Biochemical Basis And Molecular Mechanisms Underlying Gossypium Stocksii Fiber Coloration

Cotton fiber is one of the most important textile materials in the world.The use of natural colored cotton（NCC）fibers in fabrics can greatly reduce the textile processing printing and dyeing process,thereby reducing energy consumption and wastewater generation,in line with the growing demand for environmental protection and quality of life improvement.However,poor fiber quality and monotone color greatly hindered the large-scale utilization of NCC fiber.Wild cotton is a valuable reservoir for cultivated cotton genetic improvement.Given the fact that most wild cotton species produce brown fibers,the study on the mechanisms of wild cotton fiber coloration is also of great importance for improving the current cultivated NCCs.This study aimed to elucidate the biochemical basis and molecular mechanism of fiber pigmentation in diploid wild cotton Gossypium stocksii（E₁E₁）.This work analyzed the dynamic accumulation of proanthocyanidins（PAs）in the developing brown fibers of G.stocksii and characterized the expression of the genes potentially involved in PA biosynthesis.In addition,via a comparative analysis between the gene expression profiles of developing fibers of G.stocksii and diploid cotton（G.arboreum,A₂A₂）,the potential regulator Gs TT8 of PA biosynthesis was identified in G.stocksii and and biological functions of the gene was preliminary analyzed by VIGS.The following are the main findings:（1）The observation of the microstructure of developing G.stocksii fibers by transmission electron microscopy technique clarified the main stages of their fiber development,which provides the necessary information for studying and discussing the biochemical and molecular mechanisms of pigmentation.The growth of primary cell walls of G.stocksii fibers was observed at 6-12 DPA,and the formation of the secondary wall was observed at 15 DPA.The secondary wall gradually thickened during the subsequent period.The changes in microstructure suggest that the transition from fiber elongation to secondary wall biosynthesis occurs at approximately 12-15 DPA.（2）This work characterized the process of PA dynamic accumulation in developing fibers by TBO staining and colorimetry.The results indicated that the PAs had been synthesized and accumulated in the fibers at 6 DPA and 9 DPA.The content of PAs peaked around 15 DPA and 18 DPA and significantly decreased after that.Combined with the fact that the brown fiber could only be observed from 18 DPA,the dynamics accumulation of PAs suggested that the PAs were closely related to fiber coloration but might not be the direct biochemical basis.（3）Several key genes involved in PA biosynthesis in G.stocksii fiber were identified and characterized via comparative analysis of the gene expression profiles in the developing fibers of G.arboreum and G.stocksii.This work found that the flavonoid synthesis pathway underwent a longer period of up-regulation during the fiber development of G.stocksii than G.arboreum,which might confer the TT8 gene might play an important regulatory role in it.In addition,expression analysis indicated that ANR but not LAR represented the main flow of PA biosynthesis in G.stocksii fibers.Moreover,we found that AHA10,and TT10 may play critical roles in PA transport,and oxidative condensation.（4）The involvement of TT8 in the coloration of NCC fibers was preliminarily determined.After silencing the TT8 by VIGS,this work found that the PA content in the ZX1（brown fiber cotton）fiber was significantly reduced,and the color of mature fiber turned lighter,which tentatively indicated that TT8 was an important regulator of the PA biosynthesis and fiber coloration in NCCs.In summary,this study determined the essential role of PAs and derivatives during the accumulation and maturation of in the G.stocksii fiber development coloration and identified several key structural and regulatory genes involved in PA synthesis,transport,and oxidation,and the function of TT8 gene was initially investigated.The results of this paper further expand the existing understanding of the molecular mechanisms underlying cotton fiber coloration and promote the application of wild cotton genetic resources in the improvement of NCC fiber quality.