Ultrastructural changes and defense strategy of yellow lupine during silver nanoparticle and Fusarium oxysporum interaction

This study attempted to elucidate the alterations in ultrastructure of root cap cells and some metabolic reactions in yellow lupine (Lupinus luteus L.) root cells induced by silver nanoparticles (AgNPs) during the seed germination stage and seedling development. Also, the role of AgNPs in defense mechanisms against hemibiotrofic patogen Fusarium oxysporum f.sp. lupini was studied. Therefore, ultrastructural analysis of root cap cells of L. luteus L. cvs. Diament and Mister were performed by transmission electron microscopy (TEM). Additionally, intracellular reactive oxygen species (ROS), particularly hydrogen peroxide (H2O2) was detected in root cells using confocal microscopy. The obtained research results showed distinct alterations in ultrastructure of root cap cells after pretreated with AgNPs or during cross-talk between AgNPs and F. oxysporum f.sp. lupini. It was surprising that the application of AgNPs caused local and clearly visible thickenings of the cell wall (CW) with a wide electron-dense apoplast space (AS) in the root cap cells. Also, an interesting observation was an increased presence of dictyosomes (D), characterized by a highly developed cisternae system in cells of the AgNPs and AgNPs + F. oxysporum variants in comparison to the control. An increase in the processes of endocytosis and exocytosis was noticed, which is associated with the transport of material enclosed or inserted into membrane vesicles. Besides, F. oxysporum infection induced strong autophagic activity and the occurrence of multivesicular bodies (MVBs) and multilamellar bodies (MLBs). Furthermore, in response to AgNPs or F. oxysporum alone, or in combination with AgNPs and F. oxysporum, increase in green fluorescence emission associated with ROS, especially H2O2 in relation to the control.