Biomics

Plant growth-promoting rhizobacteria increase leaf size. This causes plants to lose more water, requiring them to develop mechanisms to mitigate the effects of possible water deficit. Research has revealed a variety of mechanisms for adapting the water balance of these plants. Understanding the causes of these differences is crucial for the successful application of bacteria in agriculture. This mini-review discusses the effects of PGPR on stomatal conductance, transpiration, relative leaf water content and water potential, root growth, and hydraulic conductivity. We evaluate the importance of these parameters for maintaining water balance and for stimulating plant growth under the influence of bacterial inoculation. We also analyze the role of modifications of hormonal balance influenced by PGPR in regulating these processes. PGPR have varying effects on plant water balance. There is evidence of both increases and decreases in stomatal conductance and relative water content under the influence of bacteria. The effectiveness of these alternative responses is discussed in the context of soil water availability. Plant responses to PGPR can be classified by the following traits: 1) stomatal closure and reduced transpiration; 2) increased water uptake capacity by stimulating root growth to compensate for increased transpiration; 3) increased root water channel activity, leading to greater hydraulic conductivity; and 4) osmotic adaptation, which helps maintain leaf turgor during reduced leaf water content.

PGPR, water relations, stomatal conductance, hydraulic conductivity, auxins, cytokinins, ethylene, abscisic acid, root growth, aquaporins

1. Akhtyamova Z, Arkhipova T, Sharipova G et al. The effect of plant growth-promoting bacteria Bacillus subtilis IB-22 on the hydraulic conductivity and abundance of PIP2 aquaporins in the roots of an abscisic acid-deficient barley mutant. J. Mol. Sci. 2024. 25. 10706. https://doi.org/10.3390/ijms251910706
2. Akhtyamova ZA, Arkhipova TN, Martynenko EV et al. Influence of Bacillus subtilis strain on abscisic acid content in ABA-deficient barley mutant and its wild type. Taurida Herald of the Agrarian Sciences. 2021. 2(26). 28–40. https://doi.org/10.33952/2542-0720-2021-2-26-28-40 (In Russian)
3. Alarcón MV, Salguero J, Lloret PG. Auxin modulated initiation of lateral roots is linked to pericycle cell length in maize. Plant Sci. 2019. 10. 11. https://doi.org/10.3389/fpls.2019.00011
4. Ali B, Wang X, Saleem MH et al. PGPR-Mediated Salt Tolerance in Maize by Modulating Plant Physiology, Antioxidant Defense, Compatible Solutes Accumulation and Bio-Surfactant Producing Genes. Plants (Basel). 2022. 11. 345. https://doi.org/10.3390/plants11030345
5. Arkhipova TN, Prinsen E, Veselov SU et al. Cytokinin producing bacteria enhance plant growth in drying soil. Plant Soil. 2007. 292. 305–315. https://doi.org/10.1007/s11104-007-9233-5
6. Arkhipova TN, Evseeva NV, Tkachenko OV et al. Rhizobacteria inoculation effects on phytohormone status of potato microclones cultivated in vitro under osmotic stress. Biomolecules. 2020. 10. 1231. https://doi.org/10.3390/biom10091231
7. Arkhipova T, Sharipova G, Akhiyarova G et al. The Effects of Rhizosphere Inoculation with Pseudomonas mandelii on formation of apoplast barriers, HvPIP2 aquaporins and hydraulic conductance of barley. Microorganisms. 2022. 10. 935. https://doi.org/10.3390/microorganisms10050935
8. Arteca RN, Arteca JM. Effects of brassinosteroid, auxin, and cytokinin on ethylene production in Arabidopsis thaliana J. Exp. Bot. 2008. 59. 3019–3026. https://doi.org/10.1093/jxb/ern159
9. Asari S, Tarkowská D, Rolčík J et al. Analysis of plant growth-promoting properties of Bacillus amyloliquefaciens UCMB5113 using Arabidopsis thaliana as host plant. Planta. 2017. 245. 15–30. https://doi.org/10.1007/s00425-016-2580-9
10. Barnawal D, Bharti N, Pandey SS et al. Plant growth-promoting rhizobacteria enhance wheat salt and drought stress tolerance by altering endogenous phytohormone levels and TaCTR1/TaDREB2 expression. Plant. 2017. 161. 502–514. https://doi.org/10.1111/ppl.12614
11. Belimov AA, Dodd IC, Safronova VI et al. Abscisic acid metabolizing rhizobacteria decrease ABA concentrations in planta and alter plant growth. Plant Physiol. Biochem. 74. 84–91. https://doi.org/10.1016/j.plaphy.2013.10.032
