Figure 1. Creeping bentgrass inoculated with PGPR WSF23 demonstrated higher turf quality and canopy density.
With the effects of climate change, drought stress in turfgrass may be exacerbated in many regions, severely limiting plant growth and productivity. Any approaches that can mitigate drought damages and promote rapid recovery of plants from the stress
when water is available are critically important for plant adaptation to the changing climates. One promising strategy is to utilize naturally occurring plant-growth-promoting rhizobacteria (PGPR) to improve stress tolerance and plant growth in adverse
environments. Drought-induced leaf senescence and growth inhibition have been associated with the increased production of ethylene, which is known as a stress hormone.
Certain species of PGPR can produce a deaminase enzyme (1-aminocyclopropane-1-carboxylic acid deaminase [ACCd]) that breaks down the precursors of ethylene (1-aminocyclopropane-1-carboxylic acid [ACC]) into ammonia and alpha-ketobutyrate, leading to the
elimination or suppression of ethylene accumulation in plant tissues that have been colonized (3).
Paraburkholderia aspalathi is an endophytic PGPR producing ACCd enzymes that colonize plant roots to facilitate root growth (1). We have found some new strains of P. aspalathi with high ACCd enzyme activity naturally colonizing roots of creeping bentgrass
and other grass species in the New Jersey Pine Barrens. Our previous study (2) found the inoculation of creeping bentgrass with our novel strains of P. aspalathi significantly improved drought tolerance and post-stress recovery of creeping bentgrass,
as manifested by increased tiller production and root proliferation and elongation during drought and re-watering. Plant adaptation to drought stress involves changes in metabolic processes for the synthesis of various metabolites, such as carbohydrates,
amino acids, organic acids and sugar alcohols that are known to have key metabolic functions in regulating plant growth and stress tolerance.
This study determined metabolic pathways and mechanisms of P. aspalathi, conferring the improved drought tolerance and post-drought recovery in creeping bentgrass through metabolomic analysis of leaf and root tissues.
Research methodology
Creeping bentgrass (Agrostis Stolonifera L. cv. Penncross) plants were inoculated with P. aspalathi WSF23 and exposed to drought stress in controlled environment growth chambers. Drought stress was initiated seven days after the first inoculation treatment
by withholding all water from the plants for 35 days. After the 35-day drought stress period, the plants were re-watered for 15 days to evaluate post-drought recovery. The drought stress and re-watering treatments were applied to both inoculated and
non-inoculated plants that were used for a control group. Full irrigation was also supplied to a non-stress control group of both inoculated and non-inoculated plants.
Turf visual quality and relative water content, canopy density, dark green color index (DGCI) and the content of metabolites of leaves and roots were measured during drought and post-drought recovery.
Figure 2. Creeping bentgrass inoculated with PGPR WSF23 demonstrated improved root growth.
Results and discussion
Inoculation of plants with P. aspalathi WSF23 was effective in improving creeping bentgrass tolerance to drought stress and post-stress recovery, as manifested by increased turf quality, canopy density and root growth in plants inoculated with ACCd bacteria
during drought stress and re-watering (Figures 1, 2).
While drought stress resulted in an overall decline in turf quality, inoculated plants were able to maintain a higher turf quality for a longer period than that of non-inoculated plants during 28-35 days of stress. After 35 days of drought stress, all
plants were trimmed down to the base and re-watered for a period of 15 days. Inoculated plants demonstrated higher turf quality than non-inoculated plants after 15 days of re-watering, based on turf color, uniformity and density.
After 35 days of drought stress, inoculated plants had 32.0% green cover, while non-inoculated plants only had 17.4% green cover. Inoculation also resulted in greater canopy density during the re-watering period. Drought-stressed plants that had been
inoculated with the ACCd bacteria had higher canopy density after 10 days of re-watering (26% versus 16.7%) and higher canopy density after 15 days of re-watering (32.8% versus 19.3%) when compared to non-inoculated plants that had been subjected
to the same degree of water deficit. Similarly, while drought stress reduced DGCI values, inoculated plants exhibited higher DGCI throughout the drought stress and re-watering periods.
