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RESUMEN Los hongos formadores de micorrizas arbusculares (HFMA) son simbiontes obligados presentes en la rizósfera de plantas de cacao y la diversidad de sus comunidades se modifica, dependiendo de diversos factores como la presencia de cadmio (Cd) en el suelo. La persistencia de HFMA en suelos enriquecidos naturalmente con Cd podría ser un indicador de su capacidad para tolerar esta condición. Esta investigación caracterizó la estructura de la comunidad de HFMA locales presentes en la rizósfera de plantas de cacao en dos suelos con baja (B-Cd: 0,1 mg kg-1) y alta (A-Cd: 20,9 mg kg-1) concentración de Cd. Esporas de HFMA se identificaron mediante claves taxonómicas y su abundancia, riqueza y diversidad se determinó en muestras de suelo originales y después de su multiplicación mediante cultivos trampa. Las comunidades de HFMA se compararon usando análisis de componentes principales (ACP) e índices de diversidad alfa y beta. Los resultados indican que A-Cd presentó valores significativamente menores de abundancia (21 %), riqueza (20 %) y diversidad (11 %) de morfoespecies de HFMA con respecto a B-Cd. Las dos comunidades de HFMA presentaron cinco de siete géneros en común, pero solo cuatro de las 23 morfoespecies descritas se encontraron en ambas comunidades. El análisis de diversidad beta y el ACP determinaron baja similaridad y tasa de recambio entre las comunidades de HFMA. La dominancia de Diversispora spurca, Rhizoglomus sp. y Claroideoglomus etunicatum en A-Cd sugiere que estas morfoespecies son estrés-tolerantes y candidatos potenciales para el desarrollo de estrategias de mitigación en suelos con Cd.

ABSTRACT Arbuscular mycorrhizae fungi (AMF) are obligate symbionts present in rhizosphere of cocoa plants and their community diversity is modified depending on several factors, such as cadmium (Cd) presence in soil. AMF persistence on Cd natural enriched soils might be an indicator of their tolerance and their potential in biotechnological applications. In this research we characterized local AMF community structure present in cocoa rhizosphere soils with low (B-Cd: 0.1 mg kg-1) and high (A-Cd: 20.9 mg kg-1) natural Cd concentrations. AMF spore identification was carried out using taxonomic keys and their abundance, richness and diversity were determined in original samples and after multiplication process using onion trap cultures. AMF communities were compared using alpha and beta diversity indexes and principal component analysis (PCA). The results indicated that A-Cd presented significative lower values of abundance (21 %), richness (20 %) and diversity (11 %) of AMF morphospecies in comparison with B-Cd. Both AMF communities presented five of seven genera in common, but only four of 23 morphospecies described were found in two communities. Low similarity and turnover were found among AMF communities throughout beta diversity analysis and PCA. Dominance of Diversispora spurca, Rhizoglomus sp. and Claroideoglomus etunicatum in A-Cd suggests that these morphospecies are stress-tolerant and they are potential candidates for the development of mitigation strategies in cocoa plants under Cd stress.

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RESUMEN Las enfermedades "pudrición negra del fruto" y "moniliasis" son reportadas como limitantes en plantaciones de cacao en Colombia. Fueron evaluados los patrones espaciales de incidencia para estas dos enfermedades en un cultivo de cacao durante un monitoreo de trece semanas en campo. Semivariogramas e interpolación kriging fueron usados para analizar el comportamiento espacial de "pudrición negra del fruto", los estadísticos Join Count e Indice de Moran se emplearon para estudiar el comportamiento espacial de "moniliasis". La incidencia de "pudrición negra del fruto" presentó estructura de dependencia espacial de acuerdo a los semivariogramas durante el tiempo de evaluación, mientras que la autocorrelación espacial de sitios de infección de "moniliasis" se distribuyó de forma aleatoria entre las plantas de cacao según el Indice de Moran. El rango de dispersión estimado de "pudrición negra del fruto" varió entre 21,74 y 40,23 m y los mapas kriging evidenciaron el progreso espacio-temporal de la enfermedad durante las semanas evaluadas. Las condiciones experimentales dadas y la aproximación de los análisis espaciales, contribuyeron al entendimiento de la dinámica espacial de dos enfermedades limitantes en tres clones de cacao en la Orinoquía Colombiana.