12. Belimov AA, Dodd IC, Safronova VI et al. Rhizobacteria that produce auxins and contain 1-amino-cyclopropane-1-carboxylic acid deaminase decrease amino acid concentrations in the rhizosphere and improve growth and yield of well-watered and water-limited potato (Solanum tuberosum). Appl. Biol. 2015. 167. 11–25. https://doi.org/10.1111/aab.12203
13. Bhat MA, Kumar V, Bhat MA et al. Mechanistic insights of the interaction of plant growth-promoting rhizobacteria (PGPR) with plant roots toward enhancing plant productivity by alleviating salinity stress. Microbiol. 2020. 11. 1952. https://doi.org/10.3389/fmicb.2020.01952
14. Bresson J, Varoquaux F, Bontpart T et al. The PGPR strain Phyllobacterium brassicacearum STM196 induces a reproductive delay and physiological changes that result in improved drought tolerance in Arabidopsis. New Phytol. 200. 558–569. https://doi.org/10.1111/nph.12383
15. Cho SM, Kang BR, Han SH et al. 2R,3R-Butanediol, a bacterial volatile produced by Pseudomonas chlororaphis O6, is involved in induction of systemic tolerance to drought in Arabidopsis thaliana. MPMI. 2008. 21. 1067–1075. https://doi.org/10.1094/MPMI-21-8-1067
16. Cohen AC, Travaglia CN, Bottini R, Piccoli PN. Participation of abscisic acid and gibberellins produced by endophytic Azospirillum in the alleviation of drought effects in maize. Botany. 2009. 87. 455–462. https://doi.org/10.1139/B09-023
17. Colombi T, Herrmann AM, Vallenback P, Keller T. Cortical cell diameter is key to energy costs of root growth in wheat. Plant Physiol. 180. 2049–2060. https://doi.org/10.1104/pp.19.00262
18. Daszkowska-Golec A, Szarejko I. Open or close the gate — stomata action under the control of phytohormones in drought stress conditions. Plant Sci. 2013. 4. 138. https://doi.org/10.3389/fpls.2013.00138
19. de Lima BC, Moro AL, Santos ACP et al. Bacillus subtilis ameliorates water stress tolerance in maize and common bean. Plant Interact. 2019. 14. 432–439. https://doi.org/10.1080/17429145.2019.1645896
20. Hartung W, Sauter A, Hose E. Abscisic acid in the xylem: where does it come from, where does it go to? Exp. Bot. 2002. 53. 27–32. https://doi.org/10.1093/jxb/53.366.27
21. Ehinmitan E, Losenge T, Mamati E et al. BioSolutions for green agriculture: unveiling the diverse roles of plant growth-promoting rhizobacteria. J. Microbiol. 2024. 6181491. https://doi.org/10.1155/2024/6181491
22. Fatma M, Asgher M, Iqbal N et al. Ethylene Signaling under Stressful Environments: Analyzing Collaborative Knowledge. Plants. 2022. 11. 2211. https://doi.org/10.3390/plants11172211
23. Glick BR. Bacteria with ACC deaminase can promote plant growth and help to feed the world. Res. 2014. 169. 30–39. https://doi.org/10.1016/j.micres.2013.09.009
24. Grover M, Bodhankar S, Sharma A et al. PGPR mediated alterations in root traits: way toward sustainable crop production. Sustain. Food Syst. 2021. 4. 618230. https://doi.org/10.3389/fsufs.2020.618230
25. Iqbal A, Hasnain S. Aeromonas punctata PNS-1: a promising candidate to change the root morphogenesis of Arabidopsis thaliana in MS and sand system. Acta Physiol. Plant. 35. 657–665. https://doi.org/10.1007/s11738-012-1106-8
26. Kudoyarova G, Veselova S, Hartung W et al. Involvement of root ABA and hydraulic conductivity in the control of water relations in wheat plants exposed to increased evaporation demand. Planta. 2011. 233. 87–94. https://doi.org/10.1007/s00425-010-1286-7
27. Kudoyarova G, Arkhipova T, Korshunova T et al. Phytohormone Mediation of Interactions Between Plants and Non-Symbiotic Growth Promoting Bacteria Under Edaphic Stresses. Plant Sci. 2019. 10. 1368. https://doi.org/10.3389/fpls.2019.01368
28. Liu D, Mou S, Zou Y et al. Exploring the relationship between deep roots and shoot growth of wheat under different soil moisture: A large soil column experiment. Rhizosphere. 2023. 25. 100675. https://doi.org/10.1016/j.rhisph.2023.100675
29. Maksimov N.A. A Short Course in Plant Physiology. Moscow, 1958. (In Russian)
30. Maurel C, Boursiac Y, Luu D, Santoni V, Shahzad Z, Verdoucq L. Aquaporins in plants. Rev. 2015. 95. 1321–1358. https://doi.org/10.1152/physrev.00008.2015
31. Marulanda A, Azcón R, Chaumont F et al. Regulation of plasma membrane aquaporins by inoculation with a Bacillus megaterium strain in maize (Zea mays) plants under unstressed and salt-stressed conditions. Planta. 2010. 232. 533–543. https://doi.org/10.1007/s00425-010-1196-8
