Signal role for activation of caspase-3-like protease and burst of superoxide anions during Ce4+-induced apoptosis of cultured Taxus cuspidata cells

Effects of ROS and caspase-3-like protein on the growth and aerenchyma formation of Potamogeton perfoliatus stem

Aerenchyma formation plays an important role in the survival of Potamogeton perfoliatus in submerged environment. To understand the regulatory role of reactive oxygen species (ROS) and caspase 3-like protein signaling molecules in aerenchyma formation, we investigated the effects of exogenous NADPH oxidase inhibitor (diphenyleneiodonium chloride, DPI), catalase inhibitor (3-amino-1,2,4-triazole, AT), and caspase-3-like protein inhibitor (AC-DEVD-CHO, DEVD) on morphological and physiological characteristics and aerenchyma formation in P. perfoliatus. The results showed that after DPI treatment, caspase-3-like protein activity decreased, ROS-related enzyme activities increased, and H₂O₂ content decreased, thereby inhibiting aerenchyma formation. When the concentration of DPI was approximately 1 μmol/L, the inhibitory effect was the most obvious. On the contrary, after the AT treatment, caspase-3-like protein activity increased, ROS-related enzyme activities decreased, and the H₂O₂ content increased, ultimately promoting aerenchyma formation, and the promotion was the most obvious under treatment with approximately 500 μmol/L AT. After DEVD treatment, the inhibition of vegetative growth caused by DPI or AT treatment was alleviated, significantly reducing caspase-3-like activity and inhibiting aerenchyma development. The results of this study show that ROS has a positive regulatory effect on aerenchyma formation, and caspase-3-like protein is activated to promote ROS-mediated aerenchyma formation. This experiment provides a new theoretical basis for further exploration of the signal transduction effects of ROS and caspase-3-like protein in plant cells and their roles in plant development.

Transcription level differences in Taxus wallichiana var. mairei elicited by Ce3+, Ce4+ and methyl jasmonate

Taxol is a precious and effective anticancer drug. Cerium and methyl jasmonate (MJ) have been shown to increase the yield of taxol in taxus cells. However, the mechanisms of cerium-mediated and MJ-mediated taxol biosynthesis remain unknown. RNA-Seq was applied to study the overall regulation mechanism of cerium and MJ on taxol biosynthesis and analyze the differences among T. mairei cells elicited by Ce³⁺, Ce⁴⁺ and MJ on transcriptional level . Using sequence homology, 179 unigenes were identified as taxol synthesis genes. Under the condition of 100 μM MJ, taxol synthesis genes were up-regulated. Notably, taxol synthesis genes were down-regulated expression at 1 mM Ce³⁺ and 1 mM Ce⁴⁺. Differential expression genes involved in some related functions were analyzed, such as MAPK signaling pathway and plant-pathogen interaction. Sequence alignment and phylogenetic analysis of nine differentially expressed WRKYs in our data were carried out.

Proteolytic activities in cortex of apical parts of Vicia faba ssp. minor seedling roots during kinetin-induced programmed cell death

Programmed cell death (PCD) is a crucial process in plant development. In this paper, proteolytically related aspects of kinetin-induced PCD in cortex cells of Vicia faba ssp. minor seedlings were examined using morphological, fluorometric, spectrophotometric, and fluorescence microscopic analyses. Cell viability estimation after 46 μM kinetin treatment of seedling roots showed that the number of dying cortex cells increased with treatment duration, reaching maximum after 72 h. Weight of the apical root segments increased with time and was about 2.5-fold greater after 96 h, while the protein content remained unchanged, compared to the control. The total and cysteine-dependent proteolytic activities fluctuated during 1-96-h treatment, which was not accompanied by the changes in the protein amount, indicating that the absolute protein amounts decreased during kinetin-induced PCD. N-ethylmaleimide (NEM), phenylmethylsulfonyl fluoride (PMSF), and Z-Leu-Leu-Nva-H (MG115), the respective cysteine, serine, and proteasome inhibitors, suppressed kinetin-induced PCD. PMSF significantly decreased serine-dependent proteolytic activities without changing the amount of proteins, unlike NEM and MG115. More pronounced effect of PMSF over NEM indicated that in the root apical segments, the most important proteolytic activity during kinetin-induced PCD was that of serine proteases, while that of cysteine proteases may be important for protein degradation in the last phase of the process. Both NEM and PMSF inhibited apoptotic-like structure formation during kinetin-induced PCD. The level of caspase-3-like activity of β1 proteasome subunit increased after kinetin treatment. Addition of proteasome inhibitor MG-115 reduced the number of dying cells, suggesting that proteasomes might play an important role during kinetin-induced PCD.

