# Beneficial Effects of a Root-Endophytic Bacterium with Quorum-Sensing Traits on Growth and Drought Tolerance in the Vulnerable Conifer Araucaria araucana

## TL;DR

A root-endophytic bacterium helps Araucaria araucana, a vulnerable conifer, grow better and tolerate drought, offering hope for its conservation under climate change.

## Contribution

The study shows a root-endophytic bacterium with quorum-sensing traits enhances drought tolerance in Araucaria araucana.

## Key findings

- Inoculated seedlings under drought had higher biomass, chlorophyll, and water content compared to non-inoculated ones.
- Drought-induced oxidative stress was reduced in inoculated seedlings with lower malondialdehyde and proline levels.
- The bacterium produced AHLs and other growth-promoting substances, improving the plant's redox balance under stress.

## Abstract

Climate change-induced drought threatens the persistence of Araucaria araucana, an endangered and endemic conifer of the Southern Andes. Beneficial plant–microbe interactions may contribute to drought resilience. Here, we evaluated the effects of a root-endophytic bacterium with the capacity to produce N-acyl homoserine lactones (AHLs) on the growth and drought tolerance of A. araucana. For this, a root endophytic bacterium was isolated from A. araucana and identified as Erwinia billingiae. It was characterized for plant growth-promoting traits, and inoculated into A. araucana seedlings under drought conditions). The bacteria produced N-butyryl-L-homoserine lactone (C4-HSL) under control conditions and C4-HSL and N-hexanoyl-L-homoserine lactone (C6-HSL) under drought stress. The strain also produces indoleacetic acid, ammonia, siderophores and solubilizes phosphate. Under drought stress, non-inoculated seedlings showed marked reductions in shoot and root biomass, chlorophyll content, relative water content (RWC), and soluble sugars. In contrast, inoculated seedlings under drought displayed significantly higher shoot and root biomass, reaching levels comparable to those of well-watered controls. Chlorophyll content increased from 5.42 to 9.35 mg L−1, and RWC increased from 62% to 71% in inoculated plants under drought conditions. Soluble sugar content increased from 25.74 to 36.34 mg g−1 fresh weight following inoculation. Drought-induced oxidative stress was significantly alleviated in inoculated seedlings, with lower malondialdehyde and proline accumulation compared to non-inoculated drought-stressed plants. Antioxidant responses were modulated, indicating improved redox balance under water limitation. These results demonstrate that a root-endophytic bacterium with AHL production can enhance drought tolerance in A. araucana seedlings. This study provides novel evidence supporting the role of beneficial endophytes in microbiome-based strategies for conserving native forest species under climate change.