Abstract
Although the final size of plant organs is influenced by environmental cues, it is generally accepted that the primary size determinants are intrinsic factors that regulate and coordinate cell proliferation and cell expansion. Here, we show that optimal proteasome function is required to maintain final shoot organ size in Arabidopsis (Arabidopsis thaliana). Loss of function of the subunit regulatory particle AAA ATPase (RPT2a) causes a weak defect in 26S proteasome activity and leads to an enlargement of leaves, stems, flowers, fruits, seeds, and embryos. These size increases are a result of increased cell expansion that compensates for a reduction in cell number. Increased ploidy levels were found in some but not all enlarged organs, indicating that the cell size increases are not caused by a higher nuclear DNA content. Partial loss of function of the regulatory particle non-ATPase (RPN) subunits RPN10 and RPN12a causes a stronger defect in proteasome function and also results in cell enlargement and decreased cell proliferation. However, the increased cell volumes in rpn10-1 and rpn12a-1 mutants translated into the enlargement of only some, but not all, shoot organs. Collectively, these data show that during Arabidopsis shoot development, the maintenance of optimal proteasome activity levels is important for balancing cell expansion with cell proliferation rates.
Highlights
The final size of plant organs is influenced by environmental cues, it is generally accepted that the primary size determinants are intrinsic factors that regulate and coordinate cell proliferation and cell expansion
The majority of RP mutants described to date have shorter roots, which suggests that it is the down-regulation of 26S proteasome (26SP) function and not the RPT2a subunit that leads to the change in root growth (Kurepa et al, 2008)
Most of the phenotypes of the RP mutants described to date seem to reflect a general alteration in proteasome activity and not a specific defect that relates to a specific function of a particular subunit (Huang et al, 2006; Kurepa et al, 2008)
Summary
The final size of plant organs is influenced by environmental cues, it is generally accepted that the primary size determinants are intrinsic factors that regulate and coordinate cell proliferation and cell expansion. Loss of function of the subunit regulatory particle AAA ATPase (RPT2a) causes a weak defect in 26S proteasome activity and leads to an enlargement of leaves, stems, flowers, fruits, seeds, and embryos These size increases are a result of increased cell expansion that compensates for a reduction in cell number. The majority of RP mutants described to date have shorter roots, which suggests that it is the down-regulation of 26SP function and not the RPT2a subunit that leads to the change in root growth (Kurepa et al, 2008) In another example, the ae mutant, isolated as an enhancer of asymmetric leaves (as1) and as, was shown to carry a mutation in the RPN8a gene (Huang et al, 2006). This suggests that optimal proteasome function is required for the proper execution of preprogrammed cell proliferation and expansion rates
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