Genome-Wide Characterization of AspATs in Populus: Gene Expression Variation and Enzyme Activities in Response to Nitrogen Perturbations

Abstract

Aspartate aminotransferase (AspAT) catalyzes a reversible transamination reaction between glutamate and oxaloacetate to yield aspartate and 2-oxoglutarate, exerting a primary role in amino acid biosynthesis and homeostasis of nitrogen (N) and carbon metabolism within all cellular organisms. While progress in biochemical characterization of AspAT has been made for decades, the molecular and physiological characteristics of different members of the AspAT gene family remain poorly known particularly in forest trees. Here, extensive genome-wide survey of AspAT encoding genes was implemented in black cottonwood (Populus trichocarpa Torr. & A. Gray), a model species of woody plants. Thorough inspection of the phylogenies, gene structures, chromosomal distribution, cis-elements, conserved motifs, and subcellular targeting resulted in the identification of 10 AspAT isogenes (PtAspAT1-10) in the Populus genome. RNA-seq along with quantitative real-time polymerase chain reaction (qRT-PCR) validation revealed that PtAspATs displayed diverse patterns of tissue-specific expression. Spatiotemporal expressions of homologous AspATs in the poplar hybrid clone ‘Nanlin895’ were further evaluated, showing that gene expressions varied depending on source-sink dynamics. The impact on AspAT transcripts upon N starvation and seasonal senescence showed the upregulation of five AspAT in leaves concurrent with drastic downregulation of six or more AspATs in roots. Additionally, marked reductions of many more AspATs transcripts were observed in roots upon N excess. Accordingly, AspAT activities were significantly suppressed upon N starvation by an in-gel assay, prompting the argument that enzyme activity was a more direct indicator of the growth morphology under a N stress regime. Taken together, the expression profiling and enzyme activities upon stress cues provide a theoretical basis for unraveling the physiological significance of specific gene(s) in regulation of N acquisition and remobilization in woody plants.