LARP6C orchestrates posttranscriptional reprogramming of gene expression during hydration to promote pollen tube guidance

Abstract

Increasing evidence suggests that post-transcriptional regulation is a key player in the transition between mature pollen and the progamic phase (from pollination to fertilization). Nonetheless, the actors in this mRNA-based gene expression reprogramming are poorly understood. We demonstrate that the evolutionarily conserved RNA-binding protein (RBP) LARP6C is necessary for the transition from dry pollen to pollen tubes and the guided growth of pollen tubes towards the ovule in Arabidopsis thaliana. In dry pollen, LARP6C binds to transcripts encoding proteins that function in lipid synthesis and homeostasis, vesicular trafficking, and polarized cell growth. LARP6C also forms cytoplasmic granules that contain the poly(A) binding protein and possibly represent storage sites for translationally silent mRNAs. In pollen tubes, loss of LARP6C negatively affects the quantities and distribution of storage lipids, as well as vesicular trafficking. In Nicotiana benthamiana leaf cells and in planta, analysis of reporter mRNAs designed from the LARP6C target MGD2 provided evidence that LARP6C can shift from a repressor to an activator of translation when the pollen grain enters the progamic phase. We propose that LARP6C orchestrates the timely post-transcriptional regulation of a subset of mRNAs in pollen during the transition from the quiescent to active state and along the progamic phase to promote male fertilization in plants.

LARP6 proteins in plants

“…RNArecognition by 6C La-module depends on the length of the probe (with a 20-nt probe being a better substrate than a 10-nt probe) but binds RNA regardless of their 3' extremity (3'-OH or 3'-PO4) which supports that it could interact with terminal or internal sequences on RNA in vivo. In planta functional analyses, corroborated these findings, as mRNA substrates of AtLARP6C were found to share in their 5'-UTRs a (UUC/UUUC) box that AtLARP6C preferentially binds in vitro [21] .…”

“…In vitro AtLARP6C La-module shows a strong specificity towards oligo(U) homopolymers and binds with more affinity probes with uridine stretches interspersed with cytidines [16,21] . RNArecognition by 6C La-module depends on the length of the probe (with a 20-nt probe being a better substrate than a 10-nt probe) but binds RNA regardless of their 3' extremity (3'-OH or 3'-PO4) which supports that it could interact with terminal or internal sequences on RNA in vivo.…”

Lipid anchoring and electrostatic interactions target NOT-LIKE-DAD to pollen endo-plasma membrane

“…To our knowledge, only three proteins, two small GTP-binding rho of plants (PsROP1 and AtROP9) and la and related protein 6c (AtLARP6c), had previously been demonstrated to have endo-PM localization ( Lin et al, 1996 ; Li et al, 2013 ; Billey et al, 2021 ). Among these endo-PM–localized proteins, NLD is the only one to be specifically localized to the endo-PM, because both ROPs and AtLARP6c were also reported to localize in other places in the pollen tube and/or pollen grain ( Lin et al, 1996 ; Li et al, 2013 ; Billey et al, 2021 ), which is not the case for NLD (this study; Gilles et al, 2017a ; Kelliher et al, 2017 ). In this study, the importance of two putatively S -acylated cysteine residues was demonstrated for proper NLD localization to the endo-PM ( Fig.…”

Editorial: Advances in Pollen Research: Biology, Biotechnology, and Plant Breeding Applications

“…The respective Ds-GFP transposable element insertion line showed reduced transmission through the male wich was associated with altered germination dynamics and slower growth of mutant pollen tubes. Considering that LARP6C also orchestrates posttranscriptional reprogramming of gene expression during hydration in Arabidopsis (Billey et al, 2021 ), these findings highlight the general regulatory role of LARP6C in angiosperms. The original research article by Mazuecos-Aguilera et al describes the functional study of the INAPERTURATE POLLEN1 ( INP1 ) ortholog from the basal eudicot Eschscholzia californica (California poppy), a gene involved in pollen aperture formation.…”