Silicon Induces the Biosynthesis of Lignin in Wheat Cultivars Grown under Phosphorus Stress

Although silicon (Si) and lignin are accumulated on plant cell walls and both confer resistance to multiple biotic and abiotic stresses [1,2], the impact of Si on lignin production in plants grown under phosphorus (P) stress still remains unknown. We evaluated the effect of Si on the lignin accumulation pattern and expression of lignin biosynthesis-related genes in wheat plants grown at different P levels. Two wheat cultivars differing in tolerance to P deficiency (Púrpura-sensitive and Fritz-tolerant) were hydroponically grown in a continuously aerated nutrient solution, as proposed by [3]. Ten days later, plants were treated with P (0, 0.01 or 0.1 mM) in combination with Si (0, 1 or 2 mM). Twenty-one days after the initiation of treatments, plants were harvested and lignin concentration, the lignin distribution pattern and the gene expression of phenylalanine ammonia lyase (TaPAL) and cinnamyl alcohol dehydrogenase (TaCAD) were analyzed in shoots. The lignin concentration of both wheat cultivars did not vary at different P doses; nevertheless, 2 mM Si increased lignin accumulation mainly at either 0 mM P (cv. Púrpura) or 0.01 mM P (cv. Fritz), with a more noticeable effect in Púrpura than in Fritz. This increase was in agreement with the stronger intensity of Safranine O staining observed after Si was added to both cultivars grown at sufficient or deficient P levels and supports previous findings showing the alleviative role of Si by increasing lignin production under stressful conditions [4,5,6,7,8]. Such an effect may be related to either increased hydrogen peroxide production or peroxidase activity in cell walls, as well as to the modulation of the activity and/or gene expression of some key enzymes involved in lignin biosynthesis. In this way, we also found that Si induced the expression of lignin biosynthesis genes. The up-regulation of TaPAL was detected in cv. Púrpura grown at low P levels, with a further increase observed in plants treated with Si. Similar to Si addition to P-stressed plants of cv., Fritz caused a 1.5-fold increase in the transcript level of TaPAL. Similarly, the expression level of TaCAD increased 1.7-fold as a result of the Si supply to both cultivars grown at low P. Overall, our results show that Si induced the biosynthesis of lignin in shoots of wheat plants grown under P stress. Acknowledgments. FONDECYT Regular Project N° 1201257 and FONDECYT Postdoctoral Project N° 3200901.