The J-protein AtDjB1 is required for mitochondrial complex I activity and regulates growth and development through ROS-mediated auxin signalling.

AtDjB1 is a mitochondria-located J-protein in Arabidopsis thaliana It is involved in the regulation of plant growth and development; however, the exact mechanisms remain to be determined. We performed comparison analyses of phenotypes, auxin signalling, redox status, mitochondrial structure and function using wild-type plants, AtDjB1 mutants, rescued AtDjB1 mutants by AtDjB1 or YUCCA2 (an auxin synthesis gene), and AtDjB1 overexpression plants. AtDjB1 mutants (atj1-1 or atj1-4) exhibited inhibition of growth and development and reductions in the level of IAA and the expression of YUCCA genes compared to wild-type plants. The introduction of AtDjB1 or YUCCA2 into atj1-1 largely rescued phenotypic defects and the IAA level, indicating that AtDjB1 probably regulates growth and development via auxin. Furthermore, atj1-1 plants displayed a significant reduction in amount/activity of mitochondrial complex I compared to wild-type plants; this resulted in the accumulation of reactive oxygen species (ROS). Moreover, exogenous H2O2 markedly inhibited the expression of YUCCA genes in wild-type plants. In contrast, the reducing agent ascorbate increased the expression of YUCCA genes and IAA level in atj1-1 plants, indicating that the low auxin level observed in atj1-1 was probably due to the high oxidation status. Overall, the data presented here suggest that AtDjB1 is required for mitochondrial complex I activity and regulates growth and development through ROS-mediated auxin signalling in Arabidopsis.

The Arabidopsis J-protein AtDjB1 facilitates thermotolerance by protecting cells against heat-induced oxidative damage.

AtDjB1 belongs to the J-protein family in Arabidopsis thaliana. Its biological functions in plants are largely unknown. In this study, we examined the roles of AtDjB1 in resisting heat and oxidative stresses in A. thaliana using reverse genetic analysis. AtDjB1 knockout plants (atj1-1) were more sensitive to heat stress than wildtype plants, and displayed decreased concentrations of ascorbate (ASC), and increased concentrations of hydrogen peroxide (H(2)O(2)) and oxidative products after heat shock. Application of H(2)O(2) accelerated cell death and decreased seedling viability in atj1-1. Exogenous ASC conferred much greater thermotolerance in atj1-1 than in wildtype plants, suggesting that a lower concentration of ASC in atj1-1 could be responsible for the increased concentration of H(2)O(2) and decreased thermotolerance. Furthermore, AtDjB1 was found to localize to mitochondria, directly interact with a mitochondrial heat-shock protein 70 (mtHSC70-1), and stimulate ATPase activity of mtHSC70-1. AtDjB1 knockout led to the accumulation of cellular ATP and decreased seedling respiration, indicating that AtDjB1 modulated the ASC concentration probably through affecting the function of mitochondria. Taken together, these results suggest that AtDjB1 plays a crucial role in maintaining redox homeostasis, and facilitates thermotolerance by protecting cells against heat-induced oxidative damage.