Leaf Treatments with a Protein-Based Resistance Inducer Partially Modify Phyllosphere Microbial Communities of Grapevine

Abstract

Protein derivatives and carbohydrates can stimulate plant growth, increase stress tolerance, and activate plant defense mechanisms. However, these molecules can also act as a nutritional substrate for microbial communities living on the plant phyllosphere and possibly affect their biocontrol activity against pathogens. We investigated the mechanisms of action of a protein derivative (nutrient broth, NB) against grapevine downy mildew, specifically focusing on the effects of foliar treatments on plant defense stimulation and on the composition and biocontrol features of the phyllosphere microbial populations. NB reduced downy mildew symptoms and induced the expression of defense-related genes in greenhouse- and in vitro-grown plants, indicating the activation of grapevine resistance mechanisms. Furthermore, NB increased the number of culturable phyllosphere bacteria and altered the composition of bacterial and fungal populations on leaves of greenhouse-grown plants. Although, NB-induced changes on microbial populations were affected by the structure of indigenous communities originally residing on grapevine leaves, degrees of disease reduction and defense gene modulation were consistent among the experiments. Thus, modifications in the structure of phyllosphere populations caused by NB application could partially contribute to downy mildew control by competition for space or other biocontrol strategies. Particularly, changes in the abundance of phyllosphere microorganisms may provide a contribution to resistance induction, partially affecting the hormone-mediated signaling pathways involved. Modifying phyllosphere populations by increasing natural biocontrol agents with the application of selected nutritional factors can open new opportunities in terms of sustainable plant protection strategies.

Grapevine Phyllosphere Community Analysis in Response to Elicitor Application against Powdery Mildew

“…In grapevine, the role and composition of leaf microbial communities were largely ignored until recently, when the concept of microbial terroir started to spread [40,41]. For example, the grapevine can potentially host more than 80 different viral species, of which only about half are recognized as agents of economic relevant diseases [42][43][44]. For most of these viral entities, the effects and metabolic modifications caused in grapevine are still unknown.…”

“…For most of these viral entities, the effects and metabolic modifications caused in grapevine are still unknown. However, mutualistic interactions have been reported, in which some viruses conferred drought tolerance and increased resistance to downy mildew (Plasmopara viticola) [30,43,44]. Thus, grapevine-virus interactions can be studied not only according to classical host-pathogen relationships but also by considering grapevine and its viruses as a single micro-ecosystem able to adapt to the surrounding environment and to influence plant responses, including those to fungi and fungicides.…”

The Effect of Hydrolysis and Protein Source on the Efficacy of Protein Hydrolysates as Plant Resistance Inducers against Powdery Mildew

“…Likewise, artificial protein hydrolysates were demonstrated to act as resistance inducers [25][26][27] and their biocontrol activity was affected by the original protein source, hydrolysis method and degree of hydrolysis [20,28], as well as by their biochemical properties [18,56]. On the basis of these findings, we investigated and compared the impact of different plant protein sources and different hydrolysis methods on the efficacy of protein hydrolysates in controlling courgette powdery mildew, in order to clarify the possible role of specific peptide fragments and amino acids Our results showed that both enzymatic and acid methods significantly enhanced the efficacy of guar against courgette powdery mildew, in particular when hydrolysis was carried out with Alcalase 50% and with H 2 SO 4 6N condition B, respectively.…”

“…In addition, guar enzymatic hydrolysates did not affect conidia germination on leaf disks, suggesting a mode of action mainly based on the stimulation of plant resistance mechanisms, as observed for other protein extracts. Indeed, peptide fragments of different origin have been shown to elicit grapevine defence mechanisms by the up-regulation of defence-related genes [24][25][26][27]. However, further analysis will be required to demonstrate how these peptide sequences could be involved in plant immunity regulation.…”

“…For example, casein and soybean hydrolysates have been shown to elicit grapevine defence mechanisms against downy mildew and grey mould by the up-regulation of PR genes [24,25]. Likewise, a protein derivative was effective in controlling courgette powdery mildew and grapevine [26], and activated the expression of defence-related genes in grapevine, suggesting the stimulation of plant resistance mechanisms [27]. In addition, the characteristics and functional properties of hydrolysates are influenced by the method used for hydrolysis and by the choice of the original protein source [28].…”