Time-Course Analysis of Levels of Indole-3-Acetic Acid and Expression of Auxin-Responsive GH3 Genes in Betula platyphylla

Abstract

Indole-3-acetic acid (IAA) is a natural hormone that plays an important role in the essential physiological processes of plant growth, development, and environmental response. To maintain IAA homeostasis in plants, some members of the GH3 gene family may mediate the conjugation of amino acids to indole-3-acetic acid. In this study, nine birch BpGH3 genes were cloned, including three BpGH3.5 genes, three BpGH3.6 genes, two BpGH3.9 genes, and a single BpGH3.3 gene, that were likely to be auxin-responsive genes. Time-course expression analysis of these nine BpGH3 genes using real-time reverse transcriptase–polymerase chain reaction (RT-PCR) was conducted using birch plant tissues collected during a 1-year growth cycle. Further, we determined the endogenous free IAA content in birch during this period by using liquid chromatography tandem mass spectrometry. The results indicated that the IAA levels changed remarkably throughout the study period; the level remained relatively stable during the developmental stage from June 1 to July 1, and IAA was present in the birch leaves throughout the study period. The correlation between the expression of BpGH3 genes and IAA levels was analyzed using Pearson correlation analysis. The results revealed a significant negative correlation between the expressions of four BpGH3 genes, namely, BpGH3.3 (p < 0.05), BpGH3.5a (p < 0.01), BpGH3.5b (p < 0.01), and BpGH3.9a (p < 0.01) and the free IAA level, thereby suggesting that these genes might play roles in IAA regulation.