Study On The Molecular And Physiological Mechanism Of Yield And Quality Difference Between Colored And White Fiber Cotton

Low yield and quality of naturally colored cotton fiber is one of the most important problems that is required to be solved in the genetic improvement. During fiber development, besides cellulose synthesis, pigment metabolism is an additional process in naturally colored cotton fiber that differed in white cotton fiber. Pigment synthesis in fiber will consume part of carbohydrate in fiber cells affecting cellulose synthesis for fiber yield and quality. Therefore, in present paper, the molecular and physiological mechanism for the difference of yield and quality between white and naturally colored cotton fibers were analyzed from plant photosynthesis, fiber carbohydrate and pigment content, fiber respiration, and gene function and expression that related with carbohydrate metabolic enzymes aspects according to substrate competition hypothesis. The main results wereshowed as below.1. Relationship between photosynthetic property and yield and quality of naturally colored cotton fiberUsing brown fiber cotton, green fiber cotton, two brown hybrids, hybrid 1 (the parents were upland cotton) and hybrid 2 (the two parents were upland cotton and sea island cotton), white fiber cotton (control) as plant material, variation of photosynthetic pigment content, chloroplast Hill reaction activity, Mg²⁺-ATPase and Ca²⁺-ATPase activity, net photosynthetic rate, leaf starch, sucrose, fructose, and glucose content was determined. Furthermore, correlation and path analysis among yield and quality and these photosynthetic indexes were also analyzed. Result showed that yield ranking of cotton fiber was hybrid 1>white fiber cotton>hybrid 2> brown fiber cotton>green fiber cotton. Yield of green fiber cotton and brown fiber cotton was only 58% and 66% of white fiber cotton, respectively. Although leaf photosynthetic pigment content, net photosynthetic rate and other photosynthetic indexes of green fiber cotton plant was near to white fiber cotton even much higher, its yield was yet the lowest. Lower yield and quality in green fiber cotton was mainly caused by excessively nutritional plant growth, less bolls per plant, and lighter boll mass. However, the reason for low yield and quality of brown fiber cotton was its weak photosynthesis, less physiological activity of leaf chloroplast, and small amount of photosynthetic production. As for the two brown hybrids, photosynthetic ability of hybrid 1 much better than hybrid 2 and yield of hybrid 1 was significantly higher than hybrid 2. However, quality of hybrid 2 was slightly better than hybrid 1. Result of correlation and path analysis among yield and quality and photosynthetic indexes indicated that net photosynthetic rate had the most contribution to yield and the next important was carbohydrate content while had the reverse tendency for fiber length, which was possibly due to its fiber cellular osmotic regulation promoting fiber elongation besides it was used as metabolic substrate. These results indicated that it was feasible for fiber yield and quality improvement through screening for efficiency photosynthesis hybrid combination.2. Effect of fiber carbohydrate and pigment content on fiber lengthFiber carbohydrate and pigment content, cellulose content, and fiber length dynamic were investigated in white fiber cotton, brown fiber cotton, green fiber cotton, and two brown hybrids during fiber development. Result showed that fiber sucrose content was the highest at 10 DPA and then continuously decreased in white fiber cotton and naturally fiber cotton from the day of anthesis to fiber maturation. Fiber fructose content kept decreasing state from 0 DPA to fiber maturation. However fiber glucose content peaked at 20 DPA, which was consistent with the vigorous stage of the fiber cellulose synthesis both in white and naturally fiber cotton. Fiber carbohydrate content in green fiber cotton and brown hybrid 1 was similar and much higher than other type cotton. However, green fiber cotton had the lowest yield and quality. It was because higher PAL activity leading to large amount of pigment synthesis in green cotton fiber, which would consume much carbohydrate. Thus cellulose synthetic amount reduced. In addition to this reason, low carbohydrate use efficiency was other possibly important reason for low cellulose content because of its low micronaire value, which indicated much carbohydrate was not to be used in cellulose synthesis as fiber maturation. In brown cotton fiber, its lower cellulose content than white cotton fiber was due to low carbohydrate content in fiber and consumption of part carbohydrate for pigment synthesis resulting in substrate reduction. As for hybrid 1, much higher carbohydrate content in fiber could partly make up for the usage of carbohydrate in pigment synthesis. These results demonstrated the substrate ebbing and flowing phenomenon in carbohydrate consumption for pigment and cellulose synthesis. 