Megapolis such as Mexico City, have atmospheric pollutants that interact with the humidity and solar radiation. The topography of this city promotes air stagnation, generating atmospheric pollutants and episodes of acid rain, a phenomenon well recorded since the end of the 1980s. However, little we know about how urban trees respond to acid rain in the city. Here we present how simulated acid rain causes anatomical and changes in photosynthetic pigments in two of the most abundant urban trees in Mexico City: Liquidambar styraciflua L. and Fraxinus uhdei (Wenz.) Lingelsh. We first described the leaf anatomy of both species. Then, we used one-year-old trees sprayed with sulfuric acid solutions at pH 2.5 and 3.8, and evaluated visible leaf damage, anatomical alterations, and chlorophyll contents. In both species, the pH 2.5 caused cuticle alterations and areas of total tissue destruction. L. styraciflua showed greater sensitivity, but we discuss some of the tolerance mechanisms. Finally, acid rain also reduced the chlorophyll contents. These results contribute toward a catalogue of urban tree species to describe pollution-induced damages, and the identification of tolerant species useful for short- and mid-term detection of environmental crisis, in cities with similar environmental conditions and urban tree composition.

In vitro plant regeneration addresses basic questions of molecular reprogramming in the absence of embryonic positional cues. The process is highly dependent on the genotype and explant characteristics. However, the regulatory mechanisms operating during organ differentiation from in vitro cultures remain largely unknown. Recently, miRNAs have emerged as key regulators during embryogenic callus induction, plant differentiation, auxin responses and totipotency. Here, we explored how development-related miRNA switches the impact on their target regulation depending on physiological and molecular events taking place during maize Tuxpeño VS-535 in vitro plant regeneration. Three callus types with distinctive regeneration potential were characterized by microscopy and histological preparations. The embryogenic calli (EC) showed higher miRNA levels than non-embryogenic tissues (NEC). An inverse correlation for miR160 and miR166 targets was found during EC callus induction, whereas miR156, miR164 and miR394 displayed similar to their targets RNA accumulation levels. Most miRNA accumulation switches took place early at regenerative spots coincident with shoot apical meristem (SAM) establishment, whereas miR156, miR160 and miR166 increased at further differentiation stages. Our data uncover particular miRNA-mediated regulation operating for maize embryogenic tissues, supporting their regulatory role in early SAM establishment and basipetala growth during the in vitro regeneration process.

Background. Distinguishing species and populations in Crassulaceae is challenging because of the morphological variability and inter-specific hybridization. Currently our understanding of the morphological and anatomical features in Echeveria is poor, and therefore it is difficult to delimit species, and morphotypes within the species. Question. Our objective was to describe the foliar anatomy and the shape of accessions in E. gigantea. We used E. gibbiflora , another species in the series as a comparison group. Comparing the two species allowed us to evaluate the relationship between anatomy and morphology in E. gigantea .Methods. We performed a survey of anatomical features in seven accessions of E. gigantea , and one accession of E. gibbiflora . We obtained epidermal prints, leaf sections, performed geometric and morphometric analyses.Results. We found that 65 % of the anatomical traits are heterogeneous among the taxonomic units. Our analysis showed that E. gibbiflora and some E. gigantea accessions share extensive anatomical similarities. There was variation within the E. gigantea , suggesting that at least one of the accessions is an independent taxonomic group. The traits with the largest contribution to variation between the groups were related to the epidermis, the hypodermis, the type of vascular bundles and the collenchyma associated to the vascular bundles. In addition we quantified the variation in leaf shape. Interestingly, we found correlations between the organ shape and anatomical traits.Conclusion. These analyses provide information about traits towards the morpho-anatomical definition of E. gigantea variation and suggest developmental correlation yet to be explained.

