Genome-wide identification of the soybean cytokinin oxidase/dehydrogenase gene family and its diverse roles in response to multiple abiotic stress

Abstract

Cytokinin oxidase/dehydrogenase (CKX) irreversibly degrades cytokinin, regulates growth and development, and helps plants to respond to environmental stress. Although the <i>CKX</i> gene has been well characterized in various plants, its role in soybean remains elusive. Therefore, in this study, the evolutionary relationship, chromosomal location, gene structure, motifs, <i>cis</i>-regulatory elements, collinearity, and gene expression patterns of <i>GmCKXs</i> were analyzed using RNA-seq, quantitative real-time PCR (qRT-PCR), and bioinformatics. We identified 18 <i>GmCKX</i> genes from the soybean genome and grouped them into five clades, each comprising members with similar gene structures and motifs. <i>Cis</i>-acting elements involved in hormones, resistance, and physiological metabolism were detected in the promoter regions of <i>GmCKXs</i>. Synteny analysis indicated that segmental duplication events contributed to the expansion of the soybean CKX family. The expression profiling of the <i>GmCKXs</i> genes using qRT-PCR showed tissue-specific expression patterns. The RNA-seq analysis also indicated that <i>GmCKXs</i> play an important role in response to salt and drought stresses at the seedling stage. The responses of the genes to salt, drought, synthetic cytokinin 6-benzyl aminopurine (6-BA), and the auxin indole-3-acetic acid (IAA) at the germination stage were further evaluated by qRT-PCR. Specifically, the <i>GmCKX14</i> gene was downregulated in the roots and the radicles at the germination stage. The hormones 6-BA and IAA repressed the expression levels of <i>GmCKX1</i>, <i>GmCKX6</i>, and <i>GmCKX9</i> genes but upregulated the expression levels of <i>GmCKX10</i> and <i>GmCKX18</i> genes. The three abiotic stresses also decreased the zeatin content in soybean radicle but enhanced the activity of the CKX enzymes. Conversely, the 6-BA and IAA treatments enhanced the CKX enzymes' activity but reduced the zeatin content in the radicles. This study, therefore, provides a reference for the functional analysis of <i>GmCKXs</i> in soybean in response to abiotic stresses.