3. Impact of cellular redox state and exogenous respiratory inhibitors on fiber developmentIn this section, cellular water content, pH value, cytochrome c oxidase activity, NAD⁺, NADH, NADP⁺, and NADPH content was monitored as fiber cell development. Furthermore, color display, ovule fresh weight, fiber length, cytochrome c oxidase and polyphenoloxidase activity in green and white cotton fiber were also traced under two respiratory inhibitor treatment, rotenone and thiourea, with in vitro culture condition. Result showed that color display timing and manner was obviously different in green and brown cotton fiber. It displayed as "suddenly" style after 30 DPA in brown cotton fiber while as "increasingly" after 20 DPA in green cotton fiber. Therefore, it could be inferred that their physiological state was different in brown and green cotton fiber. White cotton fiber had much higher water content. However, its pH value was lower than green cotton fiber and higher than brown cotton fiber. Cytochrome c oxidase activity, NAD⁺, NADH, NADP⁺, and NADPH content in brown serials of cotton fiber was far higher than white and green cotton fiber and NAD⁺ or NADP⁺ content was much higher than NADH or NADPH demonstrating vigorous respiration in fiber cells. However, coenzyme content in white and green fiber was relatively low and little difference between them. Because green cotton fiber contained two important metabolic systems, which were pigment and cellulose synthesis, it could be inferred lots of carbohydrate might be fully used. Although high activity of respiratory metabolism in brown cotton fiber, cellulose content was higher than green cotton fiber and lower than white cotton fiber while pigment content reversely because of less amount of carbohydrate content in fiber. Ovule and fiber development would be inhibited by both rotenone and thiourea causing ovule fresh weight reduction and fiber shortening. But rotenone was more harmful to ovule and fiber development and color displayed for one week earlier in green cotton fiber. Although thiourea had less effect on ovule and fiber development, it could result in no color display in green cotton fiber indicating pigment metabolism was inhibited by thiourea. Moreover, cytochrome c oxidase activity rapidly decreased in fiber cells after rotenone treatment and polyphenoloxidase activity reduced by more than 80% in green cotton fiber after thiourea treatment.4. Activity of key enzymes in carbohydrate metabolism in naturally cotton fibers and sucrose synthase gene expressionCarbohydrate metabolic related enzymes, i.e. acidic and neutral invertase, sucrose synthase, and sucrose phosphate synthase, were measured in white, brown, green cotton fiber, and two brown hybrids when fiber was at 0-20 DPA developmental stages. Furthermore, expression amount of sucrose synthase gene was assayed using RT-PCR and real time RT-PCR method. Result showed that activity invertase and sucrose synthase was the highest in brown cotton fiber. These enzymatic activities were similar between green and white cotton fiber. Sucrose synthase activity in white and naturally colored cotton fiber varied little from 2 DPA o 20 DPA revealing high activity of carbohydrate metabolism during cotton fiber development. Furthermore, sucrose synthase gene expression in white, brown, and green cotton fiber altered little during fiber development which was in accordance with enzyme activity variation. However, expression level of sucrose synthase in brown cotton fiber was higher than white cotton fiber. Green cotton fiber had the lowest expression level in fiber cells. As for different organs, fiber cells had much higher expression level than ovule and leaf in white, brown, and green fiber cotton. Low activity of sucrose synthase and gene expression in green cotton fiber was an important reason for its low utilization efficiency of carbohydrate.5. Molecular cloning of cotton fiber development specific promoter and its applicationA cotton fiber development specific promoter of GaRDL 1 gene had been isolated from cotton genomic DNA. The promoter was connected GFP sequentially as report gene and was transformed into ovule by pollen tube pathway. Result showed that GFP could be detected in fiber after 3 days of ovary injection uncovering the specific expression of promoter in fiber. Sucrose synthase gene was reversely ligased after promoter and sucrose synthase activity and carbohydrate content of mixture of ovule and fiber was measured after 3 days of ovary injection using antisense inhibition method. Result showed that sucrose synthase activity reduced in white, brown, and green cotton fiber. However, sucrose content changes little because of its low degradation resulting in low content of glucose and fructose. And thus carbohydrate utilization efficiency was lowered.