Are drought-resistance promoting bacteria cross-compatible with different plant models?

Abstract

The association between plant and plant growth promoting bacteria (PGPB) contributes to the successful thriving of plants in extreme environments featured by water shortage. We have recently shown that, with respect to the non-cultivated desert soil, the rhizosphere of pepper plants cultivated under desert farming hosts PGPB communities that are endowed with a large portfolio of PGP traits. Pepper plants exposed to bacterial isolates from plants cultivated under desert farming exhibited a higher tolerance to water shortage, compared with untreated control. This promotion was mediated by a larger root system (up to 40%), stimulated by the bacteria, that enhanced plant ability to uptake water from dry soil. We provide initial evidence that the nature of the interaction can have a limited level of specificity and that PGPB isolates may determine resistance to water stress in plants others than the one of the original isolation. It is apparent that, in relation to plant resistance to water stress, a feature of primary evolutionary importance for all plants, a cross-compatibility between PGPB and different plant models exists at least on a short-term.

Keywords: 1-aminocyclopropane-1-carboxylic acid deaminase; ACCd; CS; Compatible Solutes; IAA; ROS; Reactive Oxygen Species; arid ecosystem; auxins; drought tolerance; endosphere; plant growth promoting bacteria; plant-bacteria cross-compatibility; rhizosphere; water stress.

Figure 1. PGP strains have a cross-efficacy in promoting drought resistance in different plant models. (A) Representative images of the promotion effect on the root system of pepper plants inoculated with R05ACCd and R16ACCd strains and field-grown for 2 months under irrigation-limited conditions, compared with the uninoculated control (labeled with “no bacteria added”). Bar = 10 cm. (B) Pepper root fresh weights determined on the plants of the experiment in (A). Values are means of 4 plants ± standard deviations. The P values according to the student t-test are indicated to evaluate the differences between the plants treated with bacteria and the non-treated controls. (C) B-O strain, isolated from the rhizosphere of an olive tree growing in South Tunisia, was used to inoculate tomato plants (3 plants for each treatment). After re-watering, following a 10-d induced drought by withholding irrigation, tomato plants treated with B-O strain exhibited an increase of root fresh biomass compared to untreated control. (+): uninoculated plants, properly supplied with water during the experiment; (-): uninoculated control plants subjected to drought; (B-O): plants treated with B-O isolate and subjected to water stress. (D) B-G strain, isolated from the root endosphere of Barbera plants, was used to inoculate SO4-grafted grapevine plants (5 plantlets for each treatment). After re-watering, following a drought event induced by reducing water irrigation to 50% of water holding capacity, grapevine plants treated with B-G strain exhibited an increase of grapevine fresh biomass compared with untreated control. (+): uninoculated plants, properly supplied with water during the experiment; (-): uninoculated control plants subjected to drought; (B-G): plants treated with B-G isolate and subjected to water stress. *: P ≤ 0.05 according to the student t-test.

Figure 2. PGPB associated to plants stimulate growth during drought. Physiological plant processes (gray arrows) and mechanisms (white arrows) change according to stimulations determined by the associated PGPB (in blue in the plant drawing) in response to drought. On the left and the right sides of the plant drawing are reported the intensities of processes and mechanisms under water stress, in the absence or presence of the PGPB. For each process and mechanism represented in the same line, the lengths of the arrows indicate reduced/increased physiological responses. The black boxes indicate the specific promoting activities exerted by the PGPB in relation to the specific processes or mechanisms.