A Plastid-Bound Ankyrin Repeat Protein Controls Gametophyte and Early Embryo Development in Arabidopsis thaliana.

Katarína Kulichová,David Honys,Vinod Kumar,Said Hafidh,Janto Pieters

doi:10.3389/fpls.2022.767339

Abstract

Proplastids are essential precursors for multi-fate plastid biogenesis, including chloroplast differentiation, a powerhouse for photosynthesis in plants. Arabidopsis ankyrin repeat protein (AKRP, AT5G66055) is a plastid-localized protein with a putative function in plastid differentiation and morphogenesis. Loss of function of akrp leads to embryo developmental arrest. Whether AKRP is critical pre-fertilization has remained unresolved. Here, using reverse genetics, we report a new allele, akrp-3, that exhibited a reduced frequency of mutant embryos (<13%) compared to previously reported alleles. akrp-3 affected both male and female gametophytes resulting in reduced viability, incompetence in pollen tube attraction, altered gametic cell fate, and embryo arrest that were depleted of chlorophyll. AKRP is widely expressed, and the AKRP-GFP fusion localized to plastids of both gametophytes, in isolated chloroplast and co-localized with a plastid marker in pollen and pollen tubes. Cell-type-specific complementation of akrp-3 hinted at the developmental timing at which AKRP might play an essential role. Our findings provide a plausible insight into the crucial role of AKRP in the differentiation of both gametophytes and coupling embryo development with chlorophyll synthesis.

Highlights

Proplastids are essential organelles for plant life cycle; they provide nutrients and monomers for energy, act as a precursor for chloroplast differentiation, and function in biosynthesis and storage of pigments, hormones, starch, fats, proteins, and terpenes
Two previous embryo-lethal alleles of ankyrin repeat protein (AKRP), emb2036-1 and emb2036-2, were reported to show 25% of pale mutant homozygous seeds containing embryos arrested at globular stage but exhibited no gametophytic defects (Garcion et al, 2006; Meinke et al, 2008)
Public RNA-seq data suggest that AKRP transcripts are higher in microspores but are drastically reduced by pollen maturity, and were detected in isolated egg cell of Arabidopsis thaliana (Julca et al, 2021)

Summary

Introduction

Proplastids are essential organelles for plant life cycle; they provide nutrients and monomers for energy, act as a precursor for chloroplast differentiation, and function in biosynthesis and storage of pigments, hormones, starch, fats, proteins, and terpenes. Proplastids differentiate into chloroplasts from the globular stage, and later, during seed maturation, differentiate into storage plastids, elaioplasts (Demarsy et al, 2012; Allorent et al, 2013; reviewed in Liebers et al, 2017). Defects in plastid function might manifest prior to embryo development in the male or female gametophyte. Energy production in mature pollen and growing pollen tubes (PTs) is mainly supplied from mitochondrial respiration and ATP production; glycolysis within plastids is a contributing source of energy (reviewed in Selinski and Scheibe, 2014). Accumulation of starch during pollen maturation is, critical, as it serves later as an energy source. Knockout mutants in enzymes catalyzing the regeneration of NAD + during glycolysis, gapcp and gapcp, are sterile males

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