Silicon increases cell wall thickening and lignification in rice (Oryza sativa) root tip under excess Fe nutrition.

Iron (Fe) as a micronutrients and silicon (Si) as a cell wall element are important in plant cell wall extension and integrity. While the interaction of exogenous Si and excess Fe on root cell wall modifications is known, the effects of these nutritional parameters on the spatial changes in the activities of genes and/or enzymes involved in the lignification of root cell walls are not well studied. Thus, these parameters were investigated in the root apical part (AP) and basal part (BP) of rice (Oryza sativa L.) plants supplied with and without Si (1.5 mM) under normal (10 mg/L) and excess Fe (150 mg/L) nutrition for 7 days. Beside growth retardation, excess Fe increased the activities of phenylalanine ammonia lyase (PAL), superoxide dismutase and NADPH-oxidase and PAL and cell wall peroxidase (POD) genes expression, along with the increased phenols and H2O2 contents in the root AP. Furthermore, the increased thickening of endodermal, exodermal and metaxylem cell walls in the root AP by excess Fe was attributed to the enhanced POD activity. POD expression, endodermal and exodermal cell wall thickenings were not affected by excess Fe in the root BP. Si application under excess Fe exaggerated the effects of excess Fe on root cell wall thickening, increased POD activity but reduced H2O2 content in the root AP. Thus, Si application under excess Fe nutrition promotes earlier initiation of lignin polymerization closer to and toward the root tip and hence restricts the entry of excess Fe into the plant.

Increased cell wall thickness of endodermis and protoxylem in Aeluropus littoralis roots under salinity: The role of LAC4 and PER64 genes.

Enhanced cell wall lignification is one of the major salinity tolerance strategies in the roots of halophytes. A deep insight into the exact root developmental system in halophytes may be of great importance for understanding plant salt tolerance mechanisms. In this work the developmental and anatomical changes in the roots of halophyte Aeluropus littoralis along with expression patterns of two genes encoding for cell wall laccase (LAC4) and peroxidase (PER64) were investigated. The plants were treated with 0, 300 and 600mM NaCl and root samples were collected 3, 6 and 9days after treatment (DAT). Upon salinity treatment, root diameter and parenchyma thickness were increased significantly in the tip and middle segments compared to upper zones, but the change trend was reversed with the time. It was interestingly revealed that protoxylem was the tissue of target for lignification at root tips, while the highest lignification rates were observed in metaxylem and endodermis in upper segments. Compared to endodermis, protoxylem is restrictively involved in early stages of salt stress in root tips as an efficient barrier against Na+ flow. Gene expression analysis revealed that LAC4 expression was higher in root tips resulting in enhanced protoxylem lignification while PER64 expression was higher in more differentiated zones leading to endodermis thickening. The overall results of this study reveal the crucial role of LAC4 as an important gene in specialized protoxylem lignification in undifferentiated root tips leading to enhanced tolerance in early stages of salt stress.