Anna Fusconi

Interactions between the soilborne root pathogenPhytophthora nicotianae var.parasitica and the arbuscular mycorrhizal fungusGlomus mosseae in tomato plants

In order to study the influence of Arbuscular Mycorrhiza (AM) on the development of root rot infection, tomato plants were raised with or withoutGlomus mosseae and/orPhytophthora nicotianae var.parasitica in a sand culture system. All plants were fed with a nutrient solution containing one of two phosphorus (P) levels, 32µM (I P) or 96µM (II P), to test the consequence of enhanced P nutrition by the AM fungus on disease dynamics. Mycorrhizal plants had a similar development to that of control plants. Treatment withPhytophthora nicotianae var.parasitica resulted in a visible reduction in plant weight and in a widespread root necrosis in plants without mycorrhiza. The presence of the AM fungus decreased both weight reduction and root necrosis. The percentage reduction of adventitious root necrosis and of necrotic root apices ranged between 63 and 89% The enhancement of P nutrition increased plant development, but did not appreciably decrease disease spread. In our system, mycorrhiza increased plant resistance toP. nicotianae var.parasitica infection. Although a contribution of P nutrition by mycorrhiza cannot be excluded, other mechanisms appear to play a crucial role.

VA mycorrhizal infection and the morphology and function of root systems

Abstract VA mycorrhiza induces morphogenetic modifications in the root system of the host plant. This topic is discussed taking into account the possible significance of functional changes, as related to nutrient absorbing strategies. Modelling, topological analysis and cytological techniques are presented as useful tools for root investigation. Modelling may reveal developmental trends, and topological analysis may help to interpret the functional significance of developmental patterns. Root meristem activity and structure have been particularly considered and related to root system architecture. Finally, the bases of the morphogenetic effects of the fungus, whether they are nutritional or hormonol, are discussed.

Colonization pattern of primary tomato roots by Pseudomonas fluorescens A6RI characterized by dilution plating, flow cytometry, fluorescence, confocal and scanning electron microscopy

Early colonization of primary tomato roots, grown in vitro, by Pseudomonas fluorescens A6RI, introduced by seed bacterization, was monitored for 7 days in three different root zones (zone A, apex+elongation+young hairy zone; zone B, hairy zone; zone C, old hairy zone+collar). Bacterial quantification was assessed by enumeration of (i) colony forming units (cfu) after dilution plating and of (ii) total bacterial cells by flow cytometry. Bacterial distribution and organization in the root zones were analyzed by fluorescence, confocal and scanning electron microscopy. For all sampling dates and zones, the densities of total bacterial cells were significantly higher than those of the cfu. The kinetics of cfu densities varied according to the root zone. Their density decreased with time in zone A, while no variation with time was recorded in zones B and C. Densities of total bacterial cells did not show any significant temporal variation for any of the root zones. Microscopic analyses allowed the characterization of the distribution and organizational patterns of the bacterial cells according to time and space. In 3-day-old plants, bacteria were mostly present as single cells and were evenly distributed in the two root zones analyzed (A and B). In 5- and 7-day-old plants, distribution and organization differed according to the root zone. In zone A, only few single cells were observed, whereas zones B and C were mostly covered by cells localized between epidermal root cells and organized in pairs and strings, respectively.

Regulation of root morphogenesis in arbuscular mycorrhizae: what role do fungal exudates, phosphate, sugars and hormones play in lateral root formation?

BACKGROUND Arbuscular mycorrhizae (AMs) form a widespread root-fungus symbiosis that improves plant phosphate (Pi) acquisition and modifies the physiology and development of host plants. Increased branching is recognized as a general feature of AM roots, and has been interpreted as a means of increasing suitable sites for colonization. Fungal exudates, which are involved in the dialogue between AM fungi and their host during the pre-colonization phase, play a well-documented role in lateral root (LR) formation. In addition, the increased Pi content of AM plants, in relation to Pi-starved controls, as well as changes in the delivery of carbohydrates to the roots and modulation of phytohormone concentration, transport and sensitivity, are probably involved in increasing root system branching. SCOPE This review discusses the possible causes of increased branching in AM plants. The differential root responses to Pi, sugars and hormones of potential AM host species are also highlighted and discussed in comparison with those of the non-host Arabidopsis thaliana. CONCLUSIONS Fungal exudates are probably the main compounds regulating AM root morphogenesis during the first colonization steps, while a complex network of interactions governs root development in established AMs. Colonization and high Pi act synergistically to increase root branching, and sugar transport towards the arbusculated cells may contribute to LR formation. In addition, AM colonization and high Pi generally increase auxin and cytokinin and decrease ethylene and strigolactone levels. With the exception of cytokinins, which seem to regulate mainly the root:shoot biomass ratio, these hormones play a leading role in governing root morphogenesis, with strigolactones and ethylene blocking LR formation in the non-colonized, Pi-starved plants, and auxin inducing them in colonized plants, or in plants grown under high Pi conditions.

