Germination screen for microalgae-generated plant growth biostimulants

Abstract

Greenhouse (controlled environments) horticulture is expected to rise to ~USD 30 billion by 2023. In such climate-controlled systems, crop yields can increase ~10-fold compared with field production, due to higher water and nutrient efficiency and reduced external environmental impacts. The application of plant growth regulators (PGRs) can promote seed germination and plant development, speed up the production cycle and increase production efficiency which is of particular significance for high-value crops produced in controlled environments. Here, a novel in-vivo germination and plant growth assay was developed to rapidly identify microalgae strains that naturally produce PGRs was developed using spinach (Spinacia oleracea L) as test crop. Four newly isolated, fast growing microalgae strains in the genera Chlorococcum, Micractinium, Scenedesmus and Chlorella were screened to evaluate their potential as reliable and cost-effective sources of PGRs. Spinach seeds were primed with soluble and insoluble microalgae fractions from whole-cell and lysed-cell extracts and compared to controls (water and commercial seaweed extract). After a 5-day incubation, algae extracts improved seed germination up to 1.7-fold and 1.6-fold compared to water and seaweed extract, respectively. Green cotyledon emergence improved up to 2-fold (day 6), while seedling biomass improved up to 2.1-fold and 1.9-fold (day 9) compared to water and seaweed extract, respectively. The quantification of cytokinins (trans-Zeatin, DHZR, tZMP, iP, iPA, iPAMP), gibberellins (GA1, GA3, GA4, GA20 and GA29), auxin (IAA) and abscisic acid (ABA) validated that the four uncharacterized microalgal strains produced relevant PGRs. To boost plant propagation, PGR-producing microalgae may offer natural and cost-effective alternative biostimulants to synthetically produced agrochemicals.