Local and Systemic Regulation of Plant Root System Architecture and Symbiotic Nodulation by a Receptor-Like Kinase

In plants, root system architecture is determined by the activity of root apical meristems, which control the root growth rate, and by the formation of lateral roots. In legumes, an additional root lateral organ can develop: the symbiotic nitrogen-fixing nodule. We identified in Medicago truncatula ten allelic mutants showing a compact root architecture phenotype (cra2) independent of any major shoot phenotype, and that consisted of shorter roots, an increased number of lateral roots, and a reduced number of nodules. The CRA2 gene encodes a Leucine-Rich Repeat Receptor-Like Kinase (LRR-RLK) that primarily negatively regulates lateral root formation and positively regulates symbiotic nodulation. Grafting experiments revealed that CRA2 acts through different pathways to regulate these lateral organs originating from the roots, locally controlling the lateral root development and nodule formation systemically from the shoots. The CRA2 LRR-RLK therefore integrates short- and long-distance regulations to control root system architecture under non-symbiotic and symbiotic conditions.     

A H+-ATPase That Energizes Nutrient Uptake during Mycorrhizal Symbioses in Rice and Medicago truncatula

Most plant species form symbioses with arbuscular mycorrhizal (AM) fungi, which facilitate the uptake of mineral nutrients such as phosphate from the soil. Several transporters, particularly proton-coupled phosphate transporters, have been identified on both the plant and fungal membranes and contribute to delivering phosphate from fungi to plants. The mechanism of nutrient exchange has been studied in plants during mycorrhizal colonization, but the source of the electrochemical proton gradient that drives nutrient exchange is not known. Here, we show that plasma membrane H⁺-ATPases that are specifically induced in arbuscule-containing cells are required for enhanced proton pumping activity in membrane vesicles from AM-colonized roots of rice (Oryza sativa) and Medicago truncatula. Mutation of the H⁺-ATPases reduced arbuscule size and impaired nutrient uptake by the host plant through the mycorrhizal symbiosis. Overexpression of the H⁺-ATPase Os-HA1 increased both phosphate uptake and the plasma membrane potential, suggesting that this H⁺-ATPase plays a key role in energizing the periarbuscular membrane, thereby facilitating nutrient exchange in arbusculated plant cells.     

Flexible functional interactions between G‐protein subunits contribute to the specificity of plant responses

Plants being sessile integrate information from a variety of endogenous and external cues simultaneously to optimize growth and development. This necessitates the signaling networks in plants to be highly dynamic and flexible. One such network involves heterotrimeric G‐proteins comprised of Gα, Gβ, and Gγ subunits, which influence many aspects of growth, development, and stress response pathways. In plants such as Arabidopsis, a relatively simple repertoire of G‐proteins comprised of one canonical and three extra‐large Gα, one Gβ and three Gγ subunits exists. Because the Gβ and Gγ proteins form obligate dimers, the phenotypes of plants lacking the sole Gβ or all Gγ genes are similar, as expected. However, Gα proteins can exist either as monomers or in a complex with Gβγ, and the details of combinatorial genetic and physiological interactions of different Gα proteins with the sole Gβ remain unexplored. To evaluate such flexible, signal‐dependent interactions and their contribution toward eliciting a specific response, we have generated Arabidopsis mutants lacking specific combinations of Gα and Gβ genes, performed extensive phenotypic analysis, and evaluated the results in the context of subunit usage and interaction specificity. Our data show that multiple mechanistic modes, and in some cases complex epistatic relationships, exist depending on the signal‐dependent interactions between the Gα and Gβ proteins. This suggests that, despite their limited numbers, the inherent flexibility of plant G‐protein networks provides for the adaptability needed to survive under continuously changing environments.     

Plant growth-promoting rhizobacteria systemically protect Arabidopsis thaliana against Cucumber mosaic virus by a salicylic acid and NPR1-independent and jasmonic acid-dependent signaling pathway

