The mitochondrial fission regulator DRP3B does not regulate cell death in plants

Mitochondrial Fission Complex Is Required for Long-Term Heat Tolerance of Arabidopsis

Plants are often exposed not only to short-term (S) heat stress but also to long-term (L) heat stress over several consecutive days. A few Arabidopsis mutants defective in L-heat tolerance have been identified, but the molecular mechanisms involved are less well understood than those involved in S-heat tolerance. To elucidate the mechanisms, we isolated the new sensitive to long-term heat5 (sloh5) mutant from EMS-mutagenized seeds of L-heat-tolerant Col-0. The sloh5 mutant was hypersensitive to L-heat but not to S-heat, osmo-shock, salt-shock or oxidative stress. The causal gene, SLOH5, is identical to elongatedmitochondria1 (ELM1), which plays an important role in mitochondrial fission in conjunction with dynamin-related proteins DRP3A and DRP3B. Transcript levels of ELM1, DRP3A and DRP3B were time-dependently increased by L-heat stress, and drp3a drp3b double mutants were hypersensitive to L-heat stress. The sloh5 mutant contained massively elongated mitochondria. L-heat stress caused mitochondrial dysfunction and cell death in sloh5. Furthermore, WT plants treated with a mitochondrial myosin ATPase inhibitor were hypersensitive to L-heat stress. These findings suggest that mitochondrial fission and function are important in L-heat tolerance of Arabidopsis.

Protein partners of plant ubiquitin-specific proteases (UBPs)

As one type of deubiquitinases (DUBs), ubiquitin-specific proteases (UBPs) play an extensive and significant role in plant life involving the regulation of plant development and stress responses. However, comprehensive studies are still needed to determine the functional mechanisms, which are largely unclear. Here, we summarized recent progress of plant UBPs' functional partners, particularly the molecular mechanisms by which UBPs work with their partners. We believe that functional analyses of UBPs and their partners will provide new insights into protein deubiquitination and lead to a better understanding of the physiological roles of UBPs in plants.

Qualitative and quantitative modifications of root mitochondria during senescence of above-ground parts of Arabidopis thaliana

This work studied modifications experienced by root mitochondria during whole plant senescence or under light deprivation, using Arabidopsis thaliana plants with YFP tagged to mitochondria. During post-bolting development, root respiratory activity started to decline after aboveground organs (i.e., rosette leaves) had senesced. This suggests that carbohydrate starvation may induce root senescence. Similarly, darkening the whole plant induced a decrease in respiration of roots. This was partially due to a decrease in the number of total mitochondria (YFP-labelled mitochondria) and most probably to a decrease in the quantity of mitochondria with a developed inner membrane potential (ΔΨm, i.e., Mitotracker red- labelled mitochondria). Also, the lower amount of mitochondria with ΔΨm compared to YFP-labelled mitochondria at 10d of whole darkened plant, suggests the presence of mitochondria in a "standby state". The experiments also suggest that small mitochondria made the main contribution to the respiratory activity that was lost during root senescence. Sugar supplementation partially restored the respiration of mitochondria after 10d of whole plant dark treatment. These results suggest that root senescence is triggered by carbohydrate starvation, with loss of ΔΨm mitochondria and changes in mitochondrial size distribution.

Mitochondrial energy and redox signaling in plants

SIGNIFICANCE

For a plant to grow and develop, energy and appropriate building blocks are a fundamental requirement. Mitochondrial respiration is a vital source for both. The delicate redox processes that make up respiration are affected by the plant's changing environment. Therefore, mitochondrial regulation is critically important to maintain cellular homeostasis. This involves sensing signals from changes in mitochondrial physiology, transducing this information, and mounting tailored responses, by either adjusting mitochondrial and cellular functions directly or reprogramming gene expression.

RECENT ADVANCES

Retrograde (RTG) signaling, by which mitochondrial signals control nuclear gene expression, has been a field of very active research in recent years. Nevertheless, no mitochondrial RTG-signaling pathway is yet understood in plants. This review summarizes recent advances toward elucidating redox processes and other bioenergetic factors as a part of RTG signaling of plant mitochondria.

CRITICAL ISSUES

Novel insights into mitochondrial physiology and redox-regulation provide a framework of upstream signaling. On the other end, downstream responses to modified mitochondrial function have become available, including transcriptomic data and mitochondrial phenotypes, revealing processes in the plant that are under mitochondrial control.

FUTURE DIRECTIONS

Drawing parallels to chloroplast signaling and mitochondrial signaling in animal systems allows to bridge gaps in the current understanding and to deduce promising directions for future research. It is proposed that targeted usage of new technical approaches, such as quantitative in vivo imaging, will provide novel leverage to the dissection of plant mitochondrial signaling.

Arabidopsis sphingolipid fatty acid 2-hydroxylases (AtFAH1 and AtFAH2) are functionally differentiated in fatty acid 2-hydroxylation and stress responses

2-Hydroxy fatty acids (2-HFAs) are predominantly present in sphingolipids and have important physicochemical and physiological functions in eukaryotic cells. Recent studies from our group demonstrated that sphingolipid fatty acid 2-hydroxylase (FAH) is required for the function of Arabidopsis (Arabidopsis thaliana) Bax inhibitor-1 (AtBI-1), which is an endoplasmic reticulum membrane-localized cell death suppressor. However, little is known about the function of two Arabidopsis FAH homologs (AtFAH1 and AtFAH2), and it remains unclear whether 2-HFAs participate in cell death regulation. In this study, we found that both AtFAH1 and AtFAH2 had FAH activity, and the interaction with Arabidopsis cytochrome b₅ was needed for the sufficient activity. 2-HFA analysis of AtFAH1 knockdown lines and atfah2 mutant showed that AtFAH1 mainly 2-hydroxylated very-long-chain fatty acid (VLCFA), whereas AtFAH2 selectively 2-hydroxylated palmitic acid in Arabidopsis. In addition, 2-HFAs were related to resistance to oxidative stress, and AtFAH1 or 2-hydroxy VLCFA showed particularly strong responses to oxidative stress. Furthermore, AtFAH1 interacted with AtBI-1 via cytochrome b₅ more preferentially than AtFAH2. Our results suggest that AtFAH1 and AtFAH2 are functionally different FAHs, and that AtFAH1 or 2-hydroxy VLCFA is a key factor in AtBI-1-mediated cell death suppression.