Multi-Level Investigation in Arabidopsis of the Growth-Stimulating Effects of a Protein Hydrolysate Biostimulant.

Biostimulant use is a promising agricultural strategy to maintain yield while decreasing mineral fertilisers and pesticides. Whereas the positive effect of protein hydrolysates (PH) on growth and yield has already been described, the underlying processes are not well understood. To better identify and characterise the physiological and molecular targets of PH, we first carried out in vitro experiments using the model species Arabidopsis thaliana. PH stimulated Arabidopsis root growth in a dose-dependent manner, with high concentrations inhibiting growth. After having determined a stimulating PH concentration, we performed metabolomic and transcriptomic analyses. We observed that PH application increased amino acid levels in the plants, including a high level of glutamine, although it was only present in trace amounts in PH. These results suggest that amino acids are taken up and metabolized in the plant. PH also had a profound impact on the Arabidopsis transcriptome, with genes involved in nitrate and amino acid transport and assimilation showing a clear up- and down-regulation, respectively. For example, the expression of the high-affinity nitrate transporter NRT2.1 (NITRATE TRANSPORTER 2) was decreased, and consistently high-affinity nitrate uptake and the development of lateral roots were diminished. The PH contained high levels of branched amino acids, which are known to induce the TOR kinase activity, a major driver of growth. Finally, high-throughput phenotyping also showed that PH supply increased shoot growth. In conclusion, our findings show that PH has a clear and robust effect on Arabidopsis growth, but also on the transcriptome, metabolite levels, and physiological processes like nitrogen metabolism.