Iron deprivation-induced reactive oxygen species generation leads to non-autolytic PCD in Brassica napus leaves

Using iron-deprived (-Fe) chlorotic as well as green iron-deficient (5 μM Fe) and iron-sufficient supplied (50 μM Fe) leaves of young hydroponically reared Brassica napus plants, we explored iron deficiency effects on triggering programmed cell death (PCD) phenomena. Iron deficiency increased superoxide anion but decreased hydroxyl radical (•OH) formation (TBARS levels). Impaired photosystem II efficiency led to hydrogen peroxide accumulation in chloroplasts; NADPH oxidase activity, however, remained on the same level in all treatments. Non-autolytic PCD was observed especially in the chlorotic leaf of iron-deprived plants, to a lesser extent in iron-deficient plants. It correlated with higher DNAse-, alkaline protease- and caspase-3-like activities, DNA fragmentation and chromatin condensation, hydrogen peroxide accumulation and higher superoxide dismutase activity. A significant decrease in catalase activity together with rising levels of dehydroascorbic acid indicated a strong disturbance of the redox homeostasis, which, however, was not caused by •OH formation in concordance with the fact that iron is required to catalyse the Fenton reaction leading to •OH generation. This study documents the chain of events that contributes to the development of non-autolytic PCD in advanced stages of iron deficiency in B. napus leaves.

The proteasome is responsible for caspase-3-like activity during xylem development

Xylem development is a process of xylem cell terminal differentiation that includes initial cell division, cell expansion, secondary cell wall formation and programmed cell death (PCD). PCD in plants and apoptosis in animals share many common characteristics. Caspase-3, which displays Asp-Glu-Val-Asp (DEVD) specificity, is a crucial executioner during animal cells apoptosis. Although a gene orthologous to caspase-3 is absent in plants, caspase-3-like activity is involved in many cases of PCD and developmental processes. However, there is no direct evidence that caspase-3-like activity exists in xylem cell death. In this study, we showed that caspase-3-like activity is present and is associated with secondary xylem development in Populus tomentosa. The protease responsible for the caspase-3-like activity was purified from poplar secondary xylem using hydrophobic interaction chromatography (HIC), Q anion exchange chromatography and gel filtration chromatography. After identification by liquid chromatography-tandem mass spectrometry (LC-MS/MS), it was revealed that the 20S proteasome (20SP) was responsible for the caspase-3-like activity in secondary xylem development. In poplar 20SP, there are seven α subunits encoded by 12 genes and seven β subunits encoded by 12 genes. Pharmacological assays showed that Ac-DEVD-CHO, a caspase-3 inhibitor, suppressed xylem differentiation in the veins of Arabidopsis cotyledons. Furthermore, clasto-lactacystin β-lactone, a proteasome inhibitor, inhibited PCD of tracheary element in a VND6-induced Arabidopsis xylogenic culture. In conclusion, the 20S proteasome is responsible for caspase-3-like activity and is involved in xylem development.

Cerium elicitor-induced phosphatidic acid triggers apoptotic signaling development in Taxus cuspidata cell suspension cultures

Degradation of membrane phospholipids is associated with apoptotic responses, but the signaling development of this degradation is not well understood. Cerium (Ce(4+)), an important rare earth element, induces cellular apoptosis and taxol biosynthesis in Taxus cuspidata suspension cultures. Here, using mass spectrometry and biochemical technique, we demonstrated that the phospholipase D (PLD) was rapidly activated by Ce(4+) and hydrolyzed structural phospholipids to generate lipid signal molecule, phosphatidic acid (PA). 1-Butanol, an antagonist of PLD-dependent PA production, blocked the biphasic burst of superoxide anions (O2(*-)) and thus mitigated cellular apoptosis. The time-course analysis of PA accumulation and ERK-like mitogen-activated protein kinase (MAPK) regulation indicated PA generation preceded MAPK activation, suggesting that the rapid accumulation of PA might be required for the initial MAPK activity. After 2h of Ce(4+) elicitation, however, PA-induced O2(*-) burst, forming a negative regulation to MAPK activity, which in turn led to apoptotic signaling development.

What happened to plant caspases?

