Silicon modifies both a local response and a systemic response to mechanical stress in tobacco leaves

Both lignin and silicon (Si) are major players in the resistance of plants to mechanical stress (MS). Focusing on the phenolic metabolism, here we studied the short-term effects of a local MS on tobacco ( Nicotiana rustica L. cv. Basmas) plants with Si (+Si, 1 mM Na 2 SiO 3 ) and without Si (‒Si) treatments in order to see how Si may modify local and systemic responses. One week after starting the Si treatment, a half of the plants were exposed to a mechanical pressure applying 980 Pa for 24 h on the upper side of the 3 rd leaf of each plant (+MS). The rest of the plants remained unstressed (‒MS). Plants were harvested 24 h and 72 h after starting the MS and the leaves directly exposed to the mechanical stress (DMS) and those indirectly exposed to the mechanical stress (IMS) from below and above the DMS leaf were analyzed for phenolic metabolism along with the corresponding leaves from‒MS plants. In the DMS leaf, the activities of polyphenol oxidase, phenylalanine ammonia lyase, and cytosolic and covalently-bound peroxidases increased by the MS, while decreased by Si. In accordance with this in the DMS leaf, the content of soluble and cell wall-bound phenolics and lignin were enhanced by the MS but decreased by Si. Interestingly, Si influenced the pattern of response to the MS depending on whether the leaves were directly treated by the MS or not. Silicon treatment augmented MS-induced lignin accumulation in the DMS leaf while rather inhibited lignin formation in the IMS leaves. These data show that Si modified MS-mediated changes in the phenolic metabolism differently in local and systemic leaves.