SMALL AUXIN UP RNA62/75 Are Required for the Translation of Transcripts Essential for Pollen Tube Growth

Successful pollen tube elongation is critical for double fertilization, but the biological functions of pollen tube genes and the regulatory machinery underlying this crucial process are largely unknown. A previous translatomic study revealed two Arabidopsis (Arabidopsis thaliana) SAUR (SMALL AUXIN UP RNA) genes, SAUR62 and SAUR75, whose expression is up-regulated by pollination. Here, we found that both SAUR62 and SAUR75 localized mainly to pollen tube nuclei. The siliques of homozygous saur62 (saur62/-), saur75 (saur75/-), and the SAUR62/75 RNA interference (RNAi) knockdown line had many aborted seeds. These lines had normal pollen viability but defective in vitro and in vivo pollen tube growth, with branching phenotypes. Immunoprecipitation with transgenic SAUR62/75-GFP flowers revealed ribosomal protein RPL12 family members as potential interacting partners, and their individual interactions were confirmed further by yeast two-hybrid and bimolecular fluorescence complementation assays. Polysome profiling showed reduced 80S ribosome abundance in homozygous saur62, saur75, ribosomal large subunit12c, and SAUR62/75 RNAi flowers, suggesting that SAUR62/75 play roles in ribosome assembly. To clarify their roles in translation, we analyzed total proteins from RNAi versus wild-type flowers by isobaric tags for relative and absolute quantitation, revealing significantly reduced expression of factors participating in pollen tube wall biogenesis and F-actin dynamics, which are critical for the elastic properties of tube elongation. Indeed, RNAi pollen tubes showed mislocalization of deesterified and esterified pectins and F-actin organization. Thus, the biological roles of SAUR62/75 and their RPL12 partners are critical in ribosomal pre-60S subunit assembly for efficient pollen tube elongation and subsequent fertilization.