Overexpression of a new putative membrane protein gene AtMRB1 results in organ size enlargement in Arabidopsis

A putative origin of the insect chemosensory receptor superfamily in the last common eukaryotic ancestor

The insect chemosensory repertoires of Odorant Receptors (ORs) and Gustatory Receptors (GRs) together represent one of the largest families of ligand-gated ion channels. Previous analyses have identified homologous 'Gustatory Receptor-Like' (GRL) proteins across Animalia, but the evolutionary origin of this novel class of ion channels is unknown. We describe a survey of unicellular eukaryotic genomes for GRLs, identifying several candidates in fungi, protists and algae that contain many structural features characteristic of animal GRLs. The existence of these proteins in unicellular eukaryotes, together with ab initio protein structure predictions, provide evidence for homology between GRLs and a family of uncharacterized plant proteins containing the DUF3537 domain. Together, our analyses suggest an origin of this protein superfamily in the last common eukaryotic ancestor.

PCA22 acts as a suppressor of atrzf1 to mediate proline accumulation in response to abiotic stress in Arabidopsis

Proline metabolism is important for environmental responses, plant growth, and development. However, its precise roles in plant abiotic stress tolerance are not well understood. Mutants are valuable for the identification of new genes and for elucidating their roles in physiological mechanisms. We applied a suppressor mutation approach to identify novel genes involved in the regulation of proline metabolism in Arabidopsis. Using the atrzf1 (Arabidopsis thaliana ring zinc finger 1) mutant as a parental line for activation tagging mutagenesis, we selected several mutants with suppressed induction of proline accumulation under dehydration conditions. One of the selected mutants [proline content alterative 22 (pca22)] appeared to have reduced proline contents compared with the atrzf1 mutant under drought stress. Generally, pca22 mutant plants displayed suppressed atrzf1 insensitivity to dehydration and abscisic acid during early seedling growth. Additionally, the pca22 mutant exhibited shorter pollen tube length than wild-type (WT) and atrzf1 plants. Furthermore, PCA22-overexpressing plants were more sensitive to dehydration stress than the WT and RNAi lines. Green fluorescent protein-tagged PCA22 was localized to the cytoplasm of transgenic Arabidopsis cells. Collectively, these results suggest that pca22 acts as dominant suppressor mutant of atrzf1 in the abiotic stress response.

Curd development associated gene (CDAG1) in cauliflower (Brassica oleracea L. var. botrytis) could result in enlarged organ size and increased biomass

The curd is a specialized organ and the most important product organ of cauliflower (Brassica oleracea L. var. botrytis). However, the mechanism underlying the regulation of curd formation and development remains largely unknown. In the present study, a novel homologous gene containing the Organ Size Related (OSR) domain, namely, CDAG1 (Curd Development Associated Gene 1) was identified in cauliflower. Quantitative analysis indicated that CDAG1 showed significantly higher transcript levels in young tissues. Functional analysis demonstrated that the ectopic overexpression of CDAG1 in Arabidopsis and cauliflower could significantly promote organ growth and result in larger organ size and increased biomass. Organ enlargement was predominantly due to increased cell number. In addition, 228 genes involved in the CDAG1-mediated regulatory network were discovered by transcriptome analysis. Among these genes, CDAG1 was confirmed to inhibit the transcriptional expression of the endogenous OSR genes, ARGOS and ARL, while a series of ethylene-responsive transcription factors (ERFs) were found to increased expression in 35S:CDAG1 transgenic Arabidopsis plants. This implies that CDAG1 may function in the ethylene-mediated signal pathway. These findings provide new insight into the function of OSR genes, and suggest potential applications of CDAG1 in breeding high-yielding crops.

Molecular and physiological characterization of the Arabidopsis thaliana Oxidation-related Zinc Finger 2, a plasma membrane protein involved in ABA and salt stress response through the ABI2-mediated signaling pathway

CCCH-type zinc finger proteins are important for developmental and environmental responses. However, the precise roles of these proteins in plant stress tolerance are poorly understood. Arabidopsis thaliana Oxidation-related Zinc Finger 2 (AtOZF2) (At4g29190) is an AtOZF1 homolog previously isolated from Arabidopsis, which confers oxidative stress tolerance on plants. The AtOZF2 protein is localized in the plasma membrane, as is AtOZF1. Disruption expression of AtOZF2 led to reduced root length and leaf size. AtOZF2 was implicated to be involved in the ABA and salinity responses. atozf2 antisense lines were more sensitive to ABA and salt stress during the seed germination and cotyledon greening processes. In contrast, AtOZF2-overexpressing plants were more insensitive to ABA and salt stress than the wild type. Interestingly, in the presence of ABA and salt stress, the transcript level of ABA insensitive 2 (ABI2), but not that of ABI1, in AtOZF2-overexpressing plants was lower than that in the wild type, whereas the expression of ABI2 in atozf2 was significantly enhanced. Thus, AtOZF2 is involved in the ABA and salt stress response through the ABI2-mediated signaling pathway. Taken together, these findings provide compelling evidence that AtOZF2 is an important regulator for plant tolerance to abiotic stress.

A nuclear-encoded mitochondrial gene AtCIB22 is essential for plant development in Arabidopsis

Complex I (the NADH:ubiquinone oxidoreductase) of the mitochondrial respiratory chain is a complicated, multi-subunit, membrane-bound assembly and contains more than 40 different proteins in higher plants. In this paper, we characterize the Arabidopsis homologue (designated as AtCIB22) of the B22 subunit of eukaryotic mitochondrial Complex I. AtCIB22 is a single-copy gene and is highly conserved throughout eukaryotes. AtCIB22 protein is located in mitochondria and the AtCIB22 gene is widely expressed in different tissues. Mutant Arabidopsis plants with a disrupted AtCIB22 gene display pleiotropic phenotypes including shorter roots, smaller plants and delayed flowering. Stress analysis indicates that the AtCIB22 mutants' seed germination and early seedling growth are severely inhibited by sucrose deprivation stress but more tolerant to ethanol stress. Molecular analysis reveals that in moderate knockdown AtCIB22 mutants, genes including cell redox proteins and stress related proteins are significantly up-regulated, and that in severe knockdown AtCIB22 mutants, the alternative respiratory pathways including NDA1, NDB2, AOX1a and AtPUMP1 are remarkably elevated. These data demonstrate that AtCIB22 is essential for plant development and mitochondrial electron transport chains in Arabidopsis. Our findings also enhance our understanding about the physiological role of Complex I in plants.