Comparative Proteomic and Biochemical Analyses Reveal Different Molecular Events Occurring in the Process of Fiber Initiation between Wild-Type Allotetraploid Cotton and Its Fuzzless-Lintless Mutant

To explore lint fiber initiation-related proteins in allotetraploid cotton (Gossypium hirsutum L.), a comparative proteomic analysis was performed between wild-type cotton (Xu-142) and its fuzzless-lintless mutant (Xu-142-fl) at five developmental time points for lint fiber initiation from -3 to +3 days post-anthesis (dpa). Using two-dimensional gel electrophoresis (2-DE) combined with mass spectrometry (MS) analyses, 91 differentially accumulated protein (DAP) species that are related to fiber initiation were successfully identified, of which 58 preferentially accumulated in the wild-type and 33 species in the fl mutant. These DAPs are involved in various cellular and metabolic processes, mainly including important energy/carbohydrate metabolism, redox homeostasis, amino acid and fatty acid biosynthesis, protein quality control, cytoskeleton dynamics, and anthocyanidin metabolism. Further physiological and biochemical experiments revealed dynamic changes in the carbohydrate flux and H2O2 levels in the cotton fiber initiation process. Compared with those in the fl mutant, the contents of glucose and fructose in wild-type ovules sharply increased after anthesis with a relatively higher rate of amino acid biosynthesis. The relative sugar starvation and lower rate of amino acid biosynthesis in the fl mutant ovules may impede the carbohydrate/energy supply and cell wall synthesis, which is consistent with the proteomic results. However, the H2O2 burst was only observed in the wild-type ovules on the day of anthesis. Cotton boll injection experiments in combination with electron microscope observation collectively indicated that H2O2 burst, which is negatively regulated by ascorbate peroxidases (APx), plays an important role in the fiber initiation process. Taken together, our study demonstrates a putative network of DAP species related to fiber initiation in cotton ovules and provides a foundation for future studies on the specific functions of these proteins in fiber development.