Background: Beneficial microorganisms modify root architecture through different mechanisms mostly related to plant hormones synthesis. Less information is available regarding the emission of microbial volatile organic compounds (VOCs) and their effects on root architecture. Numerous studies describe the release of VOCs by bacteria and filamentous fungi, but little information is available regarding soil yeasts. Methods: The VOCs released by the soil yeast Solicoccozyma terrea were evaluated in an experiment with tomato seedlings. Four treatments (tomato seedlings, T; S. terrea inoculum without tomato seedlings, St; tomato seedlings exposed to the volatiles of S. terrea inoculum, TSt; and a control without tomato seedlings or yeast inoculum, C) were assessed in terms of root morphology. VOCs were analyzed by GC–MS, and in a second experiment, the response of tomato seedlings and root nitrate uptake transporters to specific compounds identified in the first experiment was evaluated under low and high N supply. Results: A mixture with 57 VOCs was identified which promoted lateral roots formation in tomato plants. Three compounds (isopropyl alcohol, 2-pentylfuran and trans-1,3-pentadiene) were exclusively present in the TSt treatment. All three compounds stimulated lateral roots formation and increased root volume. These compounds showed a stimulatory effect on the expression of NRT1.2 , NRT2.1 and NRT2.3 nitrate transporters under N deficiency conditions. Conclusions: This study provides, for the first time, information on a soil yeast capable of promoting lateral roots formation in tomato through VOCs and that isopropyl alcohol, 2-pentylfuran and trans-1,3-pentadiene modulate root nitrate transporters expression under N deficiency conditions.

Beneficial microorganisms promote plant growth through different mechanisms, such as the production of plant hormones, the synthesis of 1-aminocyclopropane-1-carboxylic acid deaminase (ACCD), or, through volatile organic compounds (VOCs) emission. There are numerous studies describing a biostimulant action mediated by VOCs released by bacteria and filamentous fungi. However, in soil yeasts, research in this regard is still incipient. In a previous study, we present the stimulatory action of Solicoccozyma aeria YCPUC79 on tomato seedling growth by inoculating the rootzone with this yeast. The positive effect of S. aeria was related to the synthesis of indole acetic acid (IAA) and the presence of ACCD activity by the yeast. In the present study, we evaluated whether S. aeria is capable of emitting VOCs with biostimulant activity. For this, an experiment was conducted to test the release of VOCs in four treatments: S. aeria (Sa), tomato seedlings (T), tomato seedlings sharing the ambient with S. aeria but with no physical contact (TSa) plus a control with no yeast nor tomato seedlings (C). Tomato seedlings exposed to S. aeria in-oculum (TSa) presented 1.2-fold shorter main roots but increased the number of lateral roots by 80% compared to T. Regarding the analysis of VOCs, 59 compounds were identified excluding those found in the control treatment. These compounds represent twelve chemical families, in-cluding alcohols, esters, furans, hydrocarbons, ketones and terpenes. The treatment TSa shows an increased abundance of ketones, alcohols, esters, sulfur-containing compounds, and pyrazines in comparison to T treatment. Three compounds (butyl hept-4-yl ester-phthalic acid, (E)1.3-pentadiene and 1-propenylthiol) were exclusively present in the TSa treatment. This study provides, for the first time, information on a soil yeast capable of promoting the production of lateral roots in tomato through VOCs.