Regulation of a Chemical Defense against Herbivory Produced by Symbiotic Fungi in Grass Plants      

Abstract

Neotyphodium uncinatum and Neotyphodium siegelii are fungal symbionts (endophytes) of meadow fescue (MF; Lolium pratense), which they protect from insects by producing loline alkaloids. High levels of lolines are produced following insect damage or mock herbivory (clipping). Although loline alkaloid levels were greatly elevated in regrowth after clipping, loline-alkaloid biosynthesis (LOL) gene expression in regrowth and basal tissues was similar to unclipped controls. The dramatic increase of lolines in regrowth reflected the much higher concentrations in young (center) versus older (outer) leaf blades, so LOL gene expression was compared in these tissues. In MF-N. siegelii, LOL gene expression was similar in younger and older leaf blades, whereas expression of N. uncinatum LOL genes and some associated biosynthesis genes was higher in younger than older leaf blades. Because lolines are derived from amino acids that are mobilized to new growth, we tested the amino acid levels in center and outer leaf blades. Younger leaf blades of aposymbiotic plants (no endophyte present) had significantly higher levels of asparagine and sometimes glutamine compared to older leaf blades. The amino acid levels were much lower in MF-N. siegelii and MF-N. uncinatum compared to aposymbiotic plants and MF with Epichloë festucae (a closely related symbiont), which lacked lolines. We conclude that loline alkaloid production in young tissue depleted these amino acid pools and was apparently regulated by availability of the amino acid substrates. As a result, lolines maximally protect young host tissues in a fashion similar to endogenous plant metabolites that conform to optimal defense theory.

Genotypic and Chemotypic Diversity of Neotyphodium Endophytes in Tall Fescue from Greece

“…4B) as well as pathway intermediates N 1 -acetylnorloline (NANL), N 1 -acetylloline (NAL), N 1 -methylloline (NML). The total loline concentrations detected for all lines were comparable to levels reported previously for other Neotyphodium species (69). Consistent with the LTM gene profiles of the G2 and G3 isolates (Table 1), lolitrem B and terpendole intermediates were detected in G2-infected plant lines, whereas only terpendole intermediates, such as terpendole C, were observed in the G3-infected plants (Fig.…”

Fungal grass endophytes and arthropod communities: lessons from plant defence theory and multitrophic interactions

“…Endophytic alkaloids can also be induced when plants are under attack, at least in agronomic grasses (e.g., Bultmanet al 2004). Like constitutive defences, the genetic bases for these 'on-demand' defences, particularly for alkaloids, are well-established for both plants (e.g., Anssour & Baldwin 2010) and for endophytic fungi (Sullivan et al, 2007 andZhang et al, 2009). So, again PDT includes induced responses and applies directly to endophyte-mediated resistance to herbivores.…”

Measurement of 13C and 15N isotope labeling by gas chromatography/combustion/isotope ratio mass spectrometry to study amino acid fluxes in a plant–microbe symbiotic association

“…metabolite profiling [2] ) has been proposed as a methodological approach for understanding responses to physiological or environmental changes of biological systems. [3] Amino acids are the most important targets for metabolic profiling because they are involved in the synthesis of protein, secondary metabolism, plant defense, [4] osmotic adjustment [5] and the synthesis of cell components. [6] Due to the key role of amino acids in plants, precise and reproducible methods are required for their analysis.…”