ABSTRACT "Black pod" and "frosty pod" diseases are reported as limiting in cocoa plantations in Colombia. Spatial patterns of incidence for these two diseases were evaluated during a thirteen weeks long period of monitoring in the field. Semivariogram and kriging interpolation were used to analyze the spatial behavior of "black pod" and Join Count and Moran's Index statistics were used to analyze the spatial behavior of "frosty pod". The incidence of "Black pod" has a spatial dependence structure according to semivariogramas during the evaluation time, while spatial autocorrelation of "moniliasis" infection sites was randomly distribuited among cocoa plants according to the Moran's Index. The estimated dispersion range of "Black pod" was from 21.74 to 40.23 m y the kriging maps showed the spatio-temporal progression of the disease during the evaluated weeks. The experimental conditions given and spatial analysis approach, contributed to the understanding of the spatial dynamics of two limiting diseases in three clones of cocoa in the Colombian Orinoquia.

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Arbuscular mycorrhizal fungi (AMF) and phosphate solubilizing Pseudomonas bacteria (PSB) could potentially interact synergistically because PSB solubilize phosphate into a form that AMF can absorb and transport to the plant. However, very little is known about the interactions between these two groups of microorganisms and how they influence the growth of each other. We tested whether different strains of bacteria, that have the capacity to solubilize phosphate, are able to grow along AMF hyphae and differentially influence the growth of AMF both outside the roots of carrot in in vitro conditions and inside the roots of potato in the presence of a microbial community. We found strong effects of AMF on the growth of the different bacterial strains. Different bacterial strains also had very strong effects on the growth of AMF extraradical hyphae outside the roots of carrot and on colonization of potato roots by AMF. The differential effects on colonization occurred in the presence of a microbial community. Our results show that these two important groups of rhizosphere microorganisms indeed interact with each other. Such interactions could potentially lead to synergistic effects between the two groups but this could depend on whether the bacteria truly solubilize phosphate in the rhizosphere in the presence of microbial communities.

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The global human population is expected to reach ∼9 billion by 2050. Feeding this many people represents a major challenge requiring global crop yield increases of up to 100%. Microbial symbionts of plants such as arbuscular mycorrhizal fungi (AMF) represent a huge, but unrealized resource for improving yields of globally important crops, especially in the tropics. We argue that the application of AMF in agriculture is too simplistic and ignores basic ecological principals. To achieve this challenge, a community and population ecology approach can contribute greatly. First, ecologists could significantly improve our understanding of the determinants of the survival of introduced AMF, the role of adaptability and intraspecific diversity of AMF and whether inoculation has a direct or indirect effect on plant production. Second, we call for extensive metagenomics as well as population genomics studies that are crucial to assess the environmental impact that introduction of non-local AMF may have on native AMF communities and populations. Finally, we plead for an ecologically sound use of AMF in efforts to increase food security at a global scale in a sustainable manner.

