Improvement of Drought Tolerance and Grain Yield in Common Bean by Overexpressing Trehalose-6-Phosphate Synthase in Rhizobia

Abstract

Improving stress tolerance and yield in crops are major goals for agriculture. Here, we show a new strategy to increase drought tolerance and yield in legumes by overexpressing trehalose-6-phosphate synthase in the symbiotic bacterium Rhizobium etli. Phaseolus vulgaris (common beans) plants inoculated with R. etli overexpressing trehalose-6-phosphate synthase gene had more nodules with increased nitrogenase activity and higher biomass compared with plants inoculated with wild-type R. etli. In contrast, plants inoculated with an R. etli mutant in trehalose-6-phosphate synthase gene had fewer nodules and less nitrogenase activity and biomass. Three-week-old plants subjected to drought stress fully recovered whereas plants inoculated with a wild-type or mutant strain wilted and died. The yield of bean plants inoculated with R. etli overexpressing trehalose-6-phosphate synthase gene and grown with constant irrigation increased more than 50%. Macroarray analysis of 7,200 expressed sequence tags from nodules of plants inoculated with the strain overexpressing trehalose-6-phosphate synthase gene revealed upregulation of genes involved in stress tolerance and carbon and nitrogen metabolism, suggesting a signaling mechanism for trehalose. Thus, trehalose metabolism in rhizobia is key for signaling plant growth, yield, and adaptation to abiotic stress, and its manipulation has a major agronomical impact on leguminous plants.

A Tale of Two Sugars: Trehalose 6-Phosphate and Sucrose

“…Trehalose is also implicated as a signal molecule during formation of rhizobial and mycorrhizal symbioses (Fig. 1) and in interactions with other beneficial microbes (Suárez et al, 2008;Rodríguez-Salazar et al, 2009;Fernandez et al, 2010Fernandez et al, , 2012. As these topics have been covered in a recent review , and no specific function for Tre6P in plant-microbe interactions (beyond the synthesis of trehalose) has so far been clearly identified, we shall focus here on the role of Tre6P in plant responses to abiotic stress.…”

Trehalose and Abiotic Stress in Biological Systems

“…Rhizobacteria engineered to overexpress trehalose biosynthetic genes improved the drought tolerance of the plants inoculated with the modified rhizobia strains. Common bean plants (Phaseolus vulgaris) inoculated with Rhizobium etli overexpressing trehalose-6-phosphate synthase recovered completely when exposed to drought stress as opposed to plants inoculated with the wild type strain, which wilted and died (Suarez et al, 2008). Maize plants inoculated with Azospirillum braziliense overexpressing trehalose biosynthetic genes had an 85% of survival following drought stress compared to 55% survival rate in the case of the plants inoculated with the untransformed strain (Rodriguez-Salazar et al, 2009).…”

The Physiological Mechanisms of Desiccation Tolerance in Rhizobia

“…Trehalose accumulation has also been correlated with improved stress tolerance in free-living cells of R. etli (Suarez et al, 2008). Suarez et al (2008) found that a strain of R. etli that overexpresses OstA (Ox) grew to significantly higher numbers under hyperosmotic conditions than the wild-type (WT) and an ostA deficient mutant (ostA -).…”

“…Suarez et al (2008) found that a strain of R. etli that overexpresses OstA (Ox) grew to significantly higher numbers under hyperosmotic conditions than the wild-type (WT) and an ostA deficient mutant (ostA -). Exponentially grown cells of the Ox strain were also reported to survive significantly better after exposure to heat shock and when freeze-thawed prior to plating.…”

“…The specific activity of nitrogenase (nmol /min/g of fresh nodule) measured using the acetylene-reduction assay was found to increase by 38% and decrease by 45% after inoculation with Ox and ostA -respectively when compared to inoculation with the WT. Suarez et al (2008) concluded that the overexpression of OstA and increased accumulation of trehalose in R. etli increases its capacity not only to survive stress but to also infect or escape defence mechanisms in nodulating plants leading to higher number of nodules and nitrogenase activity.…”