Abstract
Boron (B), an essential micronutrient, causes adverse effects on the growth and development of plants when highly accumulated. By the analysis of Arabidopsis mutants hypersensitive to high-boron (high-B) stress, we have shown that 26S proteasome (26SP) is required to maintain the morphology of the root apical meristem (RAM) under high-B stress. To further understand the molecular function of 26SP in tolerance to high-B stress in the RAM, in this study we investigated the pathways regulated by 26SP using a 26SP subunit mutant, rpt5a, which is hypersensitive to high-B stress. Expression of RPT5a was induced by high-B stress in the entire RAM accompanied by its strong expression in the stele, including the stem cells. Analysis of stele organization in the rpt5a mutant revealed that 26SP is especially important for maintenance of the stele under high-B stress condition (3 mM B treatment). Expression analyses of an auxin-response reporter revealed that auxin responses were enhanced in the stele and the stem cell niche by high-B stress, especially in the rpt5a mutant. In contrast, the expression of TCS::GFP representing cytokinin signaling in the stem cell niche was unchanged in the wild type and extremely weak in the rpt5a mutant, irrespective of B condition. The drastically aberrant auxin and cytokinin responses in the rpt5a mutant under high-B stress were supported by transcriptome analysis using root tips. These results suggest that the collapse of hormonal crosstalk in the stele including the stem cells occurred in the rpt5a mutant, especially under high-B stress. Treatment with the auxin signaling inhibitor α-(phenyl ethyl-2-one)-indole-3-acetic acid (PEO-IAA) reduced sensitivity to high-B stress in the wild type and restored the RAM morphology in the rpt5a mutant under the high-B stress condition. In addition, cytokinin treatment conferred the rpt5a mutant with tolerance to high-B stress in RAM morphology. It is concluded that 26SP containing RPT5a is required for maintenance of auxin/cytokinin balance in the stele, which is crucial for preventing defects in RAM morphology under high-B stress.
Highlights
The micronutrient boron (B) performs indispensable roles for plant growth and development
In plants treated with 3 mM B, GUS expression was detected in the cells outside of the stele in the root apical meristem (RAM) and staining in the stele extended toward the stem cell niche
Treatment with 6 mM B further enhanced GUS expression in the same regions as observed in the 3 mM B condition. These results indicated that regulatory particle AAA-ATPase 5a (RPT5a) in the RAM is responsive to high-B stress
Summary
The micronutrient boron (B) performs indispensable roles for plant growth and development. B is toxic when it is accumulated to a high concentration in plant tissues, and causes growth retardation. Inhibition of cell division in the RAM is a critical cause of B toxicity in roots. In roots of Arabidopsis thaliana, high-B treatment decreases the RAM size through inhibition of cell division activity (Sakamoto et al, 2011, 2018; Aquea et al, 2012). The reduction in cell division activity is observed in root tips of Vicia faba treated with a high B concentration (Liu et al, 2000), The inhibition of cell division is attributed to DNA damage caused by high-B stress in the RAM (Sakamoto et al, 2011). We previously reported that a chromosomal protein complex (condensin II) and a regulatory particle AAA-ATPase 5a (RPT5a) of 26S proteasome (26SP) are essential for amelioration of high-B-dependent DNA damage and the maintenance of RAM size in A. thaliana (Sakamoto et al, 2011, 2018)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
