Mycorrhizal and non-mycorrhizal Lactuca sativa plants exhibit contrasting responses to exogenous ABA during drought stress and recovery

Abstract

The arbuscular mycorrhizal (AM) symbiosis enhances plant tolerance to water deficit through the alteration of plant physiology and the expression of plant genes. These changes have been postulated to be caused (among others) by different contents of abscisic acid (ABA) between AM and non-AM plants. However, there are no studies dealing with the effects of exogenous ABA on the expression of stress-related genes and on the physiology of AM plants. The aim of the present study was to evaluate the influence of AM symbiosis and exogenous ABA application on plant development, physiology, and expression of several stress-related genes after both drought and a recovery period. Results show that the application of exogenous ABA had contrasting effects on AM and non-AM plants. Only AM plants fed with exogenous ABA maintained shoot biomass production unaltered by drought stress. The addition of exogenous ABA enhanced considerably the ABA content in shoots of non-AM plants, concomitantly with the expression of the stress marker genes Lsp5cs and Lslea and the gene Lsnced. By contrast, the addition of exogenous ABA decreased the content of ABA in shoots of AM plants and did not produce any further enhancement of the expression of these three genes. AM plants always exhibited higher values of root hydraulic conductivity and reduced transpiration rate under drought stress. From plants subjected to drought, only the AM plants recovered their root hydraulic conductivity completely after the 3 d recovery period. As a whole, the results indicate that AM plants regulate their ABA levels better and faster than non-AM plants, allowing a more adequate balance between leaf transpiration and root water movement during drought and recovery.

Fig. 1.

Percentage of mycorrhizal root length in lettuce plants inoculated with G. intraradices. See legend for Fig. 2.

Fig. 2.

Shoot dry weight (g plant⁻¹) in lettuce plants. Plants were either inoculated with G. intraradices (Gi) or remained non-inoculated (NI). White bars represent plants cultivated under well-watered conditions and black bars represent plants subjected to drought stress. Half of the plants were allowed to grow for an additional 3 d period in order to recover from drought (recovery) and the other half was harvested just after the drought stress period (–). Finally, plants were supplied with exogenous ABA just before and during the drought stress or recovery periods (+ABA) or did not receive ABA (No ABA). Bars represent means plus the standard error (n=5). Means followed by the same letter are not significantly different (P < 0.05) as determined by Duncan's multiple range test.

Fig. 3.

Root dry weight (g plant⁻¹) in lettuce plants. See legend for Fig. 2.

Fig. 4.

Root hydraulic conductivity (L_o; mg H₂O g⁻¹ root DW MPa⁻¹ h⁻¹) in lettuce plants. See legend for Fig. 2.

Fig. 5.

Transpiration rate (mg H₂O cm⁻² h⁻¹) in lettuce plants. See legend for Fig. 2.

Fig. 6.

ABA content (ng ABA g⁻¹ leaf DW) in leaves of lettuce plants. See legend for Fig. 2.

Fig. 7.

ABA content (ng ABA g⁻¹ root DW) in roots of lettuce plants. See legend for Fig. 2.

Fig. 8.

Northern blot of total RNA (15 μg) from lettuce shoots using Lsnced (accession no. AB120109), LsPIP2 (accession no. AJ937963), Lsp5cs (accession no. AJ715852), and Lslea (accession no. AJ704826) gene probes. Treatments are designed as NI, non-inoculated controls or Gi, plants inoculated with G. intraradices. Plants were cultivated under well-watered conditions (ww) or subjected to drought stress (ds) with (+ABA) or without addition of exogenous ABA (No ABA). Half of the plants were allowed to grow for an additional 3 d period in order to recover from drought (ww+R or ds+R) and the other half was harvested just after the drought stress period. The panel under each northern blot shows the amount of 26S rRNA loaded for each treatment. Numbers close to each northern blot represent the relative gene expression after normalization to rRNA.

Fig. 9.

Northern blot of total RNA (15 μg) from lettuce roots. See legend for Fig. 8.