Physiological and Morphological Basis for Reproductive Sensitivity to Glyphosate in Glyphosate-Resistant Cotton.

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Title: Physiological and Morphological Basis for Reproductive Sensitivity to Glyphosate in Glyphosate-Resistant Cotton.
Author: Pline, Wendy
Advisors: John W. Wilcut, Co-Chair
Keith L. Edmisten, Co-Chair
Randy Wells, Member
Ronald Sederoff, Member
Judith Thomas, Member
Stephen O. Duke, Member
Abstract: Transgenic, glyphosate-resistant (GR) cotton has been available to U.S. growers since 1997. Despitewide-spread acceptance, there have been performance complaints by growers citing lower boll retention in GRvarieties than in conventional varieties. Field and greenhouse studies confirmed grower observations,demonstrating a glyphosate-associated decrease in boll retention compared to non-treated GR or conventionalplants. Late (beyond the 5 leaf stage) foliar applications were the most injurious, presumably because plantswere entering early reproductive stages. Boll 'cavitation,' (dessicated bolls attached to plants) also observed bygrowers following glyphosate treatment, resulted from abnormal abscission zone formation and was related tovariety. 14C-glyphosate absorption per square centimeter was greater when applied to stem tissue than leaf tissue, but overallabsorption is likely greater when applied foliarly, due to a greater total surface area. Because glyphosatetypically moves from source tissue to sink tissue, up to 3.7% of applied 14C-glyphosate was translocated toreproductive tissues. Seedling development in both GR and conventional cotton was inhibited by root-absorbed glyphosate.Root tissues were more sensitive to glyphosate than cotyledons or hypocotyls, as was demonstrated byinhibition of lateral root formation and shikimic acid accumulation. CP4-EPSPS content was significantly lowerin GR seedling roots than cotyledons, accounting for glyphosate sensitivity. Further studies compared the accumulation of shikimic acid in response to glyphosate in reproductiveand vegetative tissues in GR and conventional cotton. Shikimic acid accumulation per mM of glyphosate, wasgreater in reproductive than vegetative leaf tissue in both GR and conventional cotton, suggesting thatreproductive tissue is innately more sensitive to glyphosate than vegetative tissue. The quantity of theglyphosate-resistant CP4-EPSPS enzyme was significantly less in stamens than in vegetative leaf tissues.Several morphological differences were apparent in flowers of glyphosate-treated GR cotton. Glyphosate applications inhibited stamen elongation, resulting in anthers not extending to the tip of the stigma,thereby limiting pollen deposition to the lower, less receptive portion of the stigma. Total pollen deposition on the stigma was less in glyphosate-treated GR cotton than non-treated GR or conventional cotton. Microscopicanalysis of pollen revealed that glyphosate arrested maturation in at least 3 pollen developmental stages. Atanthesis, pollen grains from glyphosate-treated GR plants were collapsed, highly vacuolated, and had 60% lowerviability (ability to germinate) than pollen from non-treated GR or conventional plants. Retained bolls fromglyphosate-treated plants had fewer seeds than those from non-treated GR or conventional plants. Hand crossesbetween glyphosate-treated plants demonstrated that the number of seeds per boll was decreased when the maleparent, but not the female parent, was glyphosate treated. Hand pollinations using pollen from treated plants,although overcoming the increased anther-stigma distance, did not restore the normal number of seeds per boll.Treatment of GA onto glyphosate-treated GR plants did not remediate glyphosate effects on pollen viability andfloral morphology. Therefore, presumably, several of the bolls that were shed due to glyphosate-treatments,contained an insufficient number of fertilized ovules and are thus shed. This research proposes that boll shed due to glyphosate in GR cotton is due to insufficient levels ofCP4-EPSPS in stamens, which are highly sensitive to glyphosate. As glyphosate accumulates in reproductivestructures, stamen elongation and pollen development are inhibited, resulting in poor fertilization of ovules. Because fewer ovules are fertilized, bolls are either shed or contain fewer seeds if retained. Growers shouldlimit glyphosate contact with GR cotton, especially during the reproductive stages in order to minimize thepotential for glyphosate-induced boll shed.
Date: 2002-03-25
Degree: PhD
Discipline: Crop Science

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