The extent of conservation in the programmed cell death pathways that are activated in species belonging to different kingdoms is not clear. Caspases are key components of animal apoptosis; caspase activities are detected in both animal and plant cells. Yet, while animals have caspase genes, plants do not have orthologous sequences in their genomes. It is 10 years since the first caspase activity was reported in plants, and there are now at least eight caspase activities that have been measured in plant extracts using caspase substrates. Various caspase inhibitors can block many forms of plant programmed cell death, suggesting that caspase-like activities are required for completion of the process. Since plant metacaspases do not have caspase activities, a major challenge is to identify the plant proteases that are responsible for the caspase-like activities and to understand how they relate, if at all, to animal caspases. The protease vacuolar processing enzyme, a legumain, is responsible for the cleavage of caspase-1 synthetic substrate in plant extracts. Saspase, a serine protease, cleaves caspase-8 and some caspase-6 synthetic substrates. Possible scenarios that could explain why plants have caspase activities without caspases are discussed.

Lipidomic analysis reveals differential defense responses of Taxus cuspidata cells to two elicitors, methyl jasmonate and cerium (Ce4+)

Methyl jasmonate (MeJA) and cerium (Ce(4+)) elicitation share common features of increasing taxol accumulation of Taxus cuspidata cells. Interestingly, Ce(4+) induces programmed cell death (PCD), but this phenomenon is not observed with MeJA elicitation. Here, using a lipidomic approach to measure more than 100 membrane glycerophospholipids of T. cuspidata cells quantitatively, we discovered that lysophosphatidylcholine (LysoPC), phosphatidic acid (PA) and phosphatidylcholine were three potential lipid markers that were responsible for the differences between Ce(4+)-induced cells and MeJA-induced cells. Compared with MeJA elicitation, marked increase of phospholipase D (PLD) activity was observed following Ce(4+) elicitation, suggesting that the PLD activation and high concentrations of PA production might mediate the PCD. Rapid increase of phospholipase A(2) (PLA(2)) activity caused the release of fatty acids and LysoPC following Ce(4+) elicitation, which enhanced endogenous jasmonic acid (JA) accumulation. In contrast, PLA(2) activity was poorly induced following MeJA elicitation. PLA(2) inhibitor suppressed not only JA accumulation but also taxol production, suggesting that the PLA(2) activation mediated Ce(4+)-induced taxol production partially through a JA-dependent signaling pathway. These results demonstrate that differential alternation of glycerolphospholipids caused by phospholipases constitutes an important step in cell death response to Ce(4+) and increasing taxol production.

Comparative lipidomics analysis of cellular development and apoptosis in two Taxus cell lines

A comparative lipidomics approach was employed to investigate the changes in membrane phospholipids during the procession of cellular development and apoptosis of two plant cell lines, Taxus cuspidata and Taxus chinensis var. mairei. Analysis of lipids by LC/ESI/MS(n) showed more than 90 phospholipid molecular species and indicated significant differences in the abundance throughout a 3-week period. Phosphatidic acid (PA), phosphatidylcholine (PC) and lysophosphatidylcholine (LysoPC) were three important lipid groups that were responsible for the discrimination between the apoptotic T. chinensis var. mairei and living T. cuspidata cells. Continuous increase of phospholipase D (PLD) activity led to PA production in apoptotic T. chinensis var. mairei cells suggesting that the PLD activation and PA formation mediated the apoptosis. Comparison of the profiles of phosphatidylbutanol (PtdBut) with those of PC or phosphatidylethanolamine (PE) indicated that PC rather than PE was the major substrate of PLD in vivo. These results suggest that the alternation of membrane phospholipids may regulate apoptosis, triggering an increase in taxol production of T. chinensis var. mairei cells.

Nitric oxide mediates inactivation of glutathione S-transferase in suspension culture of Taxus cuspidata during shear stress

The importance of nitric oxide (NO) in regulating plant cell responses to environmental stresses is becoming evident. Here the possible role of NO in suspension cultures of Taxus cuspidata under shear stress was investigated in a Couette-type shear reactor. It was found that shear stress with 190 s(-1) caused NO generation in 8 h. NO formation can be inhibited by N-nitro-L-arginine, a nitric oxide synthase inhibitor. Moreover, the activity of glutathione S-transferase (GST), a principal enzyme responsible for detoxification, decreased during shear stress. This inactivation partially recovered when NOS inhibitor or NO scavenger was added into cell cultures during shear stress. Treatment with reactive nitrogen species (RNS) also caused inactivation of GST in cells. The results indicate that NO plays a crucial role in GST inactivation in Taxus cuspidata cells under shear stress.