GENETIC AND EXPRESSIONAL EVIDENCE DISPLAYS FUNCTIONAL DISTINCTION OF COTTON BASIC HELIX-LOOP-HELIX PROTEINS IN ARABIDOPSIS TRICHOME INITIATION

The cultivated tetraploid cotton species (AD genomes) were originated from two ancestral diploid species (A and D genomes). While the ancestral A-genome species produce spinnable fibers, the D- genome species do not. Cotton fibers are unicellular trichomes originating from seed coat epidermal cells, and currently there is an immense interest in understanding the process of fiber initiation and development. Current knowledge demonstrates that there is a great of deal of resemblance in initiation mechanism between Arabidopsis trichome and cotton fiber. In this study, comparative functional studies between A genome and D-genome species in cotton by using Arabidopsis trichome initiation as a model was performed. Four cotton genes TTG3, MYB2, DEL61 and DEL65 were amplified from A-genome and D-genome species and transformed into their homolog trichomeless mutants Arabidopsis ttg1, gl1, and gl3egl3, respectively. Our data showed that the transgenic plants expressing TTG3 and MYB2 genes from A genome and D-genome species complement the ttg1 and gl1 mutants, respectively. It was also discovered that complete absences of two functional basic helix loop helix (bHLH) proteins (DEL65/DEL61) in D- diploid species and one (DEL65) that is functional in A-genome species, but not from D- genome species. This observation is consistent with the natural phenomenon of spinnable fiber production in A- genome species and absence in D-genome species. These results suggested that MYB2, TTG3 and DEL65, when expressed in Arabidopsis, regulated the regulatory network genes during the trichome initiation process.