The Azospirillum brasilense type VI secretion system promotes cell aggregation,biocontrol protection against phytopathogens and attachment to the microalgae Chlorella sorokiniana

The plant-growth-promoting bacterium Azospirillum brasilense is able to associate with the microalgae Chlorella sorokiniana. Attachment of A. brasilense increases the metabolic performances of the microalgae. Recent genome analyses have revealed that the A. brasilense Az39 genome contains two complete sets of genes encoding type VI secretion systems (T6SS), including the T6SS1 that is induced by the indole-3-acetic acid (IAA) phytohormone. The T6SS is a multiprotein machine, widespread in Gram-negative bacteria, that delivers protein effectors in both prokaryotic and eukaryotic cells. Here we show that the A. brasilense T6SS is required for Chlorella-Azospirillum synthetic mutualism. Our data demonstrate that the T6SS is an important determinant to promote production of lipids, carbohydrates and photosynthetic pigments by the microalgae. We further show that this is likely due to the role of the T6SS during the attachment stage and for the production of IAA phytohormones. Finally, we demonstrate that the A. brasilense T6SS provides antagonistic activities against a number of plant pathogens such as Agrobacterium, Pectobacterium, Dickeya and Ralstonia species in vitro, suggesting that, in addition to promoting growth, A. brasilense might confer T6SS-dependent bio-control protection to microalgae and plants against bacterial pathogens.     

Recent advances in point spread function engineering and related computational microscopy approaches: from one viewpoint

This personal hybrid review piece, written in light of my recipience of the UIPAB 2020 young investigator award, contains a mixture of my scientific biography and work so far. This paper is not intended to be a comprehensive review, but only to highlight my contributions to computation-related aspects of super-resolution microscopy, as well as their origins and future directions.     

Differential regulation of translation and endocytosis of alternatively spliced forms of the type II bone morphogenetic protein (BMP) receptor

The expression and function of transforming growth factor-β superfamily receptors are regulated by multiple molecular mechanisms. The type II BMP receptor (BMPRII) is expressed as two alternatively spliced forms, a long and a short form (BMPRII-LF and –SF, respectively), which differ by an ∼500 amino acid C-terminal extension, unique among TGF-β superfamily receptors. Whereas this extension was proposed to modulate BMPRII signaling output, its contribution to the regulation of receptor expression was not addressed. To map regulatory determinants of BMPRII expression, we compared synthesis, degradation, distribution, and endocytic trafficking of BMPRII isoforms and mutants. We identified translational regulation of BMPRII expression and the contribution of a 3’ terminal coding sequence to this process. BMPRII-LF and -SF differed also in their steady-state levels, kinetics of degradation, intracellular distribution, and internalization rates. A single dileucine signal in the C-terminal extension of BMPRII-LF accounted for its faster clathrin-mediated endocytosis relative to BMPRII-SF, accompanied by mildly faster degradation. Higher expression of BMPRII-SF at the plasma membrane resulted in enhanced activation of Smad signaling, stressing the potential importance of the multilayered regulation of BMPRII expression at the plasma membrane.     

Root surface phosphatase activity in ecotypes of Aegilops peregrina

The relationships between root surface phosphatase activity and the edaphic factors of their native habitats were investigated in four ecotypes of Aegilops peregrina (Hack.) Maire et Weil. In one set of experiments plants were grown in phosphate-deficient nutrient solution cultures (5 μ M ) with three pH values: 5.5, 6.5 and 7.5. In a second series, plants were grown in both P-poor and P-rich soils.
Results showed an optimal activity of the commonly-described root surface acid phosphatase of pH 4.5–5.0 in the ecotypes Meron (a P-poor montmorillonitic, typical mediterranean Terra-Rossa soil) and Har-Hurshan (a P-rich calcareous soil). However, in the ecotypes Malkiya (a P-rich kaolinitic Terra-Rossa) and Bet-Guvrin (a P-rich calcareous soil) the optimal activity of the phosphatase occurred at pH 6.0. The pH level of the growth solution had no effect on the pH of optimal activity of the phosphatase in the ecotypes Malkiya and Bet-Guvrin, but it somewhat affected their level of activity.
Phosphatase activity was stimulated when plant roots were grown in a P-poor soil, as compared to the activity of those which were grown in a P-rich soil. Plants of the Malkiya ecotype exhibited the strongest activation of phosphatase as compared to the other three ecotypes. It seems that ecotypes which have evolved in P-rich soils may regulate their root surface phosphatase activity better than those which have evolved in P-poor soils.     

Detection of genotoxicity of metallic compounds by the bacterial bioluminescence test

Twenty metallic compounds were assayed for their genotoxic mutagenic activity by the bioluminescence test restoration of the luminescence of dark mutant of the luminous bacterium Photobacterium fischeri). The activity of the metals was tested in a liquid medium as well as on a solid medium. K₂Cr₂O₇, MnCl₂, BeCl₂, KH₂AsO₄, ZnCl₂ and Na₂WO₄ showed strong activity in liquid medium while AgNO₃, Cd(OOCCH₃)₂, CoCl₂, CuCl₂, HgCl₂, Na₂SeO₃ and Pb(NO₃)₂ were more active in the solid medium test. BaCl₂, Na₂MoO₄, NaAsO₂, NiSO₄, Na₂SeO₄, RbCl, and SnCl₂ were not active in the bioluminescence test. The correlation between the genotoxic activity of the tested metallic compounds in the bioluminescence test and other bacterial tests for genotoxic agents as well as the correlation between these results and the carcinogenicity of these compounds is discussed.     

Phenols in Cotton Seedlings Resistant and Susceptible to Alternaria macrospora

In healthy cotton seedlings, stems have a lower phenol content than leaves, but resistant plants have an altogether relatively higher phenol content than susceptible plants. Phenols extracted from infected plants can inhibit the growth of A. macrospora in vitro. In cotton plants infected with A. macrospora, phenols are oxidized by polyphenoloxidase rather than peroxidase and catalase. The main oxidative activity was around the developing necrotic area but activity was detected far from, necrosis as well. Though pre–inoculation mechanical injuries operated the phenol oxidation mechanism in the plant, they neither prevented nor encouraged the increase in disease severity. Isozyme pattern showed that contribution of all participants in the pathological interaction to the oxidative mechanism occurred in the diseased plant. A negative linear correlation was found between polyphenoloxidase activity, phenol accumulation and resistance. This study suggests that the phenol oxidative mechanism, participates in cotton plant resistance to A. macrospora.     

Differential Attachment to and Subsequent Contamination of Agricultural Crops by Salmonella enterica

U.S. salmonellosis outbreaks have occurred following consumption of tomato and cantaloupe but not lettuce. We report differential contamination among agricultural seedlings by Salmonella enterica via soil. Members of the family Brassicaceae had a higher incidence of outbreak than carrot, lettuce, and tomato. Once they were contaminated, phyllosphere populations were similar, except for tomato. Contamination differences exist among tomato cultivars.