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The arbuscular mycorrhizal symbiosis is formed between arbuscular mycorrhizal fungi (AMF) and plant roots. The fungi provide the plant with inorganic phosphate (P). The symbiosis can result in increased plant growth. Although most global food crops naturally form this symbiosis, very few studies have shown that their practical application can lead to large-scale increases in food production. Application of AMF to crops in the tropics is potentially effective for improving yields. However, a main problem of using AMF on a large-scale is producing cheap inoculum in a clean sterile carrier and sufficiently concentrated to cheaply transport. Recently, mass-produced in vitro inoculum of the model mycorrhizal fungus Rhizophagus irregularis became available, potentially making its use viable in tropical agriculture. One of the most globally important food plants in the tropics is cassava. We evaluated the effect of in vitro mass-produced R. irregularis inoculum on the yield of cassava crops at two locations in Colombia. A significant effect of R. irregularis inoculation on yield occurred at both sites. At one site, yield increases were observed irrespective of P fertilization. At the other site, inoculation with AMF and 50% of the normally applied P gave the highest yield. Despite that AMF inoculation resulted in greater food production, economic analyses revealed that AMF inoculation did not give greater return on investment than with conventional cultivation. However, the amount of AMF inoculum used was double the recommended dose and was calculated with European, not Colombian, inoculum prices. R. irregularis can also be manipulated genetically in vitro, leading to improved plant growth. We conclude that application of in vitro R. irregularis is currently a way of increasing cassava yields, that there is a strong potential for it to be economically profitable and that there is enormous potential to improve this efficiency further in the future.

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El establecimiento de la simbiosis planta-hongos formadores de micorrizas Arbusculares (HFMA) requiere procesos armónicos a nivel espacio-temporal, que dependen de señales para el reconocimiento, colonización e intercambio bidireccional de nutrientes. Las plantas presentan respuestas de defensa frente a posibles organismos invasores; sin embargo, frente a HFMA estas son débiles, localizadas y no impiden la colonización del hongo. Los beneficios de la simbiosis generalmente se asocian a nutrición vegetal, aunque, también está relacionada con el incremento de la tolerancia-resistencia de plantas a los estreses bióticos. La resistencia inducida HFMA (MIR) es importante en el control de patógenos foliares, comedores de hojas y necrótrofos, encontrándose protección de plantas micorrizadas tanto a nivel local como sistémico, relacionada con los niveles de ácido jasmónico en tejidos. Un mecanismo en la MIR está asociado con el “priming”, que permite una rápida y eficiente respuesta de defensa de plantas micorrizadas. Se han planteado posibles mecanismos de atenuación de las respuestas de defensa, considerando: activación de supresores de defensa; plantas que producen respuestas de defensa frente a HFMA y otras que no las producen, y plantas que suprimen las respuestas de defensa en la simbiosis. Aunque el control de la simbiosisestá regulado básicamente por la planta, aún se desconoce el papel de los HFMA en el debilitamiento de las respuestas de defensa. Recientemente, se ha dado un avance importante en entender los mecanismos mediante los cuales se establece y mantiene la biotrofía del hongo, al describirse la proteína SP7 que interactúa con el factor de transcripción PR, ERF19 en el núcleo de la célula vegetal. Se ha sugerido que SP7 es un efector que actúa oponiéndose al programa de inmunidad de la planta.

Harmonic processes between plant and arbuscular mycorrhyzal fungi (AMF) are required for the symbiosis formation between the two organisms. These processes depend on specific signalling for the plant-fungus recognition, colonisation and bidirectional nutrient exchange. Plants show defence responses against invasive organisms, however they react weakly and localised when challenged by AMF. The benefits derived from the mycorrization are described for the nutritional aspect; however, it is known that mycorrhized plants are more tolerant to biotic stresses. Mycorrhizal induced resistance (MIR) is especially important for the control of foliar pathogens, leaf cutters and necrotrophs. There has also been found that mycorrhizal plants are protected both locally and systemically and their protection is related with jasmonic acid levels at their tissues. One of the most important mechanisms for MIR is the so called “priming” that allows plants to exert a fast and efficient defence response. Possible mechanisms to unravel mycorrhizal plants lower defence systems include: defence suppressor activation, differential plants response towards AMF from inexistent to low, and plant defence response suppression during the AMF symbiosis. The symbiosis control is known to be regulated by the plant, however, no role has been assigned to the AMF for the weakening of the plant defence system. Recently, a big step towards understanding of the fungal role has been made. A protein SP7 that interacts with a PR transcription factor ERF19, in the plant nucleus, has been described. This discovery indicates a possible mechanism to establish and maintain the biotrophic status of the AMF counteracting the immune plant system.

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