The Effects of Selenite on Filamentous Fungi Lipid Droplets Monitored in Vivo Label Free Using Advanced Nonlinear Microscopy Technique

Abstract

Third Harmonic Generation (THG) microscopy was employed as a method of choice for lipid droplet (LD) measurements and quantification of the effect of selenite on LDs. Nonlinear laser scanning microscopy (NLSM) employs ultra-short laser pulses for imaging. THG microscopy is the modality of NLSM. Strong THG signals can only be observed from regions with non-uniformities with respect to their refractive index. Such regions in biological samples are lipid-water interfaces, and by far the brightest features in cells are LDs. For that reason, THG microscopy is the appropriate method for imaging of LDs from live unfixed cells, without the need for additional labeling. The biological effects of spore- to- end- of- exponential- phase duration (27 - 30 h) of exposure to 1 mM selenite were monitored in vivo on the cells of filamentous fungi in liquid culture. We measured the lipid droplet density and size distribution in a model fungi Phycomyces blakesleeanus. The in-house built microscope frame complemented with Yb KGW laser (1040 nm, 200 fs pulses) was used, while detection was enabled in the transmission arm by PMT through the Hoya glass UV filter (peak at 340 nm). From THG images of control and Se+4–treated hyphae, LD size and number were measured, showing that LD density was increased by more than 60% in Se+4–treated hyphae, compared to control. The average LD size distribution seemed slightly changed by Se+4-treatment. The obtained results suggest that 1 mM selenite treatment probably induces cellular stress response in filamentous fungi.