[Phytaspases: aspartate-specific proteases involved in plant cell death]

MYB2 Is Important for Tapetal PCD and Pollen Development by Directly Activating Protease Expression in Arabidopsis

Tapetal programmed cell death (PCD) is a complex biological process that plays an important role in pollen formation and reproduction. Here, we identified the MYB2 transcription factor expressed in the tapetum from stage 5 to stage 11 that was essential for tapetal PCD and pollen development in Arabidopsis thaliana. Downregulation of MYB2 retarded tapetal degeneration, produced defective pollen, and decreased pollen vitality. EMSA and transcriptional activation analysis revealed that MYB2 acted as an upstream activator and directly regulated expression of the proteases CEP1 and βVPE. The expression of these proteases was lower in the buds of the myb2 mutant. Overexpression of either/both CEP1 or/and βVPE proteases partially recover pollen vitality in the myb2 background. Taken together, our results revealed that MYB2 regulates tapetal PCD and pollen development by directly activating expression of the proteases CEP1 and βVPE. Thus, a transcription factor/proteases regulatory and activated cascade was established for tapetal PCD during another development in Arabidopsis thaliana. Highlight: MYB2 is involved in tapetal PCD and pollen development by directly regulating expression of the protease CEP1 and βVPE and establishes a transcription factor/proteases regulatory and activated cascade.

Infectious Plant Diseases: Etiology, Current Status, Problems and Prospects in Plant Protection

In recent years, there has been an increase in the number of diseases caused by bacterial, fungal, and viral infections. Infections affect plants at different stages of agricultural production. Depending on weather conditions and the phytosanitary condition of crops, the prevalence of diseases can reach 70-80% of the total plant population, and the yield can decrease in some cases down to 80-98%. Plants have innate cellular immunity, but specific phytopathogens have an ability to evade that immunity. This article examined phytopathogens of viral, fungal, and bacterial nature and explored the concepts of modern plant protection, methods of chemical, biological, and agrotechnical control, as well as modern methods used for identifying phytopathogens.

Arabidopsis thaliana phytaspase: identification and peculiar properties

Phytaspases are plant cell death-related proteases of the subtilisin-like protease family that possess an unusual aspartate cleavage specificity. Although phytaspase activity is widespread in plants, phytaspase of Arabidopsis thaliana (L.) Heynh. has escaped detection and identification thus far. Here, we show that a single gene (At4 g10540) out of 56 A. thaliana subtilisin-like protease genes encodes a phytaspase. The recombinant phytaspase was overproduced in Nicotiana benthamiana Domin leaves, isolated, and its substrate specificity and properties were characterised. At pH 5.5, at physiological mildly acidic reaction conditions, the Arabidopsis phytaspase was shown to be strictly Asp-specific. The strongly preferred cleavage motifs of the enzyme out of a panel of synthetic peptide substrates were YVAD and IETD, while the VEID-based substrate preferred by the tobacco and rice phytaspases was almost completely resistant to hydrolysis. At neutral pH, however, the Arabidopsis phytaspase could hydrolyse peptide substrates after two additional amino acid residues, His and Phe, in addition to Asp. This observation may indicate that the repertoire of Arabidopsis phytaspase targets could possibly be regulated by the conditions of the cellular environment. Similar to tobacco and rice phytaspases, the Arabidopsis enzyme was shown to accumulate in the apoplast of epidermal leaf cells. However, in stomatal cells Arabidopsis phytaspase was observed inside the cells, possibly co-localising with vacuole. Our study thus demonstrates that the Arabidopsis phytaspase possesses both important similarities with and distinctions from the already known phytaspases, and is likely to be the most divergent member of the phytaspase family.

Tobacco phytaspase: Successful expression in a heterologous system

Phytaspase, a plant serine protease, has been demonstrated to play an important role in the programmed cell death of various plants. Phytaspase is synthesized as an inactive proenzyme containing an N-terminal signal peptide followed by a pro-domain and a mature protease catalytic domain. Pre-prophytaspase autocatalytically processes itself into a pro-domain and an active mature phytaspase enzyme. We have recently demonstrated the successful expression of mature phytaspase from tobacco in a bacterial system. Herein, we focus on the expression of pre-prophytaspase as a GST-tag fusion and on its purification by affinity chromatography.

Substrate Specificity and Possible Heterologous Targets of Phytaspase, a Plant Cell Death Protease

Plants lack aspartate-specific cell death proteases homologous to animal caspases. Instead, a subtilisin-like serine-dependent plant protease named phytaspase shown to be involved in the accomplishment of programmed death of plant cells is able to hydrolyze a number of peptide-based caspase substrates. Here, we determined the substrate specificity of rice (Oryza sativa) phytaspase by using the positional scanning substrate combinatorial library approach. Phytaspase was shown to display an absolute specificity of hydrolysis after an aspartic acid residue. The preceding amino acid residues, however, significantly influence the efficiency of hydrolysis. Efficient phytaspase substrates demonstrated a remarkable preference for an aromatic amino acid residue in the P3 position. The deduced optimum phytaspase recognition motif has the sequence IWLD and is strikingly hydrophobic. The established pattern was confirmed through synthesis and kinetic analysis of cleavage of a set of optimized peptide substrates. An amino acid motif similar to the phytaspase cleavage site is shared by the human gastrointestinal peptide hormones gastrin and cholecystokinin. In agreement with the established enzyme specificity, phytaspase was shown to hydrolyze gastrin-1 and cholecystokinin at the predicted sites in vitro, thus destroying the active moieties of the hormones.