Study of mechanisms for plant growth promotion elicited by rhizobacteria in Arabidopsis thaliana

Plant growth-promoting rhizobacteria (PGPR) colonize plant roots and exert beneficial effects on plant health and development. We are investigating the mechanisms by which PGPR elicit plant growth promotion from the viewpoint of signal transduction pathways within plants. We report here our first study to determine if well-characterized PGPR strains, which previously demonstrated growth promotion of various other plants, also enhance plant growth in Arabidopsis thaliana. Eight different PGPR strains, including Bacillus subtilis GB03, B. amyloliquefaciens IN937a, B. pumilus SE-34, B. pumilus T4, B. pasteurii C9, Paenibacillus polymyxa E681, Pseudomonas fluorescens 89B-61, and Serratia marcescens 90-166, were evaluated for elicitation of growth promotion of wild-type and mutant Arabidopsis in vitro and in vivo. In vitro testing on MS medium indicated that all eight PGPR strains increased foliar fresh weight of Arabidopsis at distances of 2, 4, and 6 cm from the site of bacterial inoculation. Among the eight strains, IN937a and GB03 inhibited growth of Arabidopsis plants when the bacteria were inoculated 2 cm from the plants, while they significantly increased plant growth when inoculated 6 cm from the plants, suggesting that a bacterial metabolite that diffused into the agar accounted for growth promotion with this strain. In vivo, eight PGPR strains promoted foliar fresh weight under greenhouse conditions 4 weeks after sowing. To define signal transduction pathways associated with growth promotion elicited by PGPR, various plant-hormone mutants of Arabidopsis were evaluated in vitro and in vivo. Elicitation of growth promotion by PGPR strains in vitro involved signaling of brassinosteroid, IAA, salicylic acid, and gibberellins. In vivo testing indicated that ethylene signaling was involved in growth promotion. Results suggest that elicitation of growth promotion by PGPR in Arabidopsis is associated with several different signal transduction pathways and that such signaling may be different for plants grown in vitro vs. in vivo.     

Arabidopsis AtBECLIN 1/AtAtg6/AtVps30 is essential for pollen germination and plant development

Pollen germination on the surface of compatible stigmatic tissues is an essential step for plant fertilization. Here we report that the Arabidopsis mutant bcll is male sterile as a result of the failure ofpollen germination. We show that the bcll mutant allele cannot be transmitted by male gametophytes and no homozygous bcll mutants were obtained. Analysis of pollen developmental stages indicates that the bcll mutation affects pollen germination but not pollen maturation. Molecular analysis demonstrates that the failure of pollen germination was caused by the disruption of AtBECLIN 1. AtBECLIN 1 is expressed predominantly in mature pollen and encodes a protein with significant homology to Beclin1/Atg6/Vps30 required for the processes of autophagy and vacuolar protein sorting （VPS） in yeast. We also show that AtBECLIN 1 is required for normal plant development, and that genes related to autophagy, VPS and the glycosylphosphatidylinositol anchor system, were affected by the deficiency of AtBECLIN 1.     

The participation of AtXPB1, the XPB/RAD25 homologue gene from Arabidopsis thaliana, in DNA repair and plant development

Nucleotide excision repair in Arabidopsis thaliana differs from other eukaryotes as it contains two paralogous copies of the corresponding XPB/RAD25 gene. In this work, the functional characterization of one copy, AtXPB1, is presented. The plant gene was able to partially complement the UV sensitivity of a yeast rad25 mutant strain, thus confirming its involvement in nucleotide excision repair. The biological role of AtXPB1 protein in A. thaliana was further ascertained by obtaining a homozygous mutant plant containing the AtXPB1 genomic sequence interrupted by a T-DNA insertion. The 3' end of the mutant gene is disrupted, generating the expression of a truncated mRNA molecule. Despite the normal morphology, the mutant plants presented developmental delay, lower seed viability and a loss of germination synchrony. These plants also manifested increased sensitivity to continuous exposure to the alkylating agent MMS, thus suggesting inefficient DNA damage removal. These results indicate that, although the duplication seems to be recent, the features described for the mutant plant imply some functional or timing expression divergence between the paralogous AtXPB genes. The AtXPB1 protein function in nucleotide excision repair is probably required for the removal of lesions during seed storage, germination and early plant development.     