The mitotic cycle in root apical meristems ofAllium porrum L. is controlled by the endomycorrhizal fungusGlomus sp. strain E3

SummaryThe mitotic cycle in root apical meristems of mycorrhizal and non-mycorrhizal plants ofAllium porrum L.+Glomus sp., strain E3, has been examined by3H-TdR labelling. Significant slowing down of the mitotic cycle in the mycorrhizas through an extension of G1, S and metaphase was observed, while G2 was drastically reduced. Lengthening of the cycle was proportional to increasing infection. The ways in which the fungus could influence cell division are discussed.

Effects of arbuscular mycorrhizal colonization and phosphorus application on nuclear ploidy in Allium porrum plants

Arbuscular mycorrhizal (AM) colonization can strongly affect the plant cell nucleus, causing displacement from the periphery to the center of the cell, hypertrophy and polyploidization. The hypertrophy response has been shown in a variety of AM plants whilst polyploidization has been reported only in Lycopersicon esculentum, a multiploid species with a small genome. In order to determine whether polyploidization is a general plant response to AM colonization, analyses were performed on Allium porrum, a plant with a large genome, which is much less subject to polyploidization than L. esculentum. The ploidy status of leaves, complete root systems and four zones of the adventitious roots was investigated in relation to phosphorus content, AM colonization and root differentiation in A. porrum plants grown under two different regimes of phosphate nutrition in order to distinguish direct effects of the fungus from those of improved nutrition. Results showed the presence of two nuclear populations (2C and 4C) in all treatments and samples. Linear regression analyses suggested a general negative correlation between phosphorus content and the proportion of 2C nuclei. The percentage of 2C nuclei (and consequently that of 4C nuclei), was also influenced by AM colonization, differentiation and ageing of the root cells, which resulted in earlier occurrence, in time and space, of polyploid nuclei.

Correlation between Root Morphogenesis, Va Mycorrhizal Infection and Phosphorus Nutrition

ABSTRACT The effects of phosphorus (P) applications combined with a Glomus species strain E 3 infection on the growth and root development in leeks ( Allium porrum L.) grown in sand culture were studied. Infected roots were more branched compared with controls at low P levels. The plants also had greater fresh weights and total root lengths, shorter and more numerous adventitious roots, as well as more secondary roots per centimetre of adventitious root. Progressive P additions did not markedly influence the level of root infection by the fungus in mycorrhizal plants and induced the same root developmental pattern in controls, leading to the disappearance of differences between mycorrhizal and control plants at higher P levels. It may be argued that, in our system, the fungal influence on root architecture is mediated by nutritional effects.

Polyploidy in tomato roots as affected by arbuscular mycorrhizal colonization

Nuclear changes in roots of tomato (Lycopersicon esculentum), a plant with a small genome, during the establishment of arbuscular mycorrhizal (AM) colonization were studied using light and electron microscopy, as well as flow and static cytometry. Nuclei of mycorrhizal root cortex cells were larger and had more decondensed chromatin than those of controls. Significant ploidy distribution differences were observed between nuclei of AM colonized and control roots, and a strong correlation between nuclear polyploidization and AM colonization was found. Polyploidization and decondensation are usually associated with high metabolic activity. The metabolic activity of mycorrhizal root cells, evaluated in this work as respiratory activity by using a cytochemical assay for succinate dehydrogenase combined with image analysis, increased in comparison to controls. The meaning of polyploidization is discussed in relation to the structural and metabolic modifications induced by mycorrhization.

Root apical meristems ofAllium porrum L. as affected by arbuscular mycorrhizae and phosphorus

SummaryArbuscular mycorrhizal (AM) fungi significantly improve plant growth in soils with low phosphorus availability and cause many changes in root morphology, similar to those produced by increased P nutrition, mainly depending on root apex size and activity. The aim of this work was to discriminate between the morphogenetic role of AM fungi and P in leek (Allium porrum L.) by feeding mycorrhizal and nonmycorrhizal plants with two nutrient solutions containing 3.2 or 96 μM P and examining specific parameters related to adventitious root apices (apex size, mitotic cycle, and RNA synthesis). The results showed that AM fungi blocked meristem activity as indicated by the higher percentages of inactive apices and metaphases in the apical meristem of mycorrhizal plants, whereas the high P supply lengthened the mitotic cycle without blocking the apices, resulting in steady, slow root growth. The possible involvement of abscisic acid in the regulation of root apex activity is discussed.

Environmental Stress and Role of Arbuscular Mycorrhizal Symbiosis

Mycorrhizae are widespread symbioses between plant roots and soil fungi, involved in the absorption of nutrients from the soil. Arbuscular mycorrhizae (AMs) are made up of a wide range of land plants, including at least 80% angiosperms, and fungi belonging to the glomeromycota. They generally have a positive effect on plant growth and nutrition, and improve the absorption of relatively immobile nutrients, such as phosphate, particularly in low nutrient soils or under drought. Moreover, AMs have been shown to promote plant fitness under a variety of stress conditions. In this chapter, we provide an overview of the effects of AM colonization on host plants subjected to drought stress and soil pollution, two of the most common types of stress that limit plant growth, and of the possible mechanisms involved in the beneficial effects of AM fungi.