**1. Introduction**

Glycoside hydrolases (EC 3.2.1) are classified into a group of enzymes that hydrolyze the glycosidic bonds of carbohydrates [1]. At the end of March in 2019, 161 families have been identified and classified in the CAZy (Carbohydrate-Active enZYmes) database [2,3]. Among these families, the glycoside hydrolase (GH) family 1 is recognized for its β-glycosidase activity, which largely contributes to various developmental processes and stress responses in plants [4,5]. Genome-wide analysis of GH1 β-glycosidase genes (*BGLU*s) has been conducted in three plant species: *Arabidopsis*, with 48 genes grouped into 10 subfamilies [6]; rice, with 40 genes grouped into eight subfamilies [5]; and maize, with 26 genes grouped into four subfamilies [7,8]. Recently, a comparison between the *Arabidopsis* and rice *BGLU*s with respect to sequence identity and expression revealed that these exhibited substantial tissue specificity and differential responses to various stress treatments, although these have a high degree of similarity [9]. However, no systematic analysis of *BGLU*s in *Brassica rapa*, which is an important vegetable crop, has been performed to date.

In addition to classifications based on genomic DNA organization, *Arabidopsis* BGLUs (*AtBGLU*s) could be classified in relation to their known functions, which shows that genes within the same subfamily may function in similar processes. A large number of *AtBGLU*s are involved in flavonoid metabolism: *AtBGLU1-6* for flavonol accumulation [10,11], *AtBGLU7-11* for anthocyanin glucosyltransferase [11,12], and *AtBGLU12-17* for flavonoid utilization [10,13]. Seven genes (*AtBGLU26*, *AtBGLU34-39*) function as myrosinases for chemical defense against herbivores and pathogen attacks [14–16]. *AtBGLU18* and *AtBGLU33* regulate ABA responses by increasing ABA levels through the hydrolysis of glucose-conjugated ABA (ABA-GE) [17,18]. Scopolin, which is specifically produced in the roots, and which plays a role in a defense against pathogen attack and abiotic stresses [19,20], is controlled by *ArBGLU21-23* [21,22]. The gene products encoded by *AtBGLU45* and *AtBGLU46* hydrolyze monolignol glucosides, thereby regulating lignin biosynthesis [23]. *AtBGLU42* is involved in the induction of systemic resistance to bacterial disease, and the release of iron-mobilizing phenolic metabolites during iron deficiency [24]. However, no gene has been reported, with respect to pollen development.

During pollen development, the tapetum secretes various components, such as lipidic precursors and lipidics onto the pollen surface, leading to the formation of sculptured exine and exine cavities by hydrolyzation and other reactions [25]. In addition to lipid components, pollen wall development requires the regulation of polysaccharide metabolism [26], suggesting a possible involvement of the hydrolysis of glycosidic bonds of carbohydrates. Glycoside hydrolase has been reported involved in the cell wall polysaccharide degradation [27] and their coding genes were downregulated in the *OsTDR* (*Tapetum Degeneration Retardation*) mutant [28] and the sterile floral buds of *B. rapa* [29], indicating a possibility that β-glucosidase may play a role in pollen development.

In this study, we systematically identified *Brassica rapa* β-glycosidase genes (*BrBGLU*s) and analyzed their expression patterns and phylogenetic relationships. In addition, in silico analyses indicated that *BrBGLU10/AtBGLU20* have conserved functions during pollen development, and knocking down *AtBGLU20* using antisense oligos in *Arabidopsis* results in the production of aborted pollen grains. Furthermore, bioinformatics and molecular analyses provide valuable information on the function of *BrBGLUs* during pollen development.
