Effects of Exogenous Auxin on the Transition From Fiber Elongation to Cellulose Biosynthesis Stages During Cotton Fiber Development

Auxin is a major phytohormone that is known to be involved in the entire process of cotton fiber development.  A very recent report showed that increasing auxin level through genetically engineering in the cotton ovule epidermis at the fiber initiation stage increased numbers of lint fibers, and substantially improved fiber yield and quality.  However, molecular mechanisms of auxin involved in fiber initiation, fiber elongation, and secondary wall cellulose biosynthesis have not been characterized in cotton fibers.  Toward this goal, we studied exogenous auxin effect on expression levels of Gossypium hirsutum cellulose synthase catalytic subunit 4 (GhCesA4) that is mainly responsible for secondary wall biosynthesis in developing cotton fibers.  In untreated cotton fibers, GhCesA4 is normally induced at the transition stage when secondary wall biosynthesis begins in developing cotton fibers as the fiber elongation is completed.  In the cotton fibers treated with exogenous auxin, GhCesA4 levels were suppressed in auxin does-dependent manner.  Consistently, GhCesA4 was down-regulated by exogenous auxin treatment in heterologous system, whereas it was up-regulated by an auxin inhibitor.  The results supporting auxin regulation on the transition from fiber elongation stage to secondary wall cellulose biosynthesis stage during cotton fiber development will be presented.