A puzzle in evolution is the understanding of how the environment might drive subtle phenotypic variation, and whether this variation is adaptive. Under the neutral evolutionary theory, subtle phenotypes are almost neutral with little adaptive value. To test this idea, we studied the infraspecific variation in flower shape and color in Mammillaria haageana, a species with a wide geographical distribution and phenotypic variation, which populations are often recognized as infraspecific taxa. We collected samples from wild populations, kept them in the greenhouse for at least one reproductive year, and collected newly formed flowers. Our first objective was to characterize tepal natural variation in M. haageana through geometric morphometric and multivariate pigmentation analyses. We used landmark-based morphometrics to quantify the trends of shape variation and tepal color-patterns in 20 M. haageana accessions, belonging to five subspecies, plus 8 M. albilanata accessions for comparison as the sister species. We obtained eight geometric morphometric traits for tepal shape and color-patterns. We found broad variation in these traits between accessions belonging to the same subspecies, without taxonomic congruence with those infraspecific units. Also the phenetic cluster analysis showed different grouping patterns among accessions. When we correlated these phenotypes to the environment, we also found that solar radiation might explain the variation in tepal shape and color, suggesting that subtle variation in flower phenotypes might be adaptive. Finally we present anatomical sections in M. haageana subsp. san-angelensis to propose some of the underlying tepal structural features that may give rise to tepal variation. Our geometric morphometric approach of flower shape and color allowed us to identify the main trends of variation in each accession and putative subspecies, but also allowed us to correlate these variation to the environment, and propose anatomical mechanisms underlying this diversity of flower phenotypes.

Urbanization modifies ecosystem conditions and evolutionary processes. This includes air pollution, mostly as tropospheric ozone (O₃), which contributes to the decline of urban and peri‐urban forests. A notable case are fir (Abies religiosa) forests in the peripheral mountains southwest of Mexico City, which have been severely affected by O₃ pollution since the 1970s. Interestingly, some young individuals exhibiting minimal O₃—related damage have been observed within a zone of significant O₃ exposure. Using this setting as a natural experiment, we compared asymptomatic and symptomatic individuals of similar age (≤15 years old; n = 10) using histologic, metabolomic, and transcriptomic approaches. Plants were sampled during days of high (170 ppb) and moderate (87 ppb) O₃ concentration. Given that there have been reforestation efforts in the region, with plants from different source populations, we first confirmed that all analyzed individuals clustered within the local genetic group when compared to a species‐wide panel (Admixture analysis with ~1.5K SNPs). We observed thicker epidermis and more collapsed cells in the palisade parenchyma of needles from symptomatic individuals than from their asymptomatic counterparts, with differences increasing with needle age. Furthermore, symptomatic individuals exhibited lower concentrations of various terpenes (ß‐pinene, ß‐caryophylene oxide, α‐caryophylene, and ß‐α‐cubebene) than asymptomatic trees, as evidenced through GC–MS. Finally, transcriptomic analyses revealed differential expression for 13 genes related to carbohydrate metabolism, plant defense, and gene regulation. Our results indicate a rapid and contrasting phenotypic response among trees, likely influenced by standing genetic variation and/or plastic mechanisms. They open the door to future evolutionary studies for understanding how O₃ tolerance develops in urban environments, and how this knowledge could contribute to forest restoration.

Antecedentes: Agave salmiana tiene dos subespecies con gran variación morfológica. Agave salmiana subsp. salmiana es nativa de zonas áridas del centro y sur de México, con importancia alimenticia, cultural y económica, pero se desconoce el nivel de variación fenotípica de sus caracteres epidérmicos. Pregunta: ¿Cuál es el grado de variación anatómica intra e interpoblacional en los caracteres epidérmicos, particularmente en el margen, la región intermedia y la vena media de la hoja? Métodos: Se compararon muestras de epidermis, de tres regiones de las hojas, de cinco individuos de tres localidades del Estado de México. Se montaron como preparaciones semipermanentes; se compararon estadísticamente catorce parámetros en los tres niveles asignados. Resultados: Se encontraron diferencias significativas entre las regiones de la hoja en 57 %; mientras que entre individuos de cada localidad hay 21, 36 y 71 % respectivamente, y entre localidades 71 %. El intervalo de plasticidad fenotípica va de 14 a 100 %, la menor entre las regiones de las hojas y la mayor en la localidad de Teotihuacán. Se infiere la relación de esta variación con la procedencia de las plantas. Los análisis multivariados evidencian que seis caracteres epidérmicos distinguen significativamente a los individuos de las tres localidades, particularmente a los de Tecámac. Conclusión: La variación en los caracteres epidérmicos es significativa a nivel intra e interpoblacional. Excepto entre las tres regiones de la hoja, la plasticidad fenotípica es alta entre los individuos de cada localidad y entre las tres localidades.