l-Tryptophan Stimulates Bioactive Metabolite Accumulation and Cell Wall Remodelling in Flax Callus Cultures

While l-tryptophan is a precursor of plant growth regulators, its effects on secondary metabolism, amino acid profile and cell wall organization in flax callus remain underexplored. This study aimed to optimize flax callus shaken cultures and evaluate the impact of l-tryptophan (0.1 mM and 1 mM) on structural properties of plant cell walls in tested callus using Fourier transform infrared spectroscopy. The impact of l-tryptophan on callus proliferation and metabolism was also determined, because amino acids (among them l-tryptophan) can promote the growth of callus. The results showed that 1 mM l-tryptophan is an effective elicitor, which stimulates flax callus to accumulate larger amounts of bioactive compounds, especially carotenoids and polyphenols, than control callus cultured without l-tryptophan. A lower concentration of l-tryptophan (0.1 mM) slightly improved the level of determined secondary metabolites (except flavonoids). The effect of l-tryptophan on polymers in plant cell walls was investigated. The data confirm that the plant cell wall is a dynamic structure, capable of remodelling in response to growth conditions and external agents. l-tryptophan (0.1 and 1 mM) reduced cellulose levels and induced structural changes in cellulose compared to the untreated control. The structural analyses also suggested a decrease in lignin level and increase in pectin amounts in flax callus after tryptophan addition in comparison to control callus. The results may reflect the relationship between tryptophan and auxins (which are derived from tryptophan) and confirm the role of these metabolites in shaping the structure of the plant cell wall. In fact, an increase in tryptophan level was confirmed in flax callus in tested experimental conditions (supplementation of cultures with both doses of l-tryptophan). These findings have practical significance, because l-tryptophan is also used as a fertilizer or component of fertilizers in plant cultivation.