TheArabidopsisProteasome RPT5 Subunits Are Essential for Gametophyte Development and Show Accession-Dependent Redundancy

Abstract

We investigated the role of the ubiquitin proteasome system (UPS), which allows proteins to be selectively degraded, during gametophyte development in Arabidopsis thaliana. Three mutant alleles altering the UPS were isolated in the Wassilewskija (Ws) accession: they affect the Regulatory Particle 5a (RPT5a) gene, which (along with RPT5b) encodes one of the six AAAATPases of the proteasome regulatory particle. In the heterozygous state, all three mutant alleles displayed 50% pollen lethality, suggesting that RPT5a is essential for male gametophyte development. However, a fourth mutant in the Columbia (Col) accession did not display such a phenotype because the RPT5b Col allele complements the rpt5a defect in the male gametophyte, whereas the RPT5b Ws allele does not. Double rpt5a rpt5b mutants showed a complete male and female gametophyte lethal phenotype in a Col background, indicating that RPT5 subunits are essential for both gametophytic phases. Mitotic divisions were affected in double mutant gametophytes correlating with an absence of the proteasome-dependent cyclinA3 degradation. Finally, we show that RPT5b expression is highly increased when proteasome functioning is defective, allowing complementation of the rpt5a mutation. In conclusion, RPT5 subunits are not only essential for both male and female gametophyte development but also display accession-dependent redundancy and are crucial in cell cycle progression.

Arabidopsis thaliana26S Proteasome Subunits RPT2a and RPT5a Are Crucial for Zinc Deficiency-Tolerance

“…[7][8][9] We found that the T-DNA insertion mutants of RPT2b and RPT5b were not Zn deficiency-sensitive (Fig. 1D).…”

“…RPT5a, RPT5b play a similar role in gametophyte development. 7) Mutation in RPT2a causes increased ploidy levels and an enlargement of the leaf organ size, whereas these phenotypes were not observed in the RPT2b mutant. 8,9) These functional differences are believed to arise from the specific association of individual RP subunits and its paralog with poly-ubiquitinated proteins.…”

Response to biotic and oxidative stress in Arabidopsis thaliana: Analysis of variably phosphorylated proteins

“…This cytochrome b 5 heme-and steroid-binding domains-containing phosphoprotein, present in thylacoid membrane, has been reported being involved in oxidative defense and stress response [41]. Phosphorylated proteasome RPT5a subunit (spot 25) showed a 5-fold increase; this protein belongs to the AAA-ATPases associated with the proteasome regulatory particle [42]. Recently, Hatsugai and coworkers have demonstrated the involvement of proteasome-dependent tonoplast and plasma membrane fusion in a novel plant defense process triggering bacteria-induced programmed cell death, which leads to a vacuolar release of antimicrobial and deathinducing compounds [43].…”

Proteasome regulation, plant growth and stress tolerance

“…For example, combining knock-out mutations in both copies of genes encoding Arabidopsis RP subunit isoforms led to the lethality of both male and female gametes. 11,12 While the analyses of null mutants of RP subunits confirmed the essential function of the 26SP in plants, studies using weak loss-of-function mutants have allowed to determine the impact of fluctuations in proteasome activity on plant growth and stress responses. [13][14][15][16][17][18][19] For example, comparative analyses of Arabidopsis 26SP mutant lines rpt2a-2, rpt2a-3, rpn10-1 and rpn12a-1 revealed that reductions in 26SP abundance invariably lead to increased cell expansion combined with decreased cell division rates.…”