Miguel Martínez-Trujillo

N-acyl-L-homoserine lactones: a class of bacterial quorum-sensing signals alter post-embryonic root development in Arabidopsis thaliana

N-acyl-homoserine lactones (AHLs) belong to a class of bacterial quorum-sensing signals important for bacterial cell-to-cell communication. We evaluated Arabidopsis thaliana growth responses to a variety of AHLs ranging from 4 to 14 carbons in length, focusing on alterations in post-embryonic root development as a way to determine the biological activity of these signals. The compounds affected primary root growth, lateral root formation and root hair development, and in particular, N-decanoyl-HL (C10-HL) was found to be the most active AHL in altering root system architecture. Developmental changes elicited by C10-HL were related to altered expression of cell division and differentiation marker lines pPRZ1:uidA, CycB1:uidA and pAtEXP7:uidA in Arabidopsis roots. Although the effects of C10-HL were similar to those produced by auxins in modulating root system architecture, the primary and lateral root response to this compound was found to be independent of auxin signalling. Furthermore, we show that mutant and overexpressor lines for an Arabidopsis fatty acid amide hydrolase gene (AtFAAH) sustained altered growth response to C10-HL. All together, our results suggest that AHLs alter root development in Arabidopsis and that plants posses the enzymatic machinery to metabolize these compounds.

Improving transformation efficiency ofArabidopsis thaliana by modifying the floral dip method

Arabidopsis thaliana transformation with the floral dip method was improved by modifying the cell density and mode of application of theAgrobacterium inoculum. Drops of inoculum were applied 4 times to closed flower buds. The inoculum OD600 was increased from 0.8 to more than 2.0. These modifications improved reproducibility and increased transformation efficiencies to 2–3%.

Transkingdom signaling based on bacterial cyclodipeptides with auxin activity in plants

Microorganisms and their hosts communicate with each other through an array of signals. The plant hormone auxin (indole-3-acetic acid; IAA) is central in many aspects of plant development. Cyclodipeptides and their derivative diketopiperazines (DKPs) constitute a large class of small molecules synthesized by microorganisms with diverse and noteworthy activities. Here, we present genetic, chemical, and plant-growth data showing that in Pseudomonas aeruginosa, the LasI quorum-sensing (QS) system controls the production of three DKPs—namely, cyclo(l-Pro-l-Val), cyclo(l-Pro-l-Phe), and cyclo(l-Pro-l-Tyr)—that are involved in plant growth promotion by this bacterium. Analysis of all three bacterial DKPs in Arabidopsis thaliana seedlings provided detailed information indicative of an auxin-like activity, based on their efficacy at modulating root architecture, activation of auxin-regulated gene expression, and response of auxin-signaling mutants tir1, tir1 afb2 afb3, arf7, arf19, and arf7arf19. The observation that QS-regulated bacterial production of DKPs modulates auxin signaling and plant growth promotion establishes an important function for DKPs mediating prokaryote/eukaryote transkingdom signaling.

Transgenic Plants: An Historical Perspective

The development of technologies that allow the introduction and functional expression of foreign genes in plant cells has extended in less than two decades to the production of transgenic plants with improved insect and disease resistance, seeds and fruits with enhanced nutritional qualities, and plants that are better adapted to adverse environmental conditions. Vaccines against serious human diseases and other important products have also been developed using transgenic plants. Many more agronomic and quality traits are currently being engineered in both academic and industrial laboratories, which are limited only by our poor knowledge of plant gene function. The emergence of new functional genomic strategies for the identification and characterization of genes promises to provide a wealth of information with an enormous potential to enhance traditional plant breeding and to genetically engineer plants for specific purposes. This chapter describes some of the highlights in the development of these technologies and some of the major achievements in production and commercialization of transgenic crops. We also discuss some of the biosafety issues related to release of this novel class of plants into the environment.

