Stem cell establishment and maintenance are essential for the largely post-embryonic developmental patterning that occurs in higher plants. Plant embryos establish two stem cell populations at the shoot and root meristems which then function to continuously generate shoot and root tissues, respectively. Research has uncovered entirely separate sets of regulators for the shoot and root stem cells, raising questions about the origin of the later-evolving root meristem and the relationship between the two meristems. We have recently demonstrated that the related Arabidopsis phosphatases POL and PLL1 are essential for both shoot and root stem cell maintenance and that they act in each population by promoting expression of related WOX transcription factors. Furthermore, pol pll1 mutant embryos exhibit defects in key asymmetric divisions in the early embryo. We hypothesize that the primary functions of POL and PLL1 are to establish and/or maintain stem cell polarity. Here we show additional data linking POL/PLL1 function to the proper polarization and localization of auxin signaling during embryogenesis.

We have isolated a gene encoding a ribosome-inactivating protein (RIP) from Phytolacca insularis, designated as P. insularis antiviral protein 2 (PIP2). The PIP2 gene contained an open reading frame encoding a polypeptide of 315 amino acids. The deduced amino acid sequence of PIP2 was similar to those of other RIPs from Phytolacca plants. Recombinant PIP2 was expressed in Escherichia coli and was used to investigate its biological activities. Recombinant PIP2 inhibited protein synthesis in rabbit reticulocyte lysate by inactivating ribosomes through N-glycosidase activity. It also exhibited antiviral activity against tobacco mosaic virus (TMV). Expression of the PIP2 gene was developmentally regulated in leaves and roots of P. insularis. Furthermore, expression of the PIP2 gene was induced in leaves by mechanical wounding. The wound induction of the PIP2 gene was systemic. Expression of the PIP2 gene also increased in leaves in a systemic manner after treatment with jasmonic acid (JA) and abscisic acid (ABA), but not with salicylic acid (SA). These results imply that plants have employed the systemic synthesis of the defensive proteins to protect themselves more efficiently from infecting viruses.

In this study, a tissue-specific GAL4/UAS activation tagging system was used for the characterization of genes which could induce lethality when ubiquitously expressed. A dominant mutant exhibiting stunted growth was isolated and named defective root development 1-D (drd1-D). The T-DNA tag was located within the promoter region of AtTX12, which is predicted to encode a truncated nucleotide-binding leucine-rich repeat (NLR) protein, containing a Toll/interleukin-1 receptor (TIR) domain. The transcript levels of AtTX12 and defense-related genes were elevated in drd1-D, and the misexpression of AtTX12 recapitulated the drd1-D phenotypes. In the presence of ENHANCED DISEASE SUSCEPTIBILITY 1 (EDS1), a key transducer of signals triggered by TIR-type NLRs, a low-level of AtTX12 misexpression induced strong defective phenotypes including seedling lethality whereas, in the absence of EDS1, a high-level of AtTX12 misexpression induced weak growth defects like dwarfism, suggesting that AtTX12 might function mainly in an EDS1-dependent and partially in an EDS1-independent manner. [BMB Reports 2016; 49(12): 693–698]

Toll/interleukin -1 receptor (TIR) domains, which have NAD+ cleavage activity, are used as signaling modules in NOD-like receptors for defense responses. It has been shown that TIR domains not only form homo- or heterodimers with TIR domain-containing proteins but also interact with various proteins. A previous study showed that overexpression of Arabidopsis thaliana (Arabidopsis) AtTX14, encoding an N-terminal TIR domain and a C-terminal domain with unknown function, resulted in dwarfism and constitutive defense signaling or autoimmunity. Transgenic Arabidopsis overexpressing AtTX14 displays enhanced defense responses and associated dwarf phenotypes at 28 °C compared with those at 22 °C, which differs from other mutant or transgenic Arabidopsis with constitutive defense responses. We found that AtTX14 is alternatively spliced to encode three different proteins, and the TIR domain itself can induce autoimmunity and elevated defense responses to the bacterial pathogen Pseudomonas syringae pv. tomato. In addition, we revealed that the transcription of AtTX14 is regulated by a positive feedback mechanism. With transient overexpression of three AtTX14 protein forms in tobacco leaves, providing a heterologous system free from the positive feedback of AtTX14 in Arabidopsis, we demonstrated that expression of a splicing variant encoding the TIR domain-only protein is sufficient to activate defense signaling. A deeper understanding of interaction networks involving AtTX14 will broaden our knowledge on how plant defense signaling is regulated in response to pathogen infection and ambient temperature changes.

Soybean SE60 belongs to the gamma-thionin family of proteins. We recently demonstrated that SE60 plays a role in defense during soybean development. Here, we show that SE60 is expressed in a tissue-specific and developmentally regulated manner. The expression of SE60 is distinct from that of the glycinin (Gy2) and extensin (SbHRGP3) genes of soybean during embryogenesis and germination. A SE60::GUS(-809) transgene, comprising -809 bp of the 5'-flanking region of SE60 fused to the GUS reporter gene, was expressed specifically in developing embryos, but not in the endosperms, from the globular stage of transgenic tobacco and Arabidopsis seeds. Furthermore, light affected the SE60::GUS(-809) expression pattern in germinating seedlings. Electrophoretic mobility shift assay (EMSA) revealed that soybean nuclear proteins as well as E. coli-expressed SB16, a high mobility group protein (HMG), were bound sequence-specifically to the fragment containing AT-rich motifs identified in the SE60 promoter. Interestingly, the soybean nuclear proteins binding to the two G-boxes and RY repeat were prevalent in seeds of 2-4 mm in size. In contrast, the nuclear proteins binding to the AT-rich motif and SE60 RNA expression were more prominent in seeds of 4-6 mm in size. Therefore, we propose that factors binding to the G-boxes or RY repeat initiate SE60 expression during embryogenesis.