模式植物拟南芥开花时间分子调控研究进展

植物从营养生长到生殖生长的转变是开花发育的关键,在合适的时间开花对植物的生长和繁衍极为重要,植物开花时间的调控对农业生产发展意义重大。植物开花是由遗传因子和环境因子协同调节的一个复杂过程。近年来,对不同植物开花调控的研究,特别是对模式植物拟南芥（Arabidopsis thaliana（L.） Heynh.）的开花调控研究取得了显著进展,已探明开花时间分子调控的6条主要途径分别是光周期途径、春化途径、自主途径、温度途径、赤霉素途径和年龄途径。各遗传调控途径既相互独立又相互联系,构成一个复杂的开花调控网络。本文综述了模式植物拟南芥开花时间调控分子机制相关研究的最新进展,并对未来的研究进行了展望。     

Arabidopsis Qa-SNARE SYP2 proteins localized to different subcellular regions function redundantly in vacuolar protein sorting and plant development

SYP2 proteins are a sub-family of Qa-SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) that may be responsible for protein trafficking between pre-vacuolar compartments (PVC) and vacuoles. Arabidopsis thaliana SYP22/VAM3/SGR3 and SYP21/PEP12 proteins function independently, but are both reported to be essential for male gametophytic viability. Here, we systematically examined the redundancy of three SYP2 paralogs (i.e. SYP21, 22 and 23) using a Col-0 ecotype harboring a SYP2 paralog (SYP23/PLP) that lacked a transmembrane domain. Surprisingly, no visible phenotypes were observed, even in the double knockout syp21/pep12 syp23/plp. Deficiency of either SYP21/PEP12 or SYP23/PLP in the syp22 background resulted in a defect in vacuolar protein sorting, characterized by abnormal accumulation of protein precursors in seeds. SYP21/PEP12 knockdown enhanced the syp22 phenotype (i.e. semi-dwarfism, poor leaf vein development and abnormal development of myrosin cells), and additional knockout of SYP23/PLP further aggravated the phenotype. A GFP-SYP23/PLP fusion localized to the cytosol, but not to the PVC or vacuolar membrane, where SYP21/PEP12 or SYP22/VAM3, respectively, were localized. Immunoprecipitation analysis showed that SYP23/PLP interacted with the vacuolar Qb- and Qc-SNAREs, VTI11 and SYP5, respectively, suggesting that SYP23/PLP is able to form a SNARE complex anchoring the membrane. Unexpectedly, we found that expression of multiple copies of a genomic fragment of SYP23/PLP suppressed the abnormal syp22-3 phenotype. Thus, SYP2 proteins, including cytosolic SYP23/PLP, appear to function redundantly in vacuolar trafficking and plant development.     

Expression of the plant cyclin-dependent kinase inhibitor ICK1 affects cell division, plant growth and morphology

The plant CDK inhibitor ICK1 was identified previously from Arabidopis thaliana with its inhibitory activity characterized in vitro. ICK1 displayed several structural and functional features that are distinct from known animal CDK inhibitors. Despite the initial characterization, there is no information on the functions of any plant CDK inhibitor in plants. To gain insight into ICK1 functions in vivo and the role of cell division during plant growth and development, transgenic plants were generated expressing ICK1 driven by the cauliflower mosaic virus 35S promoter. In comparison to control plants, growth was significantly inhibited in transgenic 35S-ICK1 plants, with some plants weighing <10% of wild-type plants at the 3 week stage. Most organs of 35S-ICK1 plants were smaller. There were also modifications in plant morphology such as shape and serration of leaves and petals. The changes were so drastic that 35S-ICK1 plants with strong phenotype no longer resembled wild-type plants morphologically. Analyses showed that increased ICK1 expression resulted in reduced CDK activity and reduced the number of cells in these plants. Cells in 35S-ICK1 plants were larger than corresponding cells in control plants. These results demonstrate that ICK1 acts as a CDK inhibitor in the plant, and the inhibition of cell division by ICK1 expression has profound effects on plant growth and development. They also suggest that alterations of plant organ shape can be achieved by restriction of cell division.     

