Cotton genes GhMML1 and GhMML2 control trichome branching when ectopically expressed in tobacco.

Trichomes exhibit extraordinary diversity in shape, ultrastructure, distribution, secretion capability, biological functions, and morphological differences, which are strongly associated with their multifunction. Previous researches showed MIXTA-like transcription factors involved in regulating trichome initiation and patterning via forming MYB-bHLH-WD40 transcriptional activator complex to induce the expression of downstream genes. Here, we report the characteristics and role of GhMML1 and GhMML2, members of subgroup 9 of the R2R3-type MYB TFs. GhMML1 and GhMML2 were preferentially targeted to the nucleus and prominently expressed in the early stage during fiber development. Ectopic expression of GhMML1 and GhMML2 respectively in the transgenic tobacco plants changed the morphological characteristics of leaf trichomes; that is, the unbranched trichomes turned into multiple branched, and in the meantime, the density of trichomes was reduced on the surface of the leaf. Y2H and LCI assay revealed that both GhMML1 and GhMML2 could physically interact with a bZIP transcription factor family protein (GhbZIP) in vivo and in vitro. It has been reported that GhbZIP's homolog TAG3 in Arabidopsis is involved in the asymmetric growth of leaves and flowers via direct interaction with BOP1. Taken together, our results demonstrated that two MYB MIXTA-like proteins, GhMML1 and GhMML2, together with GhbZIP might form a multimeric complex to involve in trichome development. This study highlights the importance of MIXTA-like genes from TF subgroup 9 and will help to uncover the molecular mechanism underlying differential trichomes and their development.

GoNe encoding a class VIIIb AP2/ERF is required for both extrafloral and floral nectary development in Gossypium.

The floral nectary, first recognized and described by Carl Linnaeus, is a remarkable organ that serves to provide carbohydrate-rich nectar to visiting pollinators in return for gamete transfer between flowers. Therefore, the nectary has indispensable biological significance in plant reproduction and even in evolution. Only two genes, CRC and STY, have been reported to regulate floral nectary development. However, it is still unknown what genes contribute to extrafloral nectary development. Here, we report that a nectary development gene in Gossypium (GoNe), annotated as an APETALA 2/ethylene-responsive factor (AP2/ERF), is responsible for the formation of both floral and extrafloral nectaries. GoNe plants that are silenced via virus-induced gene silencing technology and/or knocked out by Cas9 produce a nectariless phenotype. Point mutation and gene truncation simultaneously in duplicated genes Ne1 Ne2 lead to impaired nectary development in tetraploid cotton. There is no difference in the expression of the CRC and STY genes between the nectary TM-1 and the nectariless MD90ne in cotton. Therefore, the GoNe gene responsible for the formation of floral and extrafloral nectaries may be independent of CRC and STY. A complex mechanism might exist that restricts the nectary to a specific position with different genetic factors. Characterization of these target genes regulating nectary production has provided insights into the development, evolution, and function of nectaries and insect-resistant breeding.

The transcription factor MML4_D12 regulates fiber development through interplay with the WD40-repeat protein WDR in cotton.

In planta, a vital regulatory complex, MYB-basic helix-loop-helix (bHLH)-WD40 (MBW), is involved in trichome development and synthesis of anthocyanin and proanthocyanin in Arabidopsis. Usually, WD40 proteins provide a scaffold for protein-protein interaction between MYB and bHLH proteins. Members of subgroup 9 of the R2R3 MYB transcription factors, which includes MYBMIXTA-Like (MML) genes important for plant cell differentiation, are unable to interact with bHLH. In this study, we report that a cotton (Gossypium hirsutum) seed trichome or lint fiber-related GhMML factor, GhMML4_D12, interacts with a diverged WD40 protein (GhWDR) in a process similar to but different from that of the MBW ternary complex involved in Arabidopsis trichome development. Amino acids 250-267 of GhMML4_D12 and the first and third WD40 repeat domains of GhWDR determine their interaction. GhWDR could rescue Arabidopsis ttg1 to its wild type, confirming its orthologous function in trichome development. Our findings shed more light towards understanding the key role of the MML and WD40 families in plants and in the improvement of cotton fiber production.

Gossypium barbadense and Gossypium hirsutum genomes provide insights into the origin and evolution of allotetraploid cotton.

Allotetraploid cotton is an economically important natural-fiber-producing crop worldwide. After polyploidization, Gossypium hirsutum L. evolved to produce a higher fiber yield and to better survive harsh environments than Gossypium barbadense, which produces superior-quality fibers. The global genetic and molecular bases for these interspecies divergences were unknown. Here we report high-quality de novo-assembled genomes for these two cultivated allotetraploid species with pronounced improvement in repetitive-DNA-enriched centromeric regions. Whole-genome comparative analyses revealed that species-specific alterations in gene expression, structural variations and expanded gene families were responsible for speciation and the evolutionary history of these species. These findings help to elucidate the evolution of cotton genomes and their domestication history. The information generated not only should enable breeders to improve fiber quality and resilience to ever-changing environmental conditions but also can be translated to other crops for better understanding of their domestication history and use in improvement.