Endoplasmic Reticulum Bodies in the Lateral Root Cap Are Involved in the Direct Transport of Beta-Glucosidase to Vacuoles

Programmed cell death (PCD) in lateral root caps (LRCs) is crucial for maintaining root cap functionality. Endoplasmic reticulum (ER) bodies play important roles in plant immunity and PCD. However, the distribution of ER bodies and their communication with vacuoles in the LRC remain elusive. In this study, we investigated the ultrastructure of LRC cells of wild-type and transgenic Arabidopsis lines using an auto-acquisition transmission electron microscope (TEM) system and high-pressure freezing. Gigapixel-scale high-resolution TEM imaging of the transverse and longitudinal sections of roots followed by three-dimensional imaging identified sausage-shaped structures budding from the ER. These were subsequently identified as ER bodies using GFPh transgenic lines expressing green fluorescent protein (GFP) fused with an ER retention signal (HDEL). Immunogold labeling using an anti-GFP antibody detected GFP signals in the ER bodies and vacuoles. The fusion of ER bodies with vacuoles in LRC cells was identified using correlative light and electron microscopy. Imaging of the root tips of a GFPh transgenic line with a PYK10 promoter revealed the localization of PYK10, a member of the beta-glucosidase family with an ER retention signal, in the ER bodies in the inner layer along with a fusion of ER bodies with vacuoles in the middle layer and collapse of vacuoles in the outer layer of the LRC. These findings suggest that ER bodies in LRC directly transport beta-glucosidases to the vacuoles, and that a subsequent vacuolar collapse triggered by an unknown mechanism releases protective substances to the growing root tip to protect it from the invaders.