behaviors observed amongst the three genotypes of African Upland cotton G. hirsutum that were studied permits to hypothesize that a genetic diversity is present for defensive VOC emissions, that could permit to improve and adapt by breeding these natural defense mechanisms and the response to topping, in perspective of a more efficient natural protection of cotton fields.

Mots-clés : Écimage; génotype; génétique moléculaire; mécanisme de défense; insecta; lutte biologique; protection des plantes; gossypium hirsutum

Thématique : Ravageurs des plantes; Génétique et amélioration des plantes

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