Studies Defining the Role of Protein Disulfide Isomerase-9 in Pollen Biogenesis in Arabidopsis thaliana

Abstract

Protein disulfide isomerases (PDIs) are disulfide bond catalysts that serve a breadth of important roles in eukaryotic growth and development. Arabidopsis thaliana has fourteen PDIs with variations on the canonical domain arrangement and subcellular location. Among them, PDI9 and its homolog PDI10 are part of the unfolded protein response (UPR) and have been shown to fold proteins via disulfide bonds. Using a PDI9-specific antiserum, PDI9 was found to be expressed in mature pollen. Here, the role of PDI9 in pollen biogenesis was characterized on molecular and cellular scales. First, using transfected leaf mesophyll protoplasts, PDI9 was shown to co-localize in the endoplasmic reticulum with two proteins (Leucine Rich Repeat-Extensin 8, LRX8, and ER Membrane Complex Subunit 7, EMC7) that are both expressed in pollen. Double knockout pdi9-pdi10 seedlings were used to demonstrate constitutive up-regulation of other PDIs and chaperones in non-UPR-stressed conditions relative to wild-type. Using an improved high-throughput Alexander staining method and scanning electron microscopy, PDI9 was found to play a crucial role in pollen development under prolonged heat stress. The single pdi9 and double pdi9-pdi10 knockout plants produce less viable pollen, dehisce fewer pollen grains, have impaired silique development, and exine formation is severely disrupted. Taken together, these data suggest that PDI9 mediates the development of healthy pollen under heat stress via its role in the UPR and its interactions with secretory substrates (LRX8, EMC7).