Simultaneous improvement of fiber yield and quality in upland cotton (Gossypium hirsutum L.) by integration of auxin transport and synthesis.

Cotton is a widely planted commercial crop in the world. Enhancing fiber yield and quality is a long-term goal for cotton breeders. Our previous work has demonstrated that fine promotion of auxin biosynthesis in ovule epidermis, by overexpressing FBP7pro::iaaM, has a significant improvement on lint yield and fiber fineness. Lately, transgenic cottons overexpressing GhROP6 variants modify mature fiber length by controlling GhPIN3a-mediated polar auxin transport in ovules. Here, this study showed that all these GhROP6-related cottons displayed unsatisfactory agronomic performance in field conditions. Yet extra auxin supply could promote their fiber development, suggesting inadequate auxin supply in the ovules. Thus, these cottons were integrated with enhanced auxin synthesis by crossing with FBP7pro::iaaM cotton. All the transgene-stacked cottons exhibited synergetic effects on cotton yield (seedcotton yield, lint yield, and lint percentage) and quality (length, strength, and micronaire). Notably, comparing to the FBP7pro::iaaM background, the transgene-stacked cotton co-expressing FBP7pro::iaaM and CA-ghrop6 (constitutively active GhROP6) exhibited a 12.6% increase in seedcotton yield and a 19.0% increase in lint yield over a three-year field trial, and simultaneously resulted in further improvement on fiber length, strength, and micronaire. Collectively, our data provide a potential strategy for genetic improvement on cotton fiber yield and quality. Supplementary Information: The online version contains supplementary material available at 10.1007/s11032-024-01500-w.

GhROP6 GTPase modulates auxin accumulation in cotton fibers by regulating cell-specific GhPIN3a localization.

PIN-FORMED- (PIN) mediated polar auxin transport plays a predominant role in most auxin-triggered organogenesis in plants. Global control of PIN polarity at the plasma membrane contributes to the essential establishment of auxin maxima in most multicellular tissues. However, establishment of auxin maxima in single cells is poorly understood. Cotton fibers, derived from ovule epidermal cells by auxin-triggered cell protrusion, provide an ideal model to explore the underlying mechanism. Here, we report that cell-specific degradation of GhPIN3a, which guides the establishment of the auxin gradient in cotton ovule epidermal cells, is associated with the preferential expression of GhROP6 GTPase in fiber cells. In turn, GhROP6 reduces GhPIN3a abundance at the plasma membrane and facilitates intracellular proteolysis of GhPIN3a. Overexpression and activation of GhROP6 promote cell elongation, resulting in a substantial improvement in cotton fiber length.

An Unexpected Regulatory Sequence from Rho-Related GTPase6 Confers Fiber-Specific Expression in Upland Cotton.

Cotton fibers, single seed trichomes derived from ovule epidermal cells, are the major source of global textile fibers. Fiber-specific promoters are desirable to study gene function and to modify fiber properties during fiber development. Here, we revealed that Rho-related GTPase6 (GhROP6) was expressed preferentially in developing fibers. A 1240 bp regulatory region of GhROP6, which contains a short upstream regulatory sequence, the first exon, and the partial first intron, was unexpectedly isolated and introduced into transgenic cotton for analyzing promoter activity. The promoter of GhROP6 (proChROP6) conferred a specific expression in ovule surface, but not in the other floral organs and vegetative tissues. Reverse transcription PCR analysis indicated that proGhROP6 directed full-length transcription of the fused ß-glucuronidase (GUS) gene. Further investigation of GUS staining showed that proChROP6 regulated gene expression in fibers and ovule epidermis from fiber initiation to cell elongation stages. The preferential activity was enriched in fiber cells after anthesis and reached to peak on flowering days. By comparison, proGhROP6 was a mild promoter with approximately one-twenty-fifth of the strength of the constitutive promoter CaMV35S. The promoter responded to high-dosage treatments of auxin, gibberellin and salicylic acid and slightly reduced GUS activity under the in vitro treatment. Collectively, our data suggest that the GhROP6 promoter has excellent activity in initiating fibers and has potential for bioengineering of cotton fibers.