The combined effect of a newly designed biostimulant and a plant growth-promoting bacterium increases tomato yield under salt stress by increasing the cytokinin isopentenyladenine riboside content

There is a growing need for agricultural inputs to maintain yield under adverse conditions. Salinization is a widespread problem in agrarian land, aggravated by anthropogenic global warming. Biostimulants based on living microorganisms or natural product extracts have been proposed as valuable tools for farmers employing conventional or organic practices. However, the availability of effective products is low, and our understanding of the mechanisms explaining the effects observed is very limited. This report describes the combination of a plant growth-promoting bacterium and a novel non-microbial biostimulant previously formulated in-house which increases tomato yield under salt stress. We have also determined many physiological, biochemical, and molecular parameters to characterize the molecular mechanisms underlying the observed yield increase. Our results indicate that the combined effect of both biostimulants promoted the accumulation of proline in roots and flavonoids in leaves, as well as a decrease in the antioxidant response, with the only exception of catalase activity, which was unaltered in leaves, and the ascorbate peroxidase activity, which exhibited a slight increase in roots. In addition, the joint treatment increased the content of the cytokinin isopentenyladenine riboside (IPR) in roots and leaves and promoted a significant accumulation of Krebs cycle intermediates under salt stress. The most plausible mechanism is that cytokinins protect chloroplasts and photosynthetic function, increasing the available sugar. The resulting increase in the available energy allows plants to produce more fruit and respond better to salt stress, an energy-demanding process. The co-application of both biostimulants increases yield under salt stress. It also stimulates the increase of the cytokinin IPR, which may be involved in protecting the photosynthetic system and thus reducing the appearance of reactive oxygen species. This opens new possibilities for farmers in conventional and organic agriculture, especially in developing countries, which are more likely to suffer the consequences of climate change and the resulting increase in aridity and salinization of arable land.