Metabolic analysis demonstrated that leaf tissue of inoculated plants under drought stress had significantly higher levels of arginine-succinate, proline, allantoin, folic acid and S-adenosylmethionine (SAM) relative to the leaf tissues of non-inoculated
plants. In root tissue under drought stress, the greatest increases were observed in the content of asparagine, proline, allantoin and riboflavin. Inoculation also resulted in increased carbohydrate levels under drought stress conditions, in comparison
to non-inoculated plants, with leaf tissue of inoculated plants having higher levels of stachyose, glucose, raffinose, sucrose and fructose than non-inoculated plants, while root tissue exhibited higher concentrations of arabinose, raffinose and glucose.
During post-drought recovery at 15 days of re-watering, leaf tissue of inoculated plants had significantly higher levels of phenethylamine, arginino-succinate, riboflavin and thymidine relative to non-inoculated plants, while root tissue of inoculated
plants exhibited increases in the content of methionine, alpha-ketoglutarate, SAM and folic acid. The content of sucrose, sorbitol, arabitol and proline decreased in the leaves of inoculated plants relative to non-inoculated plants, while the carbohydrates
UDP-glucose, stachyose, sorbitol and arabitol were increased in roots, supporting more rapid regrowth of plants upon re-watering.
Conclusions
Inoculation with the novel strain of P. aspalathi WSF 23 caused metabolomic adjustment in leaves and roots to strengthen stress-defense mechanisms during drought and activated metabolic processes to support plant regrowth out of stress damages when water
became available, in addition to its roles of suppressing stress-induced ethylene production. Improved drought tolerance by the inoculation of creeping bentgrass with P. aspalathi WSF23 was mainly due to the enhanced plant metabolism related to stress
defense, including osmoregulation, cell-wall stability and antioxidant protection, as well as nitrogen metabolism. The promotion of post-stress recovery in creeping bentgrass by P. aspalathi WSF23 was mainly associated with enhanced carbohydrate metabolism
and cytokinin hormone biosynthesis, biosynthesis of DNA and proteins, and energy metabolism. P. aspalathi WSF23 could be a useful and effective constituent incorporated into biostimulants or plant-health products to promote turf and root growth through
activation and regulation of plant metabolism.
The research says
- Inoculation with the novel strain of P. aspalathi WSF23 caused metabolomic adjustment in leaves and roots to strengthen stress-defense mechanisms during drought and activated metabolic processes to support plant regrowth out of stress damages when
water became available, in addition to its roles of suppressing stress-induced ethylene production.
- Improved drought tolerance by the inoculation of creeping bentgrass with P. aspalathi WSF23 was mainly due to the enhanced plant metabolism related to stress defense, including osmoregulation, cell-wall stability and antioxidant protection, as well
as nitrogen metabolism.
- P. aspalathi WSF23 could be a useful and effective constituent incorporated into biostimulants or plant-health products to promote turf and root growth through activation and regulation of plant metabolism.
Literature cited
- Cheng, L., N. Zhang and B. Huang. 2016. Effects of 1-aminocyclopropane-1-carboxylate-deaminase-producing bacteria on perennial ryegrass growth and physiological responses to salinity stress. Journal of the American Society for Horticultural Science
141(3):233-241 (https://doi.org/10.21273/JASHS.141.3.233).
- Errickson, W., N. Zhang and B. Huang. 2023. Promotive effects of endophytic rhizobacteria on tiller and root growth in creeping bentgrass during drought stress and post-stress recovery involving regulation of hormone and sugar metabolism. Crop Science
63(4):2583-2593 (https://doi.org/10.1002/csc2.21017).
- Glick, B.R. 2005. Modulation of plant ethylene levels by the bacterial enzyme ACC deaminase. 251(1):1-7 (https://doi.org/10.1016/j.femsle.2005.07.030
Bingru Huang (huang@sebs.rutgers.edu) is a distinguished professor in the Department of Plant Biology at Rutgers University, New Brunswick, N.J., and William Errickson is a Ph.D. candidate and Agriculture & Natural Resources County Agent III at the New Jersey Agricultural Experiment Station, Rutgers University, Freehold, N.J.