High compatibility between arbuscular mycorrhizal fungal communities and seedlings of different land use types in a tropical dry ecosystem

We conducted this study to explore limitations for the establishment of mycorrhizal associations in disturbed areas of the tropical dry ecosystem in the Chamela region of Jalisco, Mexico. Specifically, we: (1) assessed the diversity and composition of arbuscular mycorrhizal fungal (AMF) communities through spore morphospecies identification in three common land uses (primary forest, secondary forest, and pasture), (2) tested the inoculum potential of the AMF communities and the effect of water stress on the establishment of mycorrhizal associations in seedlings of various plant species, and (3) explored the importance of AMF community composition on early seedling development. Soil and root samples were taken from 15 random points in each of three plots established in two primary forests, two 26-year-old secondary forests, and two 26-year-old pastures. We expected that because of soil degradation and management, pastures would have the lowest and primary forests the highest AMF species richness. We found evidence for changes in AMF species composition due to land use and for higher morphospecies richness in primary forests than in secondary forests and pastures. We expected also that water stress limited plant and mycorrhizal development and that plants and AMF communities from secondary forests and pastures would be less affected by (better adapted to) water stress than those from the primary forest. We found that although all plant species showed biomass reductions under water stress, only some of the plant species had lower mycorrhizal development under water stress, and this was regardless of the AMF community inoculated. The third hypothesis was that plant species common to all land use types would respond similarly to all AMF communities, whereas plant species found mainly in one land use type would grow better when inoculated with the AMF community of that specific land use type. All plant species were however equally responsive to the three AMF communities inoculated, indicating that all plants established functionally compatible AMF in each community, with no preferences. The results suggest that early seedling growth and mycorrhizal development in secondary forests and pastures is not likely limited by diversity, quantity, or quality of mycorrhizal propagules but by the high temperature and water stress conditions prevailing at those sites.

Improvement of rice transformation using bombardment of scutellum-derived calli

Although several reports on rice transformation have been published, producing transgenic plants ofIndica rice varieties is still problematic. We report an improved protocol for transformingIndica rice genotypes. An important agronomic MexicanIndica rice variety, Morelos A-92, was used. Calli derived from scutellum seeds were produced by using auxins and bombarded with 2 vectors, one harboring the reporteruidA gene and the other with thehptII gene conferring hygromycin resistance. The influence of the molar relation of these vectors (uidA-hptIII) in generating callus and plants expressing theuidA reporter gene was analyzed. Selection of bombarded calli was performed under 2 conditions: 50 mg/L of hygromycin with 3 subcultures and 80 mg/L of hygromycin with no subcultures. The best conditions were a 20∶1uidA-hptII molar vector relationship and selection at 80 mg/L of hygromycin, producing 14% of calli expressing GUS. The minimal callus size in regenerating plants was 3 mm. Transformed rice plants were generated with 4.6% efficiency, considering the initial number of bombarded calli. Heredity of theuidA gene behaved as a single locus in transformed rice plants. Homozygous plants were identified in the T1 generation by means of pollen staining.

Chromate induces adventitious root formation via auxin signalling and SOLITARY-ROOT/IAA14 gene function in Arabidopsis thaliana

Morphological root plasticity optimizes nutrient and water uptake by plants and is a promising target to improve tolerance to metal toxicity. Exposure to sublethal chromate [Cr(VI)] concentrations inhibits root growth, decreases photosynthesis and compromises plant development and productivity. Despite the increasing environmental problem that Cr(VI) represents, to date, the Cr tolerance mechanisms of plants are not well understood, and it remains to be investigated whether root architecture remodelling is important for plant adaptation to Cr(VI) stress. In this report, we analysed the growth response of Arabidopsis thaliana seedlings to concentrations of Cr(VI) that strongly repress primary and lateral root growth. Interestingly, adventitious roots started developing, branched and allowed seedlings to grow under highly growth-repressing Cr(VI) concentrations. Cr(VI) negatively regulates auxin transport and response gene expression in the primary root tip, as evidenced by decreased expression of auxin-related reporters DR5::GFP, DR5::uidA and PIN1::PIN1::GFP, and then, another auxin maximum is established at the site of adventitious root initiation that drives adventitious root organogenesis. Both primary root growth inhibition and adventitious root formation induced by high Cr(VI) levels are blocked by a gain-of-function mutation in the SOLITARY-ROOT/IAA14 gene of Arabidopsis. These data provide evidence that suggests a critical role for auxin transport and signalling via IAA14/SLR1 in the developmental program linking Cr(VI) to root architecture remodelling.

