Control of cell and petal morphogenesis by R2R3 MYB transcription factors

Abstract

Petals of animal-pollinated angiosperms have adapted to attract pollinators. Factors influencing pollinator attention include colour and overall size of flowers. Colour is determined by the nature of the pigments, their environment and by the morphology of the petal epidermal cells. Most angiosperms have conical epidermal cells, which enhance the colour intensity and brightness of petal surfaces. The MYB-related transcription factor MIXTA controls the development of conical epidermal cells in petals of Antirrhinum majus. Another gene encoding an R2R3 MYB factor very closely related to MIXTA, AmMYBML2, is also expressed in flowers of A. majus. We have analysed the roles of AmMYBML2 and two MIXTA-related genes, PhMYB1 from Petunia hybrida and AtMYB16 from Arabidopsis thaliana, in petal development. The structural similarity between these genes, their comparable expression patterns and the similarity of the phenotypes they induce when ectopically expressed in tobacco, suggest they share homologous functions closely related to, but distinct from, that of MIXTA. Detailed phenotypic analysis of a phmyb1 mutant confirmed the role of PhMYB1 in the control of cell morphogenesis in the petal epidermis. The phmyb1 mutant showed that epidermal cell shape affects petal presentation, a phenotypic trait also observed following re-examination of mixta mutants. This suggests that the activity of MIXTA-like genes also contributes to petal form, another important factor influencing pollinator attraction.KEY WORDS: Petal, Cell shape, Petunia, Antirrhinum, MYB transcription factor Development 134, 1691Development 134, -1701Development 134, (2007 DEVELOPMENT 1692Emerging data from the study of transcription factors belonging to large families of structurally related proteins suggest that very similar members of phylogenetically-clustered subgroups usually share closely related functions, even though functions may have diverged over the entire family. Structural similarity has led to claims of orthology and functional equivalence between MIXTA, PhMYB1 and AtMYB16 (van Houwelingen et al., 1998;Romero et al., 1998). However, to achieve a general understanding of the control of morphogenesis of petal epidermal cells, the function of new genes needs to be assayed and compared with that of the prototype, MIXTA. In addition, the relevance of these genes to cell shaping and, specifically, to their roles in adapting petals for pollinator attraction, needs to be established in different angiosperm species.We have examined the function of three of the genes encoding proteins very closely related to MIXTA; PhMYB1 from Petunia hybrida, AmMYBML2 from A. majus and AtMYB16 from Arabidopsis thaliana. Structurally, these proteins are most closely related to each other and their genes are therefore orthologous. All three proteins promote directional cell expansion in a bioassay in tobacco. The similarities between the phenotypes induced by these proteins in this bioassay (their biochemical functions) and the similarities in their expression patter...

An ortholog of MIXTA‐like2 controls epidermal cell shape in flowers of Thalictrum

“…Reverse transcriptase PCR with locus-specific primers in T. thalictroides showed expression throughout the flower, consistent with related genes in Petunia and Antirrhinum (Baumann et al, 2007). The ubiquitous expression pattern (including low levels in leaves; data not shown) apparently increasing towards the center of the flower (Fig.…”

“…6g). Leaves and stamens showed no phenotype and no ectopic trichomes were found, indicating that the phenotype induced by TtMYBML is flower-specific, as also reported for AmMYBML2, AtMYB16 and PhMYB1 (Baumann et al, 2007). Based on the high amino acid sequence similarity (Fig.…”

“…A neighbor-joining tree was generated using paup*4.0b10 (Swofford, 2001) from a fragment of the alignment comprising the first 169 amino acids, including the diagnostic motifs up to the subgroup 9 domain (Baumann et al, 2007;Bailey et al, 2008). GenBank accessions used were: CAA55725 (MIXTA), AY661653 (AmMYBML1), AY821655 (AmMYBML2), AY661654 (AmMYBML3), CAA78386 (PhMYB1), X99809 (AtMYB16), NM_115989 (AtMYB17), NM_110979 (AtMYB106) and BAA86879 (AtGL1).…”

“…It may also function redundantly with AtMYB16 in conical cell formation in petals (Baumann et al, 2007;Jakoby et al, 2008). AmMYBML3 is another snapdragon-related protein with functions similar to those of the other MIXTA-like proteins (Jaffe et al, 2007).…”

Mutagenesis in Petunia x hybrida Vilm. and isolation of a novel morphological mutant

“…Recently, with the completion of several genome and EST projects, the amount of sequence data has increased drastically and allowed the effective use of mutant populations to investigate gene function in high-throughput reverse genetic approaches (Alonso and Ecker, 2006). In petunia, gene function studies have profited from the endogenous system of transposable elements (van den Broeck et al, 1998) to characterize the genetic determinants of a wide range of biological processes (Baumann et al, 2007;Cartolano et al, 2007;Simons et al, 2007). Although highly effective, transposon tagging employing non-engineered sequences can only generate loss-of-function mutations and the unstable nature of the insertion may impair long-term genetic analyses (Alonso and Ecker, 2006).…”