Tomato ethylene sensitivity determines interaction with plant growth-promoting bacteria

Abstract

Background and aims: Plant growth-promoting bacteria (PGPB) are soil micro-organisms able to interact with plants and stimulate their growth, positively affecting plant physiology and development. Although ethylene plays a key role in plant growth, little is known about the involvement of ethylene sensitivity in bacterial inoculation effects on plant physiology. Thus, the present study was pursued to establish whether ethylene perception is critical for plant-bacteria interaction and growth induction by two different PGPB strains, and to assess the physiological effects of these strains in juvenile and mature tomato ( Solanum lycopersicum ) plants.

Methods: An experiment was performed with the ethylene-insensitive tomato never ripe and its isogenic wild-type line in which these two strains were inoculated with either Bacillus megaterium or Enterobacter sp. C7. Plants were grown until juvenile and mature stages, when biomass, stomatal conductance, photosynthesis as well as nutritional, hormonal and metabolic statuses were analysed.

Key results: Bacillus megaterium promoted growth only in mature wild type plants. However, Enterobacter C7 PGPB activity affected both wild-type and never ripe plants. Furthermore, PGPB inoculation affected physiological parameters and root metabolite levels in juvenile plants; meanwhile plant nutrition was highly dependent on ethylene sensitivity and was altered at the mature stage. Bacillus megaterium inoculation improved carbon assimilation in wild-type plants. However, insensitivity to ethylene compromised B. megaterium PGPB activity, affecting photosynthetic efficiency, plant nutrition and the root sugar content. Nevertheless, Enterobacter C7 inoculation modified the root amino acid content in addition to stomatal conductance and plant nutrition.

Conclusions: Insensitivity to ethylene severely impaired B. megaterium interaction with tomato plants, resulting in physiological modifications and loss of PGPB activity. In contrast, Enterobacter C7 inoculation stimulated growth independently of ethylene perception and improved nitrogen assimilation in ethylene-insensitive plants. Thus, ethylene sensitivity is a determinant for B. megaterium , but is not involved in Enterobacter C7 PGPB activity.

Keywords: Bacillus megaterium; Enterobacter; Solanum lycorpersicum (tomato); ethylene; plant growth-promoting bacteria (PGPB); plant nutrition.

Fig. 1.

Effects of bacterial inoculation on plant dry weights at 4 and 8 weeks post-inoculation (wpi). Total (A), shoot (C) and root (E) dry weights of wild-type ‘Pearson’ (wt) and never ripe (nr) tomato (Solanum lycopersicum) plants at 4 wpi. Total (B), shoot (D) and root (F) dry weights of wt and nr tomato plants at 8 wpi. Treatments are designed as non-inoculated controls (No, black bars), Bacillus megaterium-inoculated plants (Bm, white bars), and Enterobacter C7-inoculated plants (C7, grey bars). Data are means ± s.e. (n = 9). Data were analysed by two-way ANOVA, with plant genotype (G) and inoculum (I) as sources of variation. Significance of sources of variation as well as their interaction (G × I) were evaluated by P-value; n.s., not significant; *P ≤ 0·05; **P ≤ 0·01; ***P ≤ 0·001. In the case of a significant interaction between factors, all treatments were compared with each other. In the case of a non-significant interaction between factors, inoculum effects were evaluated, analysing wt and nr plants separately using ANOVA. Means followed by different lower case letters are significantly different (P < 0·05) according to LSD test. Plant genotype effect was evaluated by analysing wt and nr plants under the same inoculation treatment (No, Bm or C7) by Student t-test, and a significant difference (P < 0·05) is shown as (*) above nr means.

Fig. 2.