A lysine-rich arabinogalactan protein in Arabidopsis is essential for plant growth and development, including cell division and expansion

Arabinogalactan proteins (AGPs), a family of hydroxyproline-rich glycoproteins, occur throughout the plant kingdom. The lysine-rich classical AGP subfamily in Arabidopsis consists of three members, AtAGP17, 18 and 19. In this study, AtAGP19 was examined in terms of its gene expression pattern and function. AtAGP19 mRNA was abundant in stems, with moderate levels in flowers and roots and low levels in leaves. AtAGP19 promoter-controlled GUS activity was high in the vasculature of leaves, roots, stems and flowers, as well as styles and siliques. A null T-DNA knockout mutant of AtAGP19 was obtained and compared to wild-type (WT) plants. The atagp19 mutant had: (i) smaller, rounder and flatter rosette leaves, (ii) lighter-green leaves containing less chlorophyll, (iii) delayed growth, (iv) shorter hypocotyls and inflorescence stems, and (v) fewer siliques and less seed production. Several abnormalities in cell size, number, shape and packing were also observed in the mutant. Complementation of this pleiotropic mutant with the WT AtAGP19 gene restored the WT phenotypes and confirmed that AtAGP19 functions in various aspects of plant growth and development, including cell division and expansion, leaf development and reproduction.     

CKS1At overexpression in Arabidopsis thaliana inhibits growth by reducing meristem size and inhibiting cell-cycle progression

The SUC1/CKS1 proteins associate with cyclin-dependent kinases (CDKs) and play an essential role in the regulation of the cell cycle. Recently, an Arabidopsis thaliana SUC1/CKS1 homologous gene, designated CKS1At, has been cloned. Here, overexpression of CKS1At in Arabidopsis is shown to reduce leaf size and root growth rates. Reduced root growth resulted primarily from an increase of the cell-cycle duration and a shortening of the meristem. Endoreduplication was unaffected. The increased cell-cycle duration was associated with an equal extension of both the G1 and G2 phases. This inhibition was due to the binding of CDK subunits with CDKs. The reduced growth rates in response to altered cell-cycle gene expression demonstrates a direct dependence of plant growth rates on cell-cycle regulation.     

Growth and development of Arabidopsis thaliana seedlings in interaction with fungi isolated from stem-end rot of avocado fruits

Abstract

The influence of Phomopsis viticola, Diaporthe phaseolorum and Pseudofusicoccum stromaticum isolated from avocado stem-end rot on growth of Arabidopsis seedlings was investigated. We found responses that are specific for each fungus, whereas P. viticola and D. phaseolorum strongly repressed root growth, P. stromaticum induced growth and branching, and enhanced shoot biomass production. Detailed microscopy and structural analyses revealed that the fungi did not affect cell division in root meristems but rather influenced cell elongation and differentiation, and these responses were related to changes in auxin responsiveness. P. stromaticum strongly acidifies the growth medium and this correlated with induction of the jasmonic acid responsive gene construct LOX2:GUS in leaves, whereas P. viticola and D. phaseolorum showed much reduced acidification properties. Taken together, our results show that fungal isolates from the stem-end of avocado fruits interact with Arabidopsis plants in highly diverse and contrasting manners influencing growth, patterning and defence.     

拟南芥RPP1A参与幼苗生长的蛋白质组学分析

核糖体蛋白不仅参与蛋白质合成,而且参与植物生长发育的调控.利用拟南芥核糖体磷酸蛋白P1(ribosomal phosphoprotein P1,RPP1)家族基因RPP1A缺失突变体rpp1a研究RPP1A缺失对幼苗蛋白质表达水平的影响,揭示其参与调控幼苗生长的作用机制.表型分析发现,与野生型WT相比,RPP1A缺失导...     

Extensive expression regulation and lack of heterologous enzymatic activity of the Class II trehalose metabolism proteins from Arabidopsis thaliana

Trehalose metabolism has profound effects on plant growth and metabolism, but the mechanisms involved are unclear. In Arabidopsis , 21 putative trehalose biosynthesis genes are classified in three subfamilies based on their similarity with yeast TPS1 (encoding a trehalose-6-phosphate synthase, TPS) or TPS2 (encoding a trehalose-6-phosphate phosphatase, TPP). Although TPS1 (Class I) and TPPA and TPPB (Class III) proteins have established TPS and TPP activity, respectively, the function of the Class II proteins (AtTPS5-AtTPS11) remains elusive. A complete set of promoter- β -glucurinidase/green fluorescent protein reporters demonstrates their remarkably differential tissue-specific expression and responsiveness to carbon availability and hormones. Heterologous expression in yeast furthermore suggests that none of the encoded enzymes displays significant TPS or TPP activity, consistent with a regulatory rather than metabolic function for this remarkable class of proteins.