Auxin producing plant growth promoting bacteria enhance temperature stress tolerance in Chilean common bean landraces

Abstract

Current climate change challenges, including rising temperatures, reduced water availability, and increased soil salinity, pose severe threats to global agricultural productivity. While plant growth promoting bacteria (PGPB) have been studied for their role in stress mitigation, their application in enhancing heat tolerance in Chilean landraces of common bean (Phaseolus vulgaris L.) remains largely unexplored. This study evaluated the interaction between native PGPB and genetically distinct Chilean common bean landraces, which are part of the Andean gene pool and represent a socio-cultural food heritage. Auxin production and root adhesion capacity were first validated for two native Bacillus strains. Subsequently, their effects on heat-stressed plants were assessed at physiological and biochemical markers associated with thermal stress tolerance. Although PGPB inoculation improved certain stress-related responses at 30-35 °C in the Tórtola and Sapito landraces, growth responses at 40 °C in Mantequilla were observed even in control plants, indicating an inherent thermotolerance rather than a treatment-specific effect. Furthermore, although individual PGPB strains positively influenced traits such as root development and oxidative stress mitigation, the bacterial consortium did not show additive or synergistic effects under the tested conditions. These findings highlight the importance of host genotype and microbial compatibility in shaping plant responses under thermal stress. This study contributes novel insights into the role of PGPB in Chilean bean landraces and provides a foundation for future efforts in developing climate-resilient agroecosystems through targeted microbial applications.

Keywords: P. vulgaris L.; Climate change; Common beans; PGPB; Temperature stress.

Fig. 1

Auxin production by the bacterial strains under study. (A) Colorimetric production of auxin by B. proteolyticus Cyn1, B. safensis Cyn2, and their bacterial consortium, based on the intensity of red colour. (B) Quantification of auxin production under serial dilutions using a standard curve.

Fig. 2

Confocal microscopy images of roots with B. proteolyticus Cyn1, B. safensis Cyn2, consortium and control treatments in 9 bean landraces. Staining was performed with SYTO 9/Propidium Iodide (LIVE/DEAD BacLight kit, ThermoFisher, USA), and images were acquired at 60X magnification. All images share the same scale bar, represented by a white line in the lower corner of each panel, corresponding to 25 μm.

Fig. 3

ORAC and polyphenols content in Chilean common beans landraces. (A) ORAC values in leaves. (B) ORAC values in roots. (C) Total polyphenol content in leaves. (D) Visual comparison of seeds from the 10 common beans landraces used in the study. All biochemical measurements were performed after bacterial inoculation (Control, B. proteolyticus Cyn1, B. safensis Cyn2, and consortium). Antioxidant activity is expressed in µM TE/100 g dry weight (Trolox Equivalents), and polyphenol content is expressed in mg GAE/g dry weight (Gallic Acid Equivalents). Values represent mean ± SD. *p < 0.05, **p < 0.01.

Fig. 4

Multivariate analysis of growth parameters in nine bean landraces under bacterial treatments. (A) Principal Component Analysis (PCA) showing the distribution of treatments and growth parameters, with ellipses representing the 95% confidence interval. PC1 (46.76%) and PC2 (31.63%) together explain 78.39% of the variance. (B) Spearman’s correlation matrix among growth parameters of bean landraces. Correlation coefficients are represented by color intensity, positive values in orange, negative values in blue. Values with an “**” indicate significant correlations (P > 0.05).

Fig. 5

Estimated marginal means (and 95% confidence intervals) for the GLMM models comparing the effect of different treatments (PGPB and control) on the growth parameters (plumule length, root length, radicle length, number of roots, total plant length, and shoot length) of the nine common bean landraces.

Fig. 6

Effect of seed bacterization on morphology and growth of common beans in Mantequilla, Sapito, and Tórtola landraces under four temperature conditions (25 °C, 30 °C, 35 °C, and 40 °C). Treatments included control, B. proteolyticus Cyn1, B. safensis Cyn2, and their bacterial consortium. Representative images of seedlings were selected from biological replicates to illustrate the typical morphological response observed for each combination of landrace, temperature, and treatment.

Fig. 7

Growth response of three Chilean bean landraces to temperature stress and bacterial inoculation. (A) Principal Component Analysis (PCA) of bacterial treatments and growth parameters in beans landraces (Mantequilla, Sapito, and Tórtola) under three temperature conditions (25 °C, 30 °C, 35 °C). (B) Spearman’s correlation matrix showing the relationship among growth parameters under temperature variation. Significant correlations are color-coded; significant values are marked with an “**” (P > 0.05).

Fig. 8

Estimated marginal means (and 95% confidence intervals) from a generalised linear mixed model (GLMM) evaluating the effects of temperature variation (25, 30 and 35 °C) and PGPB treatments (control, B. proteolyticus Cyn1, B. safensis Cyn2, and a consortium of the two bacteria) on plant growth parameters in three common bean landraces (Mantequilla, Sapito, and Tórtola). Asterisks (**) indicate a significant effect of temperature on a plant growth parameter (p < 0.05).

Fig. 9

Chlorophyll, carotenoid, polyphenol, and ORAC contents under different bacterial treatments (B. proteolyticus Cyn1, B. safensis Cyn2, consortium, and control) across three Chilean bean landraces (A) Tórtola, (B) Sapito, and (C) Mantequilla, evaluated under temperature stress (25 °C, 30 °C, 35 °C, and 40 °C).

Fig. 10

Oxidative stress (MDA) and osmoprotectant (proline) levels in bean landraces under thermal stress and bacterial treatments. (A) MDA content and (B) Proline content in Mantequilla landrace from 35 °C to 40 °C under different treatments (Control, B. proteolyticus Cyn1, B. safensis Cyn2, consortium). (C) MDA and proline content in Sapito and Tórtola landraces under normal temperature conditions. Different letters above the bars indicate statistically significant differences between treatments (p < 0.05). The asterisk (*) denotes significant differences among temperaturetreatments within the 'Mantequilla' variety (p < 0.05).