Effects of bacterial inoculation on relative growth rates (RGRs). Total (A), shoot (C) and root (E) RGRs of wild-type ‘Pearson’(wt) and never ripe (nr) tomato (Solanum lycopersicum) plants at 8 weeks post-inoculation (wpi). Treatments are designed as non-inoculated controls (No, black bars), Bacillus megaterium-inoculated plants (Bm, white bars) and Enterobacter C7-inoculated plants (C7, grey bars). Data are means ± s.e. (n = 9). Data were analysed by two-way ANOVA with plant genotype (G) and inoculum (I) as sources of variation. Significance of sources of variation as well as their interaction (G × I) was evaluated by P-value; n.s., not significant; *P ≤ 0·05; **P ≤ 0·01; ***P ≤ 0·001. In the case of a significant interaction between factors, all treatments were compared with each other. Means followed by different upper case letters are significantly different (P < 0·05) according to LSD test. In the case of a non-significant interaction between factors, inoculum effects were evaluated by analysing wt and nr plants separately using ANOVA. Means followed by different lower case letters are significantly different (P < 0·05) according to LSD test. Plant genotype effect was evaluated by analysing wt and nr plants under the same inoculation treatment (No, Bm or C7) by Student t-test, and a significant difference (P < 0·05) is shown as (*) above nr means.

Fig. 3.

Principal component analysis (PCA) of root nutrient concentrations and dry weights. Analyses were performed based on nutrient concentration and dry weight data obtained from tomato (Solanum lycopersicum) plants. Score plot at 4 weeks post-inoculation (wpi) (A) and 8 wpi (C). Treatments: non-inoculated, Bacillus megaterium-inoculated and Enterobacter C7-inoculated wild-type plants (wt No, wt Bm and wt C7, respectively) and non-inoculated, Bacillus megaterium-inoculated and Enterobacter C7-inoculated never ripe plants (nr No, nr Bm and nr C7, respectively). Each point represents one plant, and points of the same treatment are enclosed in a different coloured ellipse: green for wt No, grey for wt Bm, purple for wt C7, red for nr No, yellow for nr Bm and blue for nr C7. Loading plot at 4 wpi (B) and 8 wpi (D). Each point represents one nutrient or dry weight.

Fig. 4.

Principal component analysis (PCA) of root metabolite levels and dry weights at 4 weeks post-inoculation (wpi). Analyses were performed based on metabolite contents and dry weight data obtained from tomato (Solanum lycopersicum) roots. (A) Score plot. Treatments: non-inoculated, Bacillus megaoterium-inoculated and Enterobacter C7-inoculated wild-type plants (wt No, wt Bm and wt C7, respectively) and non-inoculated, Bacillus megaterium-inoculated and Enterobacter C7-inoculated never ripe plants (nr No, nr Bm and nr C7, respectively). Each point represents one plant, and points of the same treatment are enclosed in a different coloured ellipse: green for wt No, grey for wt Bm, purple for wt C7, red for nr No, yellow for nr Bm and blue for nr C7. (B) Loading plot. Each point represents one nutrient or dry weight.

Fig. 5.

Principal component analysis (PCA) of root metabolite levels and dry weights at 8 weeks post-inoculation (wpi). Analyses were performed based on metabolite contents and dry weight data obtained from tomato (Solanum lycopersicum) roots. (A) Score plot. Treatments: non-inoculated, Bacillus megaoterium-inoculated and Enterobacter C7-inoculated wild-type plants (wt No, wt Bm and wt C7, respectively) and non-inoculated, Bacillus megaterium-inoculated and Enterobacter C7-inoculated never ripe plants (nr No, nr Bm and nr C7, respectively). Each point represents one plant, and points of the same treatment are enclosed in a different coloured ellipse: green for wt No, grey for wt Bm, purple for wt C7, red for nr No, yellow for nr Bm and blue for nr C7. (B) Loading plot. Each point represents one nutrient or dry weight.

Fig. 6.

Summary of plant growth-promoting bacteria (PGPB) inoculation effects on wild-type and never ripe tomato (Solanum lycopersicum) plants at 4 and 8 weeks post-inoculation (wpi). Increases or decreases in measured parameters produced by PGPB inoculation are shown with ↑ and ↓ symbols, respectively. Changes induced by PGPB inoculation are shown in black and red letters for common and specific effects, respectively. Bm, Bacillus megaterium; C7, Enterobacter C7; d. wt, dry weight; RGR, relative growth rate; g_s, stomatal conductance; ΔF_v/F_m′, photosynthetic efficiency; ABA, abscisic acid; C, carbon; N, nitrogen; Mn, manganese; Ca, calcium; Fe, iron; Cu, copper; Zn, zinc; Na, sodium; Mg, magnesium; Si, silicon; S, sulphur.