It has a large genome (about 3.3 Gbp), which has recently been sequenced. The sunflower, Helianthus annuus (Asteraceae), is widely cultivated and one of the four most important sources of vegetable oil. The regulatory role of TEs might have important implications into the switch of the host expression leading to symbiosis. Finally, transposons can alter the host’s regulatory network and gene expression, at both the structural and epigenetic levels, possibly leading to phenotypic variations. Besides affecting the genome size, transposable elements are involved in genome restructuration, the generation of new genes by rearrangements of gene segments and affect heterochromatin formation in the nucleus. Transposon dynamics contribute considerably to the evolution of genomes, having primary roles in different genome functions. LTR-REs are most abundant in plant genomes, especially those belonging to the Copia and Gypsy superfamilies, which differ in the position of the integrase domain within the open reading frames and may be distinguished into a number of lineages. LTR-REs that are missing one or more domains use enzymes produced by other LTR-REs to replicate and move. This portion encodes Pol, a polyprotein which includes four enzyme domains, protease, reverse transcriptase (RT), RNaseH and integrase, involved in the mechanism of replication and integration in the chromosomes of the host and Gag, a protein resembling that of virus-like particles. The most frequent REs in plants show two direct long terminal repeats (LTRs) flanking a coding portion. In particular, class I elements (retrotransposons or retroelements, REs) represent the main fraction of the repetitive component of eukaryotic genomes because they transpose by producing RNA intermediates which are copied to cDNA and inserted into different chromosomal sites, leading to their accumulation in the genome. Transposable elements (TEs) are DNA sequences, widespread in the nuclear genome of all eukaryotes and potentially moving across the genome. In contrast, the influence of AM symbiosis on the activity of transposable elements (TEs) is not known. It is known that besides affecting gene expression, pathogenic fungi and their elicitors, as well as compounds related to plant defence, can activate the expression of transposons, especially retrotransposons (e.g Nicotiana tabacum and Solanum lycopersicum). Moreover, AMF may protect host plants by directly inhibiting deleterious microbe propagation or through induced systemic resistance as observed in of Solanum lycopersicum, Nicotiana tabacum and Phaseolus vulgaris. Ĭompared with pathogenic fungi, many similarities occur in the molecular interaction of plant roots with beneficial microorganisms, including the activation of pathogen-related genes, radical oxygen species burst and callose deposition, as part of the AMF transiently induced early defence responses. In roots, where AM symbiosis is established, such changes in both plant and fungal transcriptomes were related to mycorrhizal establishment and development, involving post-translational regulation, signalling, transport, hormone metabolism, and biotic and abiotic stresses. These physiological modifications are, in turn, linked to the activation/repression of several genes, as observed using mRNA sequencing (RNA-Seq) analyses, in a few plant species such as Medicago truncatula, Solanum lycopersicum, Solanum tuberosum, Glycine max, Oryza sativa, Lotus japonicus and Helianthus annuus. The symbiosis induces physiological changes in the colonised plants, affecting the fundamental metabolism of the host cells, and modulates their secondary metabolism, for example, enhances the activity of the antioxidant enzymatic systems and the biosynthesis of diverse phytochemicals with health-promoting activities. AMF also provide some ecosystem services, improving plant tolerance to biotic and abiotic stresses, thereby reducing the need for chemical fertiliser and pesticide inputs in agriculture. These fungi facilitate mineral nutrient uptake, particularly phosphorus and nitrogen from soil, producing large networks of fungal hyphae, which spread from colonised roots into the soil, receiving plant carbon compounds, in exchange. AM fungi (AMF) are beneficial obligate biotrophic microorganisms, represent essential elements of soil fertility, and are involved in plant nutrition and productivity. Arbuscular mycorrhizal (AM) fungi are beneficial symbionts of roots of most plant species, including many cultivated plants.
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