Signal-transducing GTPases in plants include small G proteins, heterotrimeric G proteins, and, potentially, several unique types of GTP binding proteins that are not members of either of the aforementioned classes. This review will focus on recent discoveries concerning the roles of heterotrimeric

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Heterotrimeric and Unconventional GTP Binding Proteins in Plant Cell Signaling

Summary
• Here, we analyze long-term evolution in Nicotiana allopolyploid section Repandae (the closest living diploids are N. sylvestris, the maternal parent, and N. obtusifolia, the paternal parent). We compare data with other more recently formed Nicotiana allopolyploids.
• We investigated 35S and 5S nuclear ribosomal DNA (rDNA) chromosomal location and unit divergence. A molecular clock was applied to the Nicotiana phylogenetic tree to determine allopolyploid ages.
• N. tabacum and species of Repandae were c. 0.2 and 4.5 Myr old, respectively. In all Repandae species, the numbers of both 35S and 5S rDNA loci were less than the sum of those of the diploid progenitors. Trees based on 5S rDNA spacer sequences indicated units of only the paternal parent.
• In recent Nicotiana allopolyploids, the numbers of rDNA loci equal the sum of those of their progenitors. In the Repandae genomes, diploidization is associated with locus loss. Sequence analysis indicates that 35S and 5S units most closely resemble maternal and paternal progenitors, respectively. In Nicotiana, 4.5 Myr of allopolyploid evolution renders genomic in situ hybridization (GISH) unsuitable for the complete resolution of parental genomes.

Long-term genome diploidization in allopolyploid Nicotiana section Repandae (Solanaceae).

Heterotrimeric G proteins are implicated in diverse signaling processes in plants, but the molecular mechanisms of their function are largely unknown. Finding G protein effectors and regulatory proteins can help in understanding the roles of these signal transduction proteins in plants. A yeast two-hybrid screen was performed to search for proteins that interact with Arabidopsis G protein α-subunit (GPA1). One of the identified GPA1-interacting proteins is the cupin-domain protein AtPirin1. Pirin is a recently defined protein found because of its ability to interact with a CCAAT box binding transcription factor. The GPA1–AtPirin1 interaction was confirmed in an in vitro binding assay. We characterized two atpirin1 T-DNA insertional mutants and established that they display a set of phenotypes similar to those of gpa1 mutants, including reduced germination levels in the absence of stratification and an abscisic acid–imposed delay in germination and early seedling development. These data indicate that AtPirin1 likely functions immediately downstream of GPA1 in regulating seed germination and early seedling development.

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The Arabidopsis Cupin Domain Protein AtPirin1 Interacts with the G Protein α-Subunit GPA1 and Regulates Seed Germination and Early Seedling Development

Isolation of a novel G-protein γ-subunit from Arabidopsis thaliana and its interaction with Gβ

Boron stress is an environmental factor affecting plant development and production. Recently, microRNAs (miRNAs) have been found to be involved in several plant processes such as growth regulation and stress responses. In this study, miRNAs associated with boron stress were identified and characterized in barley. miRNA profiles were also comparatively analyzed between root and leave samples. A total of 31 known and 3 new miRNAs were identified in barley; 25 of them were found to respond to boron treatment. Several miRNAs were expressed in a tissue specific manner; for example, miR156d, miR171a, miR397, and miR444a were only detected in leaves. Additionally, a total of 934 barley transcripts were found to be specifically targeted and degraded by miRNAs. In silico analysis of miRNA target genes demonstrated that many miRNA targets are conserved transcription factors such as Squamosa promoter-binding protein, Auxin response factor (ARF), and the MYB transcription factor family. A majority of these targets were responsible for plant growth and response to environmental changes. We also propose that some of the miRNAs in barley such as miRNA408 might play critical roles against boron exposure. In conclusion, barley may use several pathways and cellular processes targeted by miRNAs to cope with boron stress.

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Boron stress responsive microRNAs and their targets in barley.

Heterotrimeric GTP-binding proteins (G-proteins) play important roles in signal transduction pathways in eukaryotic cells. Through differential screening of a hairy root cDNA library of tobacco (Nicotiana tabacum L.) against transcripts from non-root tissues of normal cuttings, we obtained a partial cDNA clone that showed abundant expression and high homology to the alpha subunit gene of plant G-protein. After RACE-PCR, a full-length cDNA clone was obtained, which was 1,677-bp in length and contained an open reading frame encoding a protein of 384 amino acids. A cDNA clone encoding a beta subunit of G-protein was also isolated from the same cDNA library based on PCR amplification and library screening. The clone was 1,600-bp in length and contained an open reading frame encoding 377 amino acids. The deduced amino acid sequences of these clones showed high homology (75.5 to 99.8% amino acid identity) with alpha and beta subunits of other plant G-proteins. Genomic Southern blot analysis showed that the amphidiploid tobacco genome possessed two major copies of both alpha and beta subunit genes and some minor homologous copies. Northern blot analysis showed that the transcript of alpha subunit gene was abundant in the root tissues, particularly in the hairy root tissues. In contrast, the level of expression of the beta subunit gene was equivalent in all the tissues studied. Possible function of tobacco G-protein was discussed.

Nicotiana tabacum cDNAs encoding α and β subunits of a heterotrimeric GTP-binding protein isolated from hairy root tissues

A heterotrimeric GTP-binding protein (G protein) plays a number of important roles in the signal- transduction pathways of eukaryotic cells. The allotetraploid tobacco genome has two α-subunit gene...

Seiji Ando

Papers

Nicotiana tabacum cDNAs encoding α and β subunits of a heterotrimeric GTP-binding protein isolated from hairy root tissues

Genomic structure and homoeologous relationship of the two α-subunit genes of a heterotrimeric GTP-binding protein in tobacco