Analysis of global transcriptomic response of lettuce seedlings (Lactuca sativa L.) to microalgae extract used as biostimulant agents

The use of new natural eco-sustainable products is becoming an interesting option in order to reduce the use of chemical fertilizers and increase crop yields. Seaweed extracts are gained major attention as plant biostimulants due to their positive effect on plant-growth as well as on improving plants' tolerance against abiotic stresses. Among the horticulture crops, lettuce (Lactuca sativa L.) is a major fresh vegetable crop in the Mediterranean area, which often requires the use of natural biostimulants to improve both the quantity and quality of production. The aim of this work was to assess the effect of either Chlorella vulgaris or Scenedesmus quadricauda extracts on lettuce seedlings (Lactuca sativa L.) by motoring the induced transcriptomic modifications using a RNASeq approach. The results showed that both C. vulgaris and S. quadricauda extracts positively influence the growth of lettuce seedlings. However, a higher reprogramming of the gene expression occurred in the case of C. vulgaris treatment than in S. quadricauda extract. Considering the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway functional enrichments, the main KEGG terms are in the “Biosynthesis of secondary metabolites”, “Metabolic pathways”, “Carbon metabolism” and “Biosynthesis of amino acids” categories. This study lays the basis for understanding the mechanisms and processes triggered by the use of microalgal extracts, which can represent an easier-to-handle and cheaper method for an eco-sustainable cultivation of lettuce plants than the application of chemicals. Overall design: Lettuce seedlings (Lactuca sativa L.) were provided by a local nursery and 10 seedlings at the “four true leaves” stage were transplanted in each box in a completely random design, performing five biological replicates for treatments. The seedlings were grown for 6 days in a growth chamber at 25 ± 2°C, with a 16-h photoperiod and they were irrigated every day with 100 mL distilled water. After this period of acclimatization (6 days), the treatment was performed by irrigating the inert substrate with Hoagland solution (500 mL) containing either Chlorella vulgaris (LsCv sample) or Scenedesmus quadricauda (LsSq sample) extracts at the concentration of 1 mg of organic carbon per liter (Corg L-1), whereas the untreated plants (LsCK) received only 500 mL of Hoagland solution.