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Metacaspases are cysteine proteases present in plants, fungi, prokaryotes, and early branching eukaryotes, although a detailed description of their cellular function remains unclear. Currently, three-dimensional (3D) structures are only available for two metacaspases: Trypanosoma brucei (MCA2) and Saccharomyces cerevisiae (Yca1). Furthermore, metacaspases diverged from animal caspases of known structure, which limits straightforward homology-based interpretation of functional data. We report for the first time the identification and initial characterization of a metacaspase of Nicotiana tabacum L., NtMC1. By combining domain search, multiple sequence alignment (MSA), and protein fold-recognition studies, we provide compelling evidences that NtMC1 is a plant metacaspase type II, and predict its 3D structure using the crystal structure of two type I metacaspases (MCA2 and Yca1) and Gsu0716 protein from Geobacter sulfurreducens as template. Analysis of the predicted 3D structure allows us to propose Asp353, at the putative p10 subunit, as a new member of the aspartic acid triad that coordinates the P1 arginine/lysine residue of the substrate. Nevertheless, site-directed mutagenesis and expression analysis in bacteria and Nicotiana benthamiana indicate the functionality of both Asp348 and Asp353. Through the co-expression of mutant and wild-type proteins by transient expression in N. benthamiana leaves we found that polypeptide processing seems to be intramolecular. Our results provide the first evidence in plant metacaspases concerning the functionality of the putative p10 subunit.

We tested the hypothesis that PAL activity in chilli plants CM-334 inoculated with Nacobbus aberrans (Na) alone or in combination with Phytophthora capsici (Pc), is lower than in those inoculated only with Pc. At 21 days after nematode inoculation, inoculated plants showed a significant (P<0.01) reduction of 48% in PAL activity compared to those non-inoculated in two separate experiments. In two other tests, where plants were inoculated with the oomycete 21 days after inoculation with the nematode, PAL activity at 2, 4, 6, 8 and 24 h after inoculation with Pc was significantly higher (Tukey, P<0.01) in plants inoculated only with Pc than in plants inoculated only with Na or both pathogens (Na+Pc).

Submergence and drought stresses are the main constraints to crop production worldwide. MicroRNAs (miRNAs) are known to play a major role in plant response to various stresses. In this study, we analyzed the expression of maize and teosinte miRNAs by high-throughput sequencing of small RNA libraries in maize and its ancestor teosinte (Zea mays ssp. parviglumis), under submergence, drought, and alternated stress. We found that the expression patterns of 67 miRNA sequences representing 23 miRNA families in maize and other plants were regulated by submergence or drought. miR159a, miR166b, miR167c, and miR169c were downregulated by submergence in both plants but more severely in maize. miR156k and miR164e were upregulated by drought in teosinte but downregulated in maize. Small RNA profiling of teosinte subject to alternate treatments with drought and submergence revealed that submergence as the first stress attenuated the response to drought, while drought being the first stress did not alter the response to submergence. The miRNAs identified herein, and their potential targets, indicate that control of development, growth, and response to oxidative stress could be crucial for adaptation and that there exists evolutionary divergence between these two subspecies in miRNA response to abiotic stresses.

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SCF-type E3 ubiquitin ligases provide specificity to numerous selective protein degradation events in plants, including those that enable survival under environmental stress. SCF complexes use F-box (FBX) proteins as interchangeable substrate adaptors to recruit protein targets for ubiquitylation. FBX proteins almost universally have structure with two domains: A conserved N-terminal F-box domain interacts with a SKP protein and connects the FBX protein to the core SCF complex, while a C-terminal domain interacts with the protein target and facilitates recruitment. The F-BOX STRESS INDUCED (FBS) subfamily of plant FBX proteins has an atypical structure, however, with a centrally located F-box domain and additional conserved regions at both the N- and C-termini. FBS proteins have been linked to environmental stress networks, but no ubiquitylation target(s) or biological function has been established for this subfamily. We have identified two WD40 repeat-like proteins in Arabidopsis that are highly conserved in plants and interact with FBS proteins, which we have named FBS INTERACTING PROTEINs (FBIPs). FBIPs interact exclusively with the N-terminus of FBS proteins, and this interaction occurs in the nucleus. FBS1 destabilizes FBIP1, consistent with FBIPs being ubiquitylation targets SCFFBS1 complexes. This work indicates that FBS proteins may function in stress-responsive nuclear events, and it identifies two WD40 repeat-like proteins as new tools with which to probe how an atypical SCF complex, SCFFBS, functions via FBX protein N-terminal interaction events.