Ligulate inflorescence of Helianthus × multiflorus, cv. Soleil d'Or, correlates with a mis‐regulation of a CYCLOIDEA gene characterised by insertion of a transposable element

Abstract

Members of CYCLOIDEA (CYC)/TEOSINTE BRANCHED1 (TB1) transcription factor family are essential to control flower symmetry and inflorescence architecture. In the Helianthus annuus genome, ten CYC/TB1 genes have been identified. Studies performed on mutants recognised HaCYC2c as one of the key players controlling zygomorphism in sunflower. We identified CYC2c genes in the diploid Helianthus decapetalus (HdCYC2c) and in the interspecific hybrid Helianthus × multiflorus (H × mCYC2cA and H × mCYC2cB), a triploid (2n = 3× = 51), originated from unreduced eggs of H. decapetalus fertilised by reduced H. annuus male gametes. Phylogenetic analysis showed that HdCYC2c and H × mCYC2c were placed within a CYC2 subclade together with HaCYC2c but distinct from it. The present data showed that in H. × multiflorus the allele derived from H. annuus is deleted or highly modified. The H. × multiflorus taxon exists as radiate and ligulate inflorescence types. We analysed CYC2c expression in H. decapetalus and in the cultivar 'Soleil d'Or' of H. × multiflorus, a ligulate inflorescence type with actinomorphic corolla of disk flowers transformed into a zygomorphic ray-like corolla. In H. decapetalus, the HdCYC2c gene showed differential expression between developing flower types, being up-regulated in the corolla of ray flowers in comparison to the disk flower corolla. In H. × multiflorus, an insertion of 865 bp, which is part of a CACTA transposable element, was found in the 5'-untranslated region (5'-UTR) of H × mCYC2cB. This insertion could promote, even with epigenetic mechanisms, ectopic expression of the gene throughout the inflorescence, resulting in the observed loss of actinomorphy and originating a ligulate head.

The Ha‐ROXL gene is required for initiation of axillary and floral meristems in sunflower

“…Steady‐state mRNA levels were normalized using Ha‐18S mRNA (GenBank accession number KF767534.1) as housekeeping gene and relative expression values were calculated using the comparative Ct method (Livak & Schmittgen, ). The Ha‐18S was preferred after comparison with other putative housekeeping genes (Fambrini et al, ). The complete list of RT‐qPCR primers employed is reported in Supporting Information Table S2.…”

The plastic genome: The impact of transposable elements on gene functionality and genomic structural variations

“…In H. × multiflorus, the cv. "Soleil d'Or" is characterized by a showy ligulate inflorescence and the insertion in the 5 0 -UTR region of the H × mCYC2cB allele of a 865 bp sequence, which is part of a CACTA element (Fambrini et al, 2018). It is assumed that the insertion of a TE in proximity of the gene can influence its expression through a variety of mechanisms, including epigenetic ones, determining a transcriptional activation of the H × mCYC2cB gene in the corolla of the disc flowers that induces the development of the typical ligulate inflorescence of "Soleil d'Or" (Fambrini et al, 2018).…”

Ray flower initiation in the Helianthus radula inflorescence is influenced by a functional allele of the HrCYC2c gene

“…The corolla shape of the disk flowers is actinomorphic (Figure 1g), and composed of five equivalent petal lobes fused to form an elongated tube (Figure 1f) (Harris, 1995;Jeffrey, 1977). A ligulate inflorescence (composed of only ray flowers) is characteristic of some sunflower mutants as well double flowered (dbl) (Chapman et al, 2012) and Chrysanthemoides2 (Chry2) and in some cultivars of the interspecific hybrid Helianthus × multiflorus (Fambrini et al, 2018;Heiser & Smith, 1960;Offenthal & Kaiser, 1999).…”

“…In the Helianthus genus, the differential transcription of CYC genes is linked to changes of both floral symmetry and inflorescence architecture (Chapman et al, 2012;Fambrini et al, 2011;Fambrini et al, 2018;.…”