Abstract
The application of protein hydrolysates (PH) biostimulants is considered a promising
approach to promote crop growth and resilience against abiotic stresses. Nevertheless,
PHs bioactivity depends on both the raw material used for their preparation and
the molecular fraction applied. The present research aimed at investigating the
molecular mechanisms triggered by applying a PH and its fractions on plants
subjected to nitrogen limitations. To this objective, an integrated transcriptomicmetabolomic
approach was used to assess lettuce plants grown under different nitrogen
levels and treated with either the commercial PH Vegamin® or its molecular
fractions PH1(>10 kDa), PH2 (1–10 kDa) and PH3 (<1 kDa). Regardless of nitrogen
provision, biostimulant application enhanced lettuce biomass, likely through a
hormone-like activity. This was confirmed by the modulation of genes involved in
auxin and cytokinin synthesis, mirrored by an increase in the metabolic levels of these
hormones. Consistently, PH and PH3 upregulated genes involved in cell wall growth
and plasticity. Furthermore, the accumulation of specific metabolites suggested the
activation of a multifaceted antioxidant machinery. Notwithstanding, the modulation
of stress-response transcription factors and genes involved in detoxification processes
was observed. The coordinated action of these molecular entities might
underpin the increased resilience of lettuce plants against nitrogen-limiting
conditions.
In conclusion, integrating omics techniques allowed the elucidation of mechanistic
aspects underlying PH bioactivity in crops. Most importantly, the comparison of PH
with its fraction PH3 showed that, except for a few peculiarities, the effects induced
were equivalent, suggesting that the highest bioactivity was ascribable to the lightest
molecular fraction.