32. Munemasa S, Hauser F, Park J et al. Mechanisms of abscisic acid-mediated control of stomatal aperture. Opin. Plant Biol. 2015. 28. 154–162. https://doi.org/10.1016/j.pbi.2015.10.010
33. Namwongsa J, Jogloy S, Vorasoot N et al. Endophytic bacteria improve root traits, biomass and yield of Helianthus tuberosus under normal and deficit water conditions. J. Microbiol. Biotechnol. 2019. 29. 1777–1789. https://doi.org/10.4014/jmb.1903.03062
34. Raish SMA, Osama M, Sourani OM, Abu-Elsaoud AM. Plant growth-promoting microorganisms as biocontrol agents: mechanisms, challenges, and future prospects. Microbiol. 2025. 5. 44. https://doi.org/10.3390/applmicrobiol5020044
35. Rivas MÁ, Friero I, Alarcón MV, Salguero J. Auxin-cytokinin balance shapes maize root architecture by controlling primary root elongation and lateral root development. Plant Sci. 2022. 13. 836592. https://doi.org/10.3389/fpls.2022.836592
36. Salomon MV, Bottini R, Filho GAS et al. Bacteria isolated from roots and rhizosphere of Vitis vinifera retard water losses, induce abscisic acid accumulation and synthesis of defense-related terpenes in in vitro cultured grapevine. Plant. 2014. 151. 359–374. https://doi.org/10.1111/ppl.12117
37. Selvakumar G, Bindu GH, Bhatt RM et al. Osmotolerant cytokinin producing microbes enhance tomato growth in deficit irrigation conditions. Natl. Acad. Sci. India B: Biol. Sci. 2016. 88. 459–465. https://doi.org/10.1007/s40011-016-0766-3
38. Serraj R, Sinclair TR. Osmolyte accumulation: can it really help increase crop yield under drought conditions? Plant Cell Environ. 25. 333–341. https://doi.org/10.1046/j.1365-3040.2002.00754.x
39. Sinclair TR. Effective Water Use Required for Improving Crop Growth Rather Than Transpiration Efficiency. Plant Sci. 2018. 9. 1442. https://doi.org/10.3389/fpls.2018.01442
40. Spaepen S, Vanderleyden J. Auxin and plant-microbe interactions. Cold Spring Harb. Perspect. Biol. 3. a001438. https://doi.org/10.1101/cshperspect.a001438
41. Starikov SN, Chetverikova DV, Chetverikov SP. Destruction of phytohormones by plant growth-promoting bacteria. Ecobiotech. 2023. 6(4). 217-226. https://doi.org/10.31163/2618-964X-2023-6-4-217-226 (In Russian)
42. Sun W, Zhou S, Yu B et al. Soil moisture-atmosphere interactions drive terrestrial carbon-water trade-offs. Earth Environ. 2025. 6. 169. https://doi.org/10.1038/s43247-025-02145-z
43. Syrova DS, Shaposhnikov AI, Yuzikhin OS, Belimov AA. Destruction and transformation of phytohormones by microorganisms. Applied Biochemistry and Microbiology. 2022. Т. 58. № 1. С. 3-22. (In Russian)
44. Turner NC. Turgor maintenance by osmotic adjustment, an adaptive mechanism for coping with plant water deficits. Plant Cell Environ. 40. 1–3. https://doi.org/10.1111/pce.12839
45. Vejan P, Abdullah R, Khadiran T et al. Role of Plant Growth Promoting Rhizobacteria in Agricultural Sustainability—A Review. Molecules. 2016. 21. 573. ttps://doi.org/10.3390/molecules21050573
46. Vysotskaya LB, Arkhipova TN, Timergalina LN et al. Effect of partial root excision on transpiration, root hydraulic conductance and leaf growth in wheat seedlings. Plant Physiol. Biochem. 42. 251–255.
47. Vysotskaya L, Martynenko E, Ryabova A et al. The Growth-Inhibitory Effect of Increased Planting Density Can Be Reduced by Abscisic Acid-Degrading Bacteria. Biomolecules. 2023. 13. 1668. https://doi.org/10.3390/biom13111668
48. Werner T, Motyka V, Laucou V et al. Cytokinin-deficient transgenic Arabidopsis plants show multiple developmental alterations indicating opposite functions of cytokinins in the regulation of shoot and root meristem activity. Plant Cell. 2003. 15. 2532–2550. https://doi.org/10.1105/tpc.014928
49. Zaheer MS, Ali HH, Iqbal MA et al. Cytokinin production by Azospirillum brasilense contributes to increase in growth, yield, antioxidant, and physiological systems of wheat (Triticum aestivum). Front. Microbiol. 2022. 13. 886041. https://doi.org/10.3389/fmicb.2022.886041
50. Zhang S, Li F, Chang L et al. Broad-spectrum applications of plant growth-promoting rhizobacteria (PGPR) across diverse crops and intricate planting systems. Spectr. 2025. 13. e0187924. https://doi.org/10.1128/spectrum.01879-24
51. Zhang J, Nguyen HT, Blum A. Genetic analysis of osmotic adjustment in crop plants. J. Exp. Bot. 1999. 50. 291–302