Plant Science

Root-associated bacteria are able to influence plant fitness and vigor. A key step in understanding the belowground plant-bacteria interactions is to quantify root colonization by the bacteria of interest. Probably, genetic engineering with fluorescence markers is the most powerful way to monitor bacterial strains in plant. However, this could have some collateral problems and some strains can be challenging to label. In this sense, bacterial inoculation under properly controlled conditions can enable reliable and reproducible quantification of natural bacterial strains. In this protocol, we describe a detailed procedure for quantification of root-associated bacteria. This method applies non-aggressive samples processed with morphological identification and PCR-based genetic fingerprinting. This easy-to-follow protocol is suitable for studying bacterial colonization of plants grown either in artificial medium or in soil.

The plant phyllosphere, which represents all plant parts that are above the ground, is considered one of the most extensive ecosystems to be colonized by microorganisms, both at the surface as epiphytes or as endophytes within the plant. These plant-associated microbial communities are reservoirs of microbial diversity and they can be important for plant health. The characterization of microbial communities in diverse plants, such as Espeletia plants that are endemic to the Paramo ecosystem in the Andes Mountains, can shed light regarding possible interactions among microorganisms and microbial functional properties. Obtaining DNA from plant endophytic microbial communities involves various steps to ensure that samples are free of contamination from microorganisms present on the plant surface (epiphytes). Plant leaves are first surface sterilized, cut into pieces, homogenized using glass beads, and then used for DNA extraction using a commercially available kit. DNA samples are then quantified and analyzed using Qubit^® 2.0 for use in PCR amplification of 16S rRNA genes.

Strigolactones (SLs) are carotenoid-derived signaling chemicals containing two lactone moieties in their structures and induce seed germination of root parasitic plants, Striga and Orobanche spp. In the rhizosphere, SLs are essential host recognition signals not only for root parasitic plants but also for arbuscular mycorrhizal fungi. In plants, SLs play important roles as plant hormones regulating shoot and root architecture. Plants produce only trace amounts of chemically unstable SLs, which makes it difficult to determine SL contents in plant tissues. Here, we describe how to extract and quantify sorgomol and 5-deoxystrigol, major SLs produced in sorghum roots.

Legumes are able to form endosymbiotic interactions with nitrogen-fixing rhizobia. Endosymbiosis takes shape in formation of a symbiotic organ, the root nodule. Medicago truncatula (M. truncatula) nodules contain several zones representing subsequent stages of development. The apical part of the nodule consists of the meristem and the infection zone. At this site, bacteria are released into the host cell from infection threads. Upon release, bacteria are surrounded by a host cell-derived membrane to form symbiosomes. After release, rhizobia grow, divide, and gradually colonize the entire host cell of the fixation zone of root nodules. Therefore, mature infected cells contain thousands of symbiosomes, which remain as individual units among other organelles. Visualization of the organization and dynamics of the symbiosomes as well as other organelles in infected cells of nodules is essential to understand mechanisms regulating the development of endosymbiosis between plants and rhizobia. To examine this highly dynamic developmental process, we designed a useful imaging technique that is based on confocal scanning microscopy combined with different fluorescent dyes and GFP-tagged proteins (Gavrin et al., 2014). Here, we describe a protocol for microscopic observation, 3D rendering, and volume/area measurements of symbiosomes and other organelles in infected cells of M. truncatula root nodules. This protocol can be applied for monitoring the development of different host-microbe interactions whether symbiotic or pathogenic.

Symbiotic orchid seed germination in an in vitro system allows the growth of mycorrhizal protocorms and plantlets for scientific purposes. Orchids in nature need to establish a mycorrhizal symbiosis with fungal partners to germinate and develop into adult plants. Here we present a protocol for symbiotic germination of the terrestrial Mediterranean green meadow orchid Serapias vomeracea. The fungal symbiont Tulasnella calospora (T. calospora) (Basidiomycetes, Cantharellales) was chosen because of its common occurrence (Girlanda et al., 2011), its ability to grow in culture and compatibility in germination assays. T. calospora is one of the most common rhizoctonia-like fungi associated with terrestrial as well as epiphytic orchids.

This protocol provides a simple and fast method of quantification for intracellular flavin content, and for flavin secretion by bacteria. Intracellular flavins are extracted from bacterial pellets, and secreted flavins are examined in the cell growth medium. Flavins are separated and measured using HPLC with fluorescence detection, and quantified based on a comparison to standards.

We describe an efficient method to obtain a sufficient quantity of RNA from nematode-induced galls with a high quality and integrity, proved to be appropriate for transcriptomic analysis, i.e. real time PCR, microarray hybridization or second generation sequencing. This protocol is efficient for small quantities of galls (organs with high protein and sugar contents). The protocol allows obtaining an RNA yield of 5-15 μg total RNA from 250-300 hand dissected galls at 3 days post infection (dpi) (Figure 1). It was proved particularly for Arabidopsis and tomato.

The mutualistic root endophyte Piriformospora indica colonizes a wide range of plants and the colonization of root cells by this fungus is very often associated with beneficial effects to its host, such as growth promotion and increased biotic and abiotic stress tolerance. These traits could be based on general mechanisms and signaling pathways common to many different plant species. One such mechanism could be the recruitment of phytohormone pathways by P. indica. It is known, that many mutualistic microorganisms are able to synthesize and secrete phytohormones during the interaction with their host plants. This protocol has been successfully utilized to analyze tryptophan (TRP)-dependent biosynthesis of indole-3-acetic acid (IAA) and its indole derivatives by P. indica (Hilbert et al., 2012).

The mutualistic root endophyte Piriformospora indica colonizes a wide range of plants and the colonization of root cells by this fungus is very often associated with beneficial effects to its host, such as growth promotion and increased biotic and abiotic stress tolerance. These traits may be based on general mechanisms and signaling pathways common to many different plant species. One such mechanism could be the recruitment of phytohormone pathways by P. indica. It is known, that many mutualistic microorganisms are able to synthesize and secrete phytohormones during the interaction with their host plants. This protocol has been successfully utilized to analyze tryptophan (TRP)-dependent biosynthesis of indole-3-acetic acid (IAA) and its indole derivatives by P. indica as well as their influence on the growth of this fungus (Hilbert et al., 2012).