Genome-wide analysis of the beta-glucosidase gene family in maize (Zea mays L. var B73).

The hydrolysis of beta-D: -glucosidic bonds which is required for the liberation of many physiologically important compounds is catalyzed by the enzyme beta-glucosidase (BGLU, EC 3.2.1.21). BGLUs are implicated in several processes in plants, such as the timely response to biotic and abiotic stresses through activation of phytohormones and defense compounds. We identified 26 BGLU isozymes in the genome of the maize inbred B73 and propose a standardized nomenclature for all Zea mays BGLU paralogs (Zmbglu1-Zmbglu26). We characterized their intron-exon structure, protein features, phylogenetic relationships, and measured their expression and activity in various tissues under different environmental conditions. Sequence alignments revealed some characteristic motifs (conserved amino acids) and specific differences among different isozymes. Analysis of putative signal peptides suggested that some BGLUs are plastidic, whereas others are mitochondrial, cytosolic, vacuolar or secreted. Microarray and RT-PCR analysis showed that each member of the Zmbglu family had a characteristic expression pattern with regard to tissue specificity and response to different abiotic conditions. The source of variance for gene expression was highest for the type of organ analyzed (tissue variance) than for the growth conditions (environmental variance) or genotype (genetic variance). Analysis of promoter sequences revealed that each Zmbglu paralog possesses a distinct set of cis elements and transcription factor binding sites. Since there are no two Zmbglu paralogs that have identical molecular properties, we conclude that gene subfunctionalization in maize occurs much more rapidly than gene duplication.