More studies needed about how plants survive water stress

To protect future crops, more research is needed on how the plant hormone melatonin helps some plants survive droughts, and how it helps others to grow in water-drenched soils.

Over the past 40 years, up to 40% of wheat and maize harvests failed because of droughts. 

Droughts are however not the only form of water stress that puts plants under pressure. Between 40% and 80% of harvests grown on over 1.7 billion hectares were lost in recent years because fields became waterlogged after too much rain. In such cases, a lack of oxygen in the soil makes it difficult for plants to thrive. 

Researchers already know that plants cope better during dry periods when they are boosted with treatments that contain melatonin. This includes adding the plant hormone to soils, in irrigation water or spraying it onto leaves.

Twenty years ago, researchers started looking in earnest into why and how melatonin helps plants survive droughts. The first such work on its influence during periods of flooding only started in 2015.

The study of how melatonin protects plants during times of water stress is therefore still a fairly new research field. Researchers are eager to learn more about the topic.  

To help them identify future projects, researchers from China, Egypt, Bangladesh and Spain first summarised what they and others already knew about the topic. They considered relevant studies completed around the world. 

These included studies on how plants change in times of water stress, and the internal processes at work.

The research group then suggested specific topics that are important to study in future.

For instance, the researchers involved believe not enough is yet known about how melatonin protects plants during periods of flooding.

Scientists must also still find out how melatonin helps plants take up nutrients.

Studies about how plants cope with water stress must also investigate how nitric oxide (NO) and hydrogen sulfide (H2S) interact with melatonin within plant cells.

They added that efforts must be made to develop products containing synthetic melatonin and plant extracts rich in melatonin. 

Advanced technology and methods from the field of genetics are handy tools to tackle these topics, they said.

Researchers from three continents, including Africa, compiled the overview together. It will hopefully allow others to strategically tackle studies on melatonin, so that farmers can one day grow more crops that can withstand droughts or flood conditions.

Abstract

Water stress (drought and waterlogging) is severe abiotic stress to plant growth and development. Melatonin, a bioactive plant hormone, has been widely tested in drought situations in diverse plant species, while few studies on the role of melatonin in waterlogging stress conditions have been published. In the current review, we analyze the biostimulatory functions of melatonin on plants under both drought and waterlogging stresses. Melatonin controls the levels of reactive oxygen and nitrogen species and positively changes the molecular defense to improve plant tolerance against water stress. Moreover, the crosstalk of melatonin and other phytohormones is a key element of plant survival under drought stress, while this relationship needs further investigation under waterlogging stress. In this review, we draw the complete story of water stress on both sides—drought and waterlogging—through discussing the previous critical studies under both conditions. Moreover, we suggest several research directions, especially for waterlogging, which remains a big and vague piece of the melatonin and water stress puzzle.

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