A Transcript Profiling Approach Reveals an Abscisic Acid-Specific Glycosyltransferase (UGT73C14) Induced in Developing Fiber of <i>Ligon lintless-2</i> Mutant of Cotton (<i>Gossypium hirsutum</i> L.)

Ligon lintless-2, a monogenic dominant cotton (Gossypium hirsutum L.) fiber mutation, causing extreme reduction in lint fiber length with no pleiotropic effects on vegetative growth, represents an excellent model system to study fiber elongation. A UDP-glycosyltransferase that was highly expressed in developing fibers of the mutant Ligon lintless-2 was isolated. The predicted amino acid sequence showed ~53% similarity with Arabidopsis UGT73C sub-family members and the UDP-glycosyltransferase was designated as UGT73C14. When expressed in Escherichia coli as a recombinant protein with a maltose binding protein tag, UGT73C14 displayed enzymatic activity toward ABA and utilized UDP-glucose and UDP-galactose as the sugar donors. The recombinant UGT73C14 converted natural occurring isoform (+)-cis, trans-ABA better than (+)-trans, trans-ABA and (-)-cis, trans-ABA. Transgenic Arabidopsis plants constitutively overexpressing UGT73C14 did not show phenotypic changes under standard growth conditions. However, the increased glycosylation of ABA resulted in phenotypic changes in post-germinative growth and seedling establishment, confirming in vivo activity of UGT73C14 for ABA. This suggests that the expression level of UGT73C14 is regulated by the observed elevated levels of ABA in developing fibers of the Li2 mutant line and may be involved in the regulation of ABA homeostasis.Ligon lintless-2是一种单基因显性的陆地棉（Gossypium hirsutum L.）皮棉纤维突变体，可导致皮棉纤维长度极度缩短，且对营养生长无多效性影响，是研究纤维伸长机制的优秀模型系统。本研究从该突变体的发育纤维中分离得到一个高表达的UDP-糖基转移酶（UDP-glycosyltransferase），其预测氨基酸序列与拟南芥UGT73C家族成员具有约53%的相似性，该UDP-糖基转移酶被命名为UGT73C14。将其与麦芽糖结合蛋白标签融合后在大肠杆菌（Escherichia coli）中重组表达，结果显示UGT73C14对脱落酸（ABA）具有酶催化活性，并可利用UDP-葡萄糖和UDP-半乳糖作为糖供体。重组UGT73C14对天然存在的(+)-顺式,反式-ABA的催化转化效率优于(+)-反式,反式-ABA与(-)-顺式,反式-ABA。组成型过表达UGT73C14的转基因拟南芥在标准生长条件下未表现出明显表型变化，但ABA糖基化水平的升高使其在萌发后生长及幼苗建成阶段出现表型改变，证实了UGT73C14在体内对ABA的催化活性。上述结果表明，Li2突变体发育纤维中升高的ABA水平调控了UGT73C14的表达量，该酶可能参与脱落酸稳态的调控。