Arabidopsis thaliana ecotype Columbia plants (Col-0) treated with plant growth-promoting rhizobacteria (PGPR) Serattia marcescens strain 90-166 and Bacillus pumilus strain SE34 had significantly reduced symptom severity by Cucumber mosaic virus (CMV). In some cases, CMV accumulation was also significantly reduced in systemically infected leaves. The signal transduction pathway(s) associated with induced resistance against CMV by strain 90-166 was determined using mutant strains and transgenic and mutant Arabidopsis lines. NahG plants treated with strains 90-166 and SE34 had reduced symptom severity indicating that the resistance did not require salicylic acid (SA). Strain 90-166 naturally produces SA under iron-limited conditions. Col-0 and NahG plants treated with the SA-deficient mutant, 90-166-1441, had significantly reduced CMV symptom severity with reduced virus accumulation in Col-0 plants. Another PGPR mutant, 90-166-2882, caused reduced disease severity in Col-0 and NahG plants. In a time course study, strain 90-166 reduced virus accumulation at 7 but not at 14 and 21 days post-inoculation (dpi) on the non-inoculated leaves of Col-0 plants. NahG and npr1-1 plants treated with strain 90-166 had reduced amounts of virus at 7 and 14 dpi but not at 21 dpi. In contrast, no decrease in CMV accumulation occurred in strain 90-166-treated fad3-2 fad7-2 fad8 plants. These data indicate that the protection of Arabidopsis against CMV by strain 90-166 follows a signaling pathway for virus protection that is independent of SA and NPR1, but dependent on jasmonic acid.     

Inhibition of Helicobacter pylori growth and its cytotoxicity by 2-hydroxy 4-methoxy benzaldehyde of Decalepis hamiltonii (Wight & Arn); a new functional attribute

Helicobacter pylori mediated gastric ulcer and cancers are common global problems since it was found to colonize in ∼50% of gastric ulcer/cancer patients. Decalepis hamiltonii, (Asclepiadaceae family) extracts have been depicted with medicinal properties supporting the traditional knowledge of health beneficial attributes of D. hamiltonii. Previously we have shown that both aqueous as well as methanol extracts of D. hamiltonii containing abundant phenolics with predominant levels (20-40% of total phenolics) of 2-hydroxy-4-methoxy benzaldehyde (HMBA). Despite higher levels, HMBA contributed very little to the antioxidant activity (<10%) when compared to other phenolic compounds in the extract. In the current study we attempted to explore antimicrobial property, particularly anti-H. pylori activity, since traditional users document D. hamiltonii as a fighter of microbial infections. HMBA was isolated from the roots of D. hamiltonii by hydrodistillation and cold crystallization method; identified by HPLC and characterized using ESI-MS and confirmed by NMR studies as a compound of molecular mass 152 Da. Isolated HMBA was found to inhibit the growth of H. pylori, a potential ulcerogen in a dose dependent manner with MIC of ∼39 μg/mL as apposed to that of amoxicillin (MIC - 26 μg/mL) for which H. pylori is susceptible. Results were further substantiated by the lysis of H. pylori by electron microscopy and electrophoretic studies. Studies on the mechanism of action indicated the counteracting effect of vacuolating toxin (VacA) of H. pylori which otherwise would lead to host cell cytotoxicity. Further the increased binding ability of HMBA to DNA and protein offered an impact on DNA protectivity and bioavailability. Results for the first time provide a direct evidence for anti-microbial attribute of HMBA. Insignificant antioxidant attribute of HMBA also reveals the anti-H. pylori activity via mechanisms other than antioxidative routes.     

The key enzyme in galactose metabolism, UDP-galactose-4-epimerase, affects cell-wall integrity and morphology in Candida albicans even in the absence of galactose

The enzyme UDP-galactose-4-epimerase (GAL10) catalyzes a key step in galactose metabolism converting UDP-galactose to UDP-glucose which then can get metabolized through glycolysis and TCA cycle thus allowing the cell to use galactose as a carbon and energy source. As in many fungi, a functional homolog of GAL10 exists in Candida albicans. The domainal organization of the homologs from Saccharomyces cerevisiae and C. albicans show high degree of homology having both mutarotase and an epimerase domain. The former is responsible for the conversion of beta-d-galactose to alpha-d-galactose and the latter for epimerization of UDP-galactose to UDP-glucose. Absence of C. albicans GAL10 (CaGAL10) affects cell-wall organization, oxidative stress response, biofilm formation and filamentation. Cagal10 mutant cells tend to flocculate extensively as compared to the wild-type cells. The excessive filamentation in this mutant is reflected in its irregular and wrinkled colony morphology. Cagal10 strain is more susceptible to oxidative stress when tested in presence of H2O2. While the S. cerevisiae GAL10 (ScGAL10), essential for survival in the presence of galactose, has not been reported to have defects in the absence of galactose, the C. albicans homolog shows these phenotypes during growth in the absence of galactose. Thus a functional CaGal10 is required not only for galactose metabolism but also for normal hyphal morphogenesis, colony morphology, maintenance of cell-wall integrity and for resistance to oxidative stress even in the absence of galactose.