| Cotton fibers are unicellular seed trichomes that differentiate from the outermost cell layer of the ovule. Cotton fiber development undergoes several distinctive but overlapping steps including fiber initiation, elongation, secondary cell wall biosynthesis, and maturation, leading to mature fibers. The length of fiber which is one of the most important characters in many fiber quality properties is influenced by many factors related with the fiber elongation. Although there are many studies on cotton fiber elongation, the mechanism is still dimness. Hence, it is crucial to research the mechanism of cotton fiber elongation.The allelic mutant Ligon lintless(Ligon) with super-short fibers (only about 5 ~ 8mm in length) and its wild variety (ligon) with normal fibers(about 30mm in length) were used to study the cotton fiber elongation from three aspects, morphology, physiology and gene expression. After investigating the content of carbohydrates, the activity of xyloglucan endotransglycosylases (XETs), the regulation of expression of XET genes by zeatin in cotton fiber elongation, and the function of XET genes in the cotton fiber elongation were studied to understand the mechanism of cotton fiber elongation.The results evolved several aspects as follows:1. Morphology of fiber mutant (Ligon) and its value in study of the mechanism of cotton fiber elongationThe result obtained by observing and measuring the morphology and agronomic traits of Ligon mutant and ligon (wild type) suggested that the monogenic and dominant mutation caused various changes of morphology and agronomic traits, especially made fiber short, and made leaf and stem tortiled. Apparently, Ligon mutant had worse agronomic traits for both yield and quality. However, it is the special morphology that makes the Ligon mutant easy to be identified, and its unique characteristic of super-short fiber make Ligon mutant play an important role in the study of mechanism of cotton fiber elongation as a check of normal cotton fibers.2. The content of carbohydrate in cotton fiber on cotton fiber elongationDuring the cotton fiber elongation, the dynamic changes of the fiber length, the water content, the content of carbohydrate (glucose, fructose, sucrose, starch, cellulose and xyloglucan) and the activity of XETs in cotton fiber cells were investigated by comparing Ligon mutant with ligon. The results indicate that for Ligon mutant, the fiber elongation is very slow and inapparent though it owns two elongation phases, 3～5DPA and 12～19DPA. However, ligon had one longer phase of rapid fiber elongation (5 ~ 19DPA). Moreover, ligon had a later phase of fiber dehydration than Ligon mutant. The saccharide test showed that these phenomenons related to short cotton fiber with high content of sucrose, starch and xyloglucan, and with low content of glucose and fructose. The second fiber elongation stage of Ligon mutant (12～19DPA) might be caused by the increase of XET activity and the decrease of content of xyloglucan in fiber. In conclusion, different kinds of carbohydrate were necessary for cotton fiber elongation, but they were effective only when their contents were suitable. The high XETs activity in cotton fiber was useful for breaking the web of cellulose-xylogiucan in cell wall and promoting the fiber elongation. Meanwhile, the decomposition of disaccharide (sucrose) and polysaccharides (starch, cellulose, xyloglucan) into glucose and fructose made the concentration of dissolvable saccharides increased in cotton fiber, which caused a high turgor and a rapid elongation of fiber cell. On the other hand, glucose and fructose might provide an advantage condition for cotton fiber elongation by enhance the biosynthesis and energy metabolize in fiber.3. The effect of expression of XETs upon cotton fiber elongationIn this study, the fiber of Ligon mutant and ligon were used to measure the dynamic changes of the cotton fiber length, the content of carbohydrate (glucose, fructose, sucrose, starch and xyloglucan), the activity of XETs, and the expression of XETs genes during the cotton fiber elongation. The result indicated that ligon fiber was not only earlier in the expression of XETs but also higher in its XETs activity than Ligon mutant. It revealed that the XETs up-regulated the cotton fiber elongation. In other word, when there were high expression of XETs and high XETs activity in cotton fiber, it was beneficial to decrease the content of xyloglucan and loosen the cell wall and finaly promote the cotton fiber elongation.4. Regulation of expression of XETs by zeatin and its effect on cotton fiber elongationUsing the in vitro ovule culture technology, Ligon mutant and ligon ovules were cultured in the BT culture media with different concentrations of zeatin to study Regulation of expression of XETs by zeatin and its effect on cotton fiber elongation by measuring many kinds of effect factors of cotton fiber elongation, such as fiber length, carbohydrate content (glucose, fructose, sucrose, starch and xyloglucan), XETs activity and expression level of XETs (RT-PCR). The result suggested that zeatin had a very significant effect on in vitro fiber elongation during fiber earlier elongation, but after the phase, zeatin effect was not significant. During the fiber earlier elongation, zeatin concentration in the medium had the best effect in 10μM for Ligon and in 5μM for ligon. It was noticed that the initial stage of fiber elongation was very important for the final length of cotton fiber. Obviously, the proper concentration of zeatin was one of the factors that resulting into long fiber. In addition, the result indicated that zeatin up-regulated the expression of XET genes in the early days of fiber elongation. Therefore, it was presumed that zeatin possibly improved the XETs activity in fiber, decreased the content of xyloglucan, sucrose and starch, increased the content of glucose and fructose with the assistance of other relative enzymes, and advanced the saccharometabolism, consequently, offered enough energy and good condition for the cotton fiber elongation.
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