Abstract
Vacuolar processing enzyme (VPE) is responsible for the maturation and activation of vacuolar proteins in plants. We found that βVPE was involved in tapetal degradation and pollen development by transforming proproteases into mature protease in Arabidopsis thaliana. βVPE was expressed specifically in the tapetum from stages 5 to 8 of anther development. The βVPE protein first appeared as a proenzyme and was transformed into the mature enzyme before stages 7-8. The recombinant βVPE protein self-cleaved and transformed into a 27 kDa mature protein at pH 5.2. The mature βVPE protein could induce the maturation of CEP1 in vitro. βvpe mutants exhibited delayed vacuolar degradation and decreased pollen fertility. The maturation of CEP1, RD19A, and RD19C was seriously inhibited in βvpe mutants. Our results indicate that βVPE is a crucial processing enzyme that directly participates in the maturation of cysteine proteases before vacuolar degradation, and is indirectly involved in pollen development and tapetal cell degradation.
Highlights
Tapetal cells are degraded through programmed cell death (PCD) to provide various nutrients for pollen development, the formation of pollen exine
A scanning electron microscopy examination revealed that mature pollen grains in wild-type plants were uniformly spheroid and had finely reticulate ornamentation on their surfaces (Figure 2F and H), while the βvpe-1 mutant produced some mature pollen grains that were similar to wild-type and some abnormal pollen grains (50.00%, 56 of 112, P
These results indicate that the absence of βVPE markedly impaired pollen development and resulted in sterile pollen grains with abnormal pollen morphology
Summary
Tapetal cells are degraded through programmed cell death (PCD) to provide various nutrients for pollen development, the formation of pollen exine. ΒVPE is essential for storage protein processing (Shimada et al, 2003), and δVPE, which is expressed in the seed coat, is associated with cell death (Nakaune et al, 2005). OsVPE1 which is a homolog of the Arabidopsis βVPE gene, is a cysteine protease that plays a crucial role in the maturation of rice glutelins (Wang et al, 2009). ΒVPE is expressed in roots, flowers, buds, and ovules, and is expressed during ovule development in Vitis vinifera (Tang et al, 2016) For these reasons, βVPE is speculated to play an essential role in flower development, but its exact function and corresponding mechanism of action remain uncertain. We present the first evidence of the activation of papain-like cysteine proteases by VPE during anther development in Arabidopsis thaliana
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