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

里贡无绒2号（Ligon lintless-2）是一种单基因显性的陆地棉（Gossypium hirsutum L.）纤维突变体，其棉纤维长度极度降低，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种高表达的UDP-糖基转移酶（UDP-glycosyltransferase）。经预测，其氨基酸序列与拟南芥（Arabidopsis）UGT73C家族成员具有约53%的相似性，该酶被命名为UGT73C14。当以带有麦芽糖结合蛋白（maltose binding protein）标签的重组蛋白形式在大肠杆菌（Escherichia coli）中表达时，UGT73C14对脱落酸（ABA）表现出酶促活性，并以UDP-葡萄糖（UDP-glucose）和UDP-半乳糖（UDP-galactose）作为糖供体。重组UGT73C14对天然存在的(+)-顺式,反式-ABA异构体的催化效果优于(+)-反式,反式-ABA和(-)-顺式,反式-ABA。在标准生长条件下，组成型过表达UGT73C14的转基因拟南芥植株未出现表型变化。但ABA糖基化水平的升高导致其在萌发后生长及幼苗建成阶段出现表型异常，证实了UGT73C14在体内对ABA的催化活性。上述结果表明，Li2突变体发育纤维中观测到的ABA水平升高调控了UGT73C14的表达量，且该酶可能参与ABA稳态的调控。