Functional analysis of the 5' untranslated region of the sucrose phosphate synthase rice gene (sps1)

Abstract The sps1 gene, encoding the sucrose phosphate synthase (SPS) in rice has an unusually long 5′ untranslated region (leader) of 368 nt with a complex predicted secondary structure. Elimination of the leader from the native sps1 promoter diminishes the in vivo expression of the GUS reporter gene by 10-fold. Insertion of the leader downstream, but not upstream, of the −46(35S) CaMV promoter in the correct orientation produces an expression enhancement equivalent to that conferred in the sps1 native promoter. This expression enhancement is observed using both in vivo transient expressions in tobacco leaves and in stably transformed Arabidopsis plants. The sps1 leader also enhances the level of reporter gene expression in the strong 35S CaMV and Cassava Vein Mosaic Virus (CVMV) promoters, albeit at a lower level than in weaker promoters. In vitro transcription and translation studies using transcripts of the uidA gene, with and without the sps1 leader, show that the sps1 5′ untranslated region enhances translation over three-fold. Deletion analysis of the sps1 leader showed that in vivo enhancement is totally conserved in the 5′Δ85 and diminished by half in the 5′Δ148; the deletion 3′Δ190 conserves two thirds of the enhancement, suggesting redundancy in sequence elements.

Effect of mineral nutrients on the uptake of Cr(VI) by maize plants

To determine the potential of maize plants for phytoextraction of chromium (Cr), and the effect that some mineral nutrients have on this process, the uptake of this metal was analyzed in vitro. 12-day-old plants were incubated in nutrient solutions with 200 or 250 μM potassium chromate and 3mM supplements of nitrate, phosphate or sulfate. The greatest accumulation of Cr after 48 hours was in plants that were supplemented with nitrate (800 and 350 mg kg(-1) DW for roots and stems, respectively) with bioaccumulation factors of 18 and 7 in the roots and in the foliage, respectively. Moreover, the translocation factor from root to leaves was 0.4 when nitrate was supplemented; thus, the addition of this nutrient is recommended when the crop is to be used for phytoextraction of Cr. Considering the results obtained, the high biomass produced by this crop and the extensive knowledge of agricultural practices for this plant, we believe that maize has the potential to be considered in the phytoextraction of Cr, based on preliminary assays of soil conditions.

Respuestas morfogénicas de las raíces de Arabidopsis thaliana (Magnoliophyta: Brassicales) al estrés de Cr(VI)

La respuesta del crecimiento de las raices ante las condiciones de estres es una estrategia que le permite a la planta ya sea evadir las condiciones adversas o bien adaptarse a estas. Las actividades humanas han ocasionado la acumulacion de algunos metales que a partir de ciertas concentraciones tienen un efecto adverso en el crecimiento de las plantas. Entre estos metales se encuentra el Cromo (Cr), el cual es un serio contaminante generado por industrias de tratamiento de metales y del curtido de pieles. Para entender de manera detallada la respuesta de las raices al estres del Cr(VI), se utilizo un sistema in vitro con Arabidopsis thaliana usando 3 condiciones experimentales. (A) Plantas germinadas y crecidas por 6 dias, sin raices laterales, transferidas a medios con diferentes concentraciones de Cr(VI). Esto permitio evaluar como se afectaba el crecimiento de la raiz primaria al pasar rapidamente del medio MS al medio MS con Cr(VI) y en que medida se estimulaba o inhibia la formacion y crecimiento de nuevas raices laterales. (B) Plantas germinadas y crecidas durante 8 dias, con raices laterales, transferidas a medios con diferentes concentraciones de Cr(VI). Esta condicion permitio evaluar como se afectaba el crecimiento de raices primarias y raices laterales previamente formadas al pasar rapidamente del medio MS a medios suplementados con Cr(VI). Ademas, permitio comparar como se comportaban las raices laterales nuevas con respecto a las ya existentes