*4.3. Transmission Electron Microscopy*

The third leaves of 30-day-old wild type and *yl* plants were collected and fixed for at least 2 h at 4 ◦C in 0.1 M phosphate buffer, pH 7.4, with 2.5% glutaraldehyde and washed with the same buffer. The samples were fixed in 1% osmium tetroxide for 2 h and then dehydrated through an ethanol gradient, infiltrated and embedded in epoxy resin. Ultra-thin sections were obtained using a diamond knife and mounted on copper grids. Then the grids were stained with uranyl acetate and lead citrate, and examined using a JEM-1400 (JEOL, Tokyo, Japan) transmission electron microscope.
