Plant Growth Promotion, Phytohormone Production and Genomics of the Rhizosphere-Associated Microalga, Micractinium rhizosphaerae sp. nov

Abstract

Microalgae are important members of the soil and plant microbiomes, playing key roles in the maintenance of soil and plant health as well as in the promotion of plant growth. However, not much is understood regarding the potential of different microalgae strains in augmenting plant growth, or the mechanisms involved in such activities. In this work, the functional and genomic characterization of strain NFX-FRZ, a eukaryotic microalga belonging to the Micractinium genus that was isolated from the rhizosphere of a plant growing in a natural environment in Portugal, is presented and analyzed. The results obtained demonstrate that strain NFX-FRZ (i) belongs to a novel species, termed Micractinium rhizosphaerae sp. nov.; (ii) can effectively bind to tomato plant tissues and promote its growth; (iii) can synthesize a wide range of plant growth-promoting compounds, including phytohormones such as indole-3-acetic acid, salicylic acid, jasmonic acid and abscisic acid; and (iv) contains multiple genes involved in phytohormone biosynthesis and signaling. This study provides new insights regarding the relevance of eukaryotic microalgae as plant growth-promoting agents and helps to build a foundation for future studies regarding the origin and evolution of phytohormone biosynthesis and signaling, as well as other plant colonization and plant growth-promoting mechanisms in soil/plant-associated Micractinium.

Keywords: Micractinium; genomics; microalgae; phytohormones; plant growth; rhizosphere.

Figure 1

Morphological characteristics of strain NFX-FRZ cultivated in algae culture broth.

Figure 2

Phylogram based on Micractinium 18S-ITS1-5.8S-ITS2 regions (2477 bp). The evolutionary history was inferred by using the maximum likelihood method and Tamura-Nei model. The tree with the highest log likelihood (−5991.31) is shown. The percentage of trees in which the associated taxa clustered together is shown next to the branches. A discrete gamma distribution was used to model evolutionary rate differences among sites (5 categories (+G, parameter = 0.2992)). The rate variation model allowed for some sites to be evolutionarily invariable ([+I], 62.93% sites). The tree is drawn to scale, with branch lengths measured in the number of substitutions per site.

Figure 3

Micractinium rhizosphaerae NFX-FRZ growth dynamics when cultivated in algae culture broth (1.5×) for 9 days.

Figure 4

Results obtained from the tomato plant growth promotion assays. Different letters above the bars represent statistical differences (p < 0.05). WA—water agar; NFX-FRZ_EA—NFX-FRZ exudates agar; ACA—algae culture agar (1.5X algae culture broth + 0.5% Agar); H2A—Hoagland’s No. 2 basal salt agar.

Figure 5

Microscopy observations of the tomato root colonization abilities of Micractinium rhizosphaerae NFX-FRZ. Arrows indicate the presence of the NFX-FRZ strain. (A) overview of the primary root tissue; (B) NFX-FRZ cells attached to plant root hairs; (C) clusters of NFX-FRZ cells bound to plant root hairs.