Chemotactic Host-Finding Strategies of Plant Endoparasites and Endophytes

Abstract

Plants interact with microorganisms in the environment during all stages of their development and in most of their organs. These interactions can be either beneficial or detrimental for the plant and may be transient or long-term. In extreme cases, microorganisms become endoparastic or endophytic and permanently reside within a plant, while the host plant undergoes developmental reprogramming and produces new tissues or organs as a response to the invasion. Events at the cellular and molecular level following infection have been extensively described, however the mechanisms of how these microorganisms locate their plant hosts via chemotaxis remain largely unknown. In this review, we summarize recent findings concerning the signalling molecules that regulate chemotaxis of endoparasitic/endophytic bacteria, fungi, and nematodes. In particular, we will focus on the molecules secreted by plants that are most likely to act as guidance cues for microorganisms. These compounds are found in a wide range of plant species and show a variety of secondary effects. Interestingly, these compounds show different attraction potencies depending on the species of the invading organism, suggesting that cues perceived in the soil may be more complex than anticipated. However, what the cognate receptors are for these attractants, as well as the mechanism of how these attractants influence these organisms, remain important outstanding questions. Host-targeting marks the first step of plant-microorganism interactions, therefore understanding the signalling molecules involved in this step plays a key role in understanding these interactions as a whole.

Differential Root Exudation and Architecture for Improved Growth of Wheat Mediated by Phosphate Solubilizing Bacteria

“…Furthermore, HPLC analysis revealed a significant increase of organic acids specifically gluconic acid (20-39%), malic acid (30%), and citric acid (21%-41%) released in exudates collected from PSB-inoculated three wheat varieties as compared to their respective uninoculated controls (Figure 5). Increased concentrations of oxalic, malic and citric acids in root exudates act as chemo-attractants for soil microbes during beneficial interactions (Pascale et al, 2020;Tsai et al, 2020).…”

Root-knot nematode chemotaxis is positively regulated by l -galactose sidechains of mucilage carbohydrate rhamnogalacturonan-I

“…In addition, l-Gal has also been detected in N-glycan, RG-II, and xyloglucan from the plant cell wall, although it is unclear whether they are present in a sufficiently high level to mediate signaling (37)(38)(39). However, it is thought that plant pathogens may rely on multiple attractants for host targeting; thus, it is feasible for one of the attractants to be of low abundance (3,14). RKNs have indeed been documented to infect flax plants (40).…”

“…The search for RKN chemoattractants has been an ongoing endeavor. Root exudates from soy, tomato, Medicago, and pea are known to attract RKN (3,(8)(9)(10). Specifically, plant metabolites including phytohormones (salicylic acid, gibberellic acid, and indole-3-acetic acid), amino acids (arginine and lysine), organic acids (vanillic acid, tannic acid, and lauric acid), and other compounds (organic diamines, mannitol, and flavonoids) have been demonstrated to attract RKN (11)(12)(13).…”

“…Chemotaxis refers to the movements of organisms along a chemical gradient. This mechanism acts as the foundation for animal behaviors such as foraging, courtship, and predator avoidance and is particularly crucial for parasites searching for a host to infect (1)(2)(3). The chemical structures and properties of chemoattractants strongly influence the behaviors of their recipients.…”

Molecular Host-Nematode Interactions and Tuber Development

“…Sucrose, glucose, arabinose, galactose, and mannitol are chemo-attractants of Meloidogyne incognita, and signal transduction may involve Mi-odr-1, Mi-odr-3, Mi-tax-4 and Mi-tax-2genes [34][35][36]. Vanillic acid, lauric acid (signal transduction may require Mi-odr-1, Mi-odr-3, Mi-tax-2 and Mi-tax-4 genes) [34][35][36][37], arginine, lysine [34][35][36] and calcium chloride [35,38], Mi-odr-3, Mi-tax-2, Mi-tax-4 genes are chemotactic genes involve in Meloidogyne incognita and predicted functions are membrane-bound guanylyl cyclase that produces secondary messenger, α protein that regulates cyclic nucleotide metabolism, subunits of cyclic nucleotide-gated cation channel involved in G-protein-mediated signalling, respectively [35,36]. Carbon dioxide (CO2) is an important attractant released by roots for RKNs [39], and lauric acid controls the chemotaxis of root-knot nematodes [37].…”