Regulation of Cotton Fiber Development by Sugar and Water

Sugar and water are two key resources pivotal for cotton fiber development. We aim to elucidate (i) the molecular basis of sugar import to, and utilization within, cotton fibers and (ii) the mechanisms underpinning water influx into and efflux from fibre cells and (iii) how different components work together to drive and terminate fiber elongation and cellulose biosynthesis. Cell biology studies revealed a transient closure of plasmodesmata (PD) during cotton fibre elongation, coinciding with high expression of sugar and K⁺ transporters and rapid cell expansion. The latter is driven by osmotic water influx facilitated by a set of aquaporins (PIPs and TIPs). To elucidate how imported sucrose is utilized in fibre cells to power their elongation and cellulose biosynthesis, we examined the role of sucrose synthease (Sus) and invertase (Inv), which degrade sucrose into hexoses. Transgenic analyses revealed the critical role that Sus plays in fibre elongation, whereas over-expression of Sus increased early fibre elongation and seed set. Notably, a significant amount of sucrose must also enter into vacuoles as indicated by high activity of vacuole Inv (VIN) during fibre elongation, which may play a key role in driving water influx into vacuoles mediated by TIPs. At the onset of cellulose synthesis, a novel Sus protein targets to cell wall matrix, which may be directly involved in channeling carbon from sucrose to cellulose. These findings provide novel insights into the molecular basis of fiber development and opportunities to improve fiber yield and quality.