Root morphology and exudate availability is shaped by particle size and chemistry in Brachypodium distachyon

Abstract

Keywords Brachypodium distachyonParticle chemistry Particle size Pseudomonas fluorescens Rhizosphere Root exudation Root morphology Abstract Root morphology and exudation define a plants sphere of influence in soils, and are in turn shaped by the physiochemical characteristics of soil. We explored how particle size and chemistry of growth substrates affect root morphology and exudation of the model grass Brachypodium distachyon. Root fresh weight and root lengths were correlated with particle size, whereas root number and shoot weight remained constant. Mass spectrometry imaging suggested that both, root length and number shape root exudation. Exudate metabolite profiles detected with liquid chromatography / mass spectrometry were comparable for plants growing in glass beads or sand with various particles sizes, but distinct for plants growing in clay. However, when exudates of clay-grown plants were collected by removing the plants from the substrate, their exudate profile was similar to sand-or glass beads-grown plants. Clay particles sorbed 20% of compounds exuded by clay-grown plants, and 70% of compounds of a defined exudate medium. The sorbed compounds belonged to a range of chemical classes, among them nucleosides/nucleotides, organic acids, sugars, and amino acids. Some of the sorbed compounds could be de-sorbed by a rhizobacterium (Pseudomonas fluorescens WCS415), supporting its growth. We show that root morphology is affected by substrate size, and that root exudation in contrast is not affected by substrate size or chemistry. The availability of exuded compounds, however, depends on the substrate present. These findings further support the critical importance of the physiochemical properties of soils are crucial to consider when investigating plant morphology, exudation, and plantmicrobe interactions. Plant roots shape their environment in various ways, and are in turn shaped by physiochemical properties of the surrounding soil. Roots shape soil by dislocating particles, by polymer production, and by release of a wide variety of compounds (root exudation). Root exudates alter pH and the chemical composition around roots. Overall, root presence in soils results in formation of larger soil aggregates and increases water-holding capacity (Six et al., 2004). The plant-induced changes in chemistry can lead to weathering of minerals (Uroz et al., 2015), and alter the composition of microbial communities (Carson et al., 2007).Soils are often characterized by their particle size and mineralogy. Typical soil particles range from small (< 50 µm) to large (> 2 mm), determining physical parameters such as water binding capacity (Six et al., 2004; Six and Paustian, 2014; Rellán-Álvarez et al., 2016). Differently sized sandstones are associated with different microbial numbers, with small sandstones (2 mm) being more densely populated by microbes than larger rocks (Certini et al., 2004). Minerals differ in their structure (e.g. accessible surface), and in their surface charge, determining if and how they interact wi...

Mass spectrometry‐based quantification and spatial localization of small organic acid exudates in plant roots under phosphorus deficiency and aluminum toxicity

“…Gelatine‐embedded tissue has been employed to study the distribution of negatively charged metabolites, including OAs, in A. thaliana roots using a DMAN matrix (Ye et al , 2013), but this technique does not allow the study of exudates. A recent work by Sasse et al (2019) used a MALDI plate covered with a thin layer of ultra‐pure agarose to recover the exudates from Brachypodium distachyon roots; then they added a mixture of α‐cyano‐4‐hydroxycinnamic acid (CHCA) and 2,3‐dihydroxybenzoic acid (DHB) as a matrix and dried the samples before performing an untargeted MALDI‐MSI analysis that allowed them to identify exudation patterns of unidentified metabolites. Nevertheless, the agarose presents several drawbacks and needs thorough optimization to avoid the presence of bubbles or cracks in the agar or sample flaking during dehydration, which can all cause sample deformation and decreased detection of ions, which would be detrimental to the analysis (Yang et al , 2012).…”

Modeling of Root Nitrate Responses Suggests Preferential Foraging Arises From the Integration of Demand, Supply and Local Presence Signals

“…Next, since in our simplified model all is per unit length, we needed to convert this value to µmol/mm/h. For this we used data from a study on Brachypodium (Sasse et al, 2019), reporting an approximately 1.2:1 ratio between 1 g freshweight and 1 cm root length, resulting in a rounded off 0.6 µmol/mm/h value for up 1 . Similarly, for K up , also depending on study and nitrate transporter studied, values ranging from 6 to 100 µmol/L have been reported (Crawford and Glass, 1998).…”

“…Finally, since we described root growth in terms of length increase, we converted these internal nitrate levels to micromole/mm. For this we again used data from a study on Brachypodium and derived a 1.2:1 ratio between 1 g freshweight and 1 cm root length (Sasse et al, 2019). After this final conversion we obtained root internal nitrate levels of 0.161, 0.151, 0.131, and 0.087 for 11,400, 550, 275, and 110 µmol/L external nitrate, respectively.…”

Modeling of root nitrate responses suggests preferential foraging arises from the integration of demand, supply and local presence signals

“…Next, since in our simplified model all is per unit length, we need to convert this value to micromol/mm/h. For this we used data from a study on Brachypodium (Sasse et al, 2019), reporting an approximately 1.2:1 ratio between 1 g freshweight and 1 cm root length, resulting in a rounded off 0.6 micromole/mm/h value for up 1 . Similarly, for K up , also depending on study and nitrate transporter studied, values ranging from 6 to 100 micromole/L have been reported (Crawford and Glass, 1998), and we again take an intermediate value of 50 micromole/L.…”

“…Finally, since we describe root growth in terms of length increase, we converted these internal nitrate levels to micromole/mm. For this we again used data from a study on Brachypodium reporting both overall root length and root system weight values, from which we derived a 1.2:1 ratio between 1 g freshweight and 1 cm root length (Sasse et al, 2019). After this final conversion we obtained root internal nitrate levels of 0.161, 0.151, 0.131 and 0.087 for 11400, 550, 275 and 110 micromole/L external nitrate respectively.…”