Biostimulant activity of Galaxaura rugosa seaweed extracts against water deficit stress in tomato seedlings involves activation of ABA signaling

Abstract

Water scarcity is a serious constraint for agriculture, and global warming and climate change can exacerbate it in many areas. Therefore, sustainable approaches must be implemented to deal with current and future water scarcity scenarios. Genetic and chemical approaches are being applied to manage this limitation and maintain crop yields. In particular, biostimulants obtained from natural sources such as marine algae are promising aids for coping with water deficit stress in agriculture. Here we present a bioprospection study of extracts of the macroalgae Bonnemaisonia hamifera, Galaxaura rugosa, Dasycladus vermicularis, Ulva clathrata, Cystoseira foeniculacea, Cystoseira humilis, Lobophora dagamae, Colpomenia sinuosa and Halopteris scoparia from the north coast of Tenerife, in the Canary Islands. The aqueous extracts of Bonnemaisonia hamifera, Galaxaura rugosa, Dasycladus vermicularis and Cystoseira humilis show biostimulant activity against water deficit stress in tomato seedlings under controlled conditions, providing higher tolerance than the mock-treated control. The Galaxaura rugosa extract showed the highest biostimulant activity against water deficit stress. We demonstrate that this positive effect involves the activation of the abscisic acid (ABA) pathway in Arabidopsis thaliana (arabidopsis) and Solanum lycopersicum (tomato). Application of G. rugosa extract to the root system by drenching tomato seedlings subjected to water deficit leads to improved CO₂ assimilation and water use efficiency (WUEp), compared to mock-treated plants. These results highlight a new potential seaweed source of substances with osmoprotectant properties, useful for biostimulant development. Future studies may provide further insight into which components of the seaweed extract induce activation of the ABA pathway.

Keywords: absicic acid signaling; bioactive natural products; biostimulants for agriculture; drought; seaweed; water deficit.

Figure 1

Treatments and experimental set-up. WW and WD mean well-watered and water deficit treatments, respectively. A-WW and A-WD mean G. rugosa treatment in well-watered and G. rugosa treatment in water deficit conditions, respectively.

Figure 2

Dry weights of tomato plants treated with different algal extracts under water-deficit stress conditions. Blue and red dashed lines represent WW and WD dry weight average respectively. ⁺ and * means significant differences at p< 0.05 with respect to WW and WD, respectively.

Figure 3

Dry weights of tomato plants treated with G. rugosa extract under well-watered and water-deficit stress conditions. (A) Dry weights of tomato plants treated with different G. rugosa extracts under water-deficit conditions. (B) Blue and red dashed lines represent WW and WD dry weight average respectively. ⁺ and * means significant differences at p< 0.05 with respect to WW and WD respectively. 1-3 means G. rugosa extracts from different seasons.

Figure 4

Plant gas exchange measurements and CO₂ assimilation during the experiment. (A) Transpiration (E). (B) Stomatal conductance (gs). (C) Net photosynthesis (Pn). Bars labeled with letters indicate significant differences at p<0.05. Bars labeled by the same letter did not show significant differences at p<0.05.

Figure 5

Enhanced WUE and CO₂ assimilation ratio by G. rugosa treatment under WD conditions. (A) Instantaneous water use efficiency, Pn/E. (B) Intrinsic water use efficiency, Pn/gs. (C) Pn/Ci index. Bars labeled with the same letter did not show significant differences at p<0.05. Ratios and standard deviations were obtained from values reported in Figure 3 .

Figure 6

Induction of ABA-responsive genes in Arabidopsis thaliana and tomato by the G. rugosa extract. (A) Incubation with G. rugosa extract induces expression of luciferase (LUC) in the pMAPKKK18 LUC reporter line. Seedlings of the Arabidopsis reporter line were mock-treated with 25 μM ABA or 0.5% G. rugosa extract in 24-well plates and imaged with a charge-coupled device (CCD) camera to detect luminescence 24 h later. Quantification of luminescence is shown in the right panel and expressed as arbitrary units (a.u.). Asterisks indicate statistical significance (p<0.01). (B) The G. rugosa extract upregulates the expression of ABA-responsive genes in tomato. Ten-day-old tomato seedlings were treated with either mock or 0.5% G. rugosa extract for 6h. Histograms indicate the relative induction of the SlRAB18 namely Sl02g084850 and SlLEA namely Sl06g067980 genes by the seaweed treatment with respect to mock treatment (value 1). Expression of SlEF1a was used to normalize the expression of ABA-responsive genes.