Investigation of stimulated growth effect by application of L-aspartic acid on poplar

Poplar is an important resource for the production of wood-based products. L-aspartic acid (Asp) is a proteinogenesis amino acid that plays a critical role in cell growth modulation and stress response. Also, Asp is widespread in the commonly used amino acid-based biostimulant products. However, its effects on poplars remain largely unknown. To address this issue, we applied different concentrations of Asp to poplar seedlings under N reduction conditions (simulating low N availability status), and assessed the effects of Asp on poplar growth and development. Our results showed that supplement of Asp at an appropriate concentration, that is, 0.5 mM in the present study, had the best growth-promoting effect on poplars. It significantly increased poplar biomass and height, and simultaneously elevated the photosynthetic parameters, the chlorophyll content and total free amino acid content, as well as the activity of N metabolism-related enzymes in poplars-treated with Asp in comparision with the control plants without Asp addition. Furthermore, the expression of Asp-metabolism pathway genes (AspAT1, AspAT6, AspAT10, AK1, ASS1 and ASS2), key chlorophyll synthesis gene (LHCB3) and cell division gene (CYCD3) was all dramatically upregulated in response to Asp application, suggesting that Asp-induced effect invoked photosynthesis activation and metabolism reprogramming. More interestingly, our results showed that the levels of various phytohormones, such as auxin and cytokinins were remarkably altered, and key auxin transport gene (PIN1c-2), auxin conjugation gene (GH3.6–2) were induced by Asp treatment, indicative of the possible involvement of plant hormones under the effect of Asp, especially fine-tuning auxin homeostasis by forming IAA-Asp conjugates to store auxin and catabolizing IAA-Asp to release auxin, may account for Asp-mediated growth modulation to an extent in poplars. Our study provides a novo insight into the growth promotion effect of Asp on poplars, especially, when the N source is limited, even though the underlying mechanism of Asp-induced stimulation action needs further in-depth investigation.