Recent events shed a light on what we possibly face in the future, as hurricanes and other weather events become more severe due to the increased energy capacity of the atmosphere from greenhouse gas influx. The colloquial term for this process is climate change. And scientists are hard at work on finding ways to mitigate the impact of these weather phenomena.
For agriculture, a high concern when it comes to hurricanes and tsunamis is flooding of growing areas, especially in the middle of the planting season. This increase in moisture in the soil negatively affects oxygen diffusion rates through the earth itself, which can in turn cause hypoxia for plant roots and kill them outright.
Researchers at the University of Kiel in Germany decided to investigate how plants deal with these situations. While a fair amount of study has been done on giving plants flood resistance, there has been far less looking at the physical (and therefore genetic) mechanisms that plant roots use during flooding.
The primary family of genes involved in regulatory and biotic/abiotic stress responses are the Ethylene Response Factors (ERFs). These all-encompassing pathways are used to deal with pathogen invasions, dehydration, mineral loss, and, indeed, flooding. Past studies have shown that it is the Group VII ERF genes that respond to low oxygen environments.
The proteins of these genes are normally degraded under regular conditions, but become stabilized under hypoxic ones. Similar to mammals in several ways, some of these genes code for anaerobic metabolism functions, such as fermentation and lactic acid production. Plants are able to keep cellular function even during low oxygen scenarios, but they also use the ethylene hormones to reprogram cells in the flooded areas, primarily root systems.
Using Arabidopsis as a model, the scientists looked into how the plants adapt their roots during flooding and how ERFVII genes are involved in the reconstructions. They specifically looked into the activity of a gene called HRE2 and used a GUS reporter transgene that allowed for a colored stain to be shown upon expression of the target gene.
What was found is that HRE2 is strongly expressed near the root tip under hypoxic conditions and that shortly after exposure to these conditions (within 12 hours), the roots began to bend and grow at a slanted angle. This angle eventually increased to 39 degrees away from vertical and appeared to be a method of searching for flooding-free soil to improve oxygen uptake.
To test the specific involvement of HRE2, the researchers used a knockout line of Arabidopsis without the gene and found that root slanting occurred even under normal oxygen conditions in the mutants. Due to this, they hypothesized that HRE2 is an inhibitor of root slanting and reduces the overall amount of bending that occurs. That means that other ERF genes must be directly responsible for the bending itself and further experiments indicated that, at minimum, another set known as the RAP2 genes were used to activate other hypoxia-related genes. Though they also helped to inhibit the amount of root bending that is expressed.
Repeatedly going back and forth between hypoxic and normoxic environments proved that the root structure is reversible, with the roots returning to downward growth once conditions normalized. This displayed that the effects of root bending are tightly controlled and regulated by developmental processes in the plant and are inherently a part of its evolutionary growth pattern.
Previous theories offered evidence that one of the primary classes of plant growth hormones, auxins, have a distributed gradient in the flooded organs, the roots, during hypoxia. As a final thing to look into, the researchers tested auxin activity at the root tips. A specially engineered transgene sensor for auxins called DII-VENUS was used for this purpose.
To determine if this auxin gradient increased root bending, they incubated the previously noted mutant Arabidopsis lines with auxin analogues, which caused significantly more bending to occur even under normoxic conditions. They concluded that auxins and the gradient control genes, known for their PIN proteins, work synergistically with hypoxia signaling to force root bending.
A last test displayed that PIN2 reduces in abundance during hypoxia and this caused auxins to gravitate toward the root tips due to no longer being transported away by PIN2 proteins. This results in enhanced bending occurring.
Creating a better understanding of what genes are favored during flooding can help scientists know what to target and alter to create crop lines better suited to surviving a flood. The mechanics of root bending may allow further options for allowing plants to obtain oxygen even when dealing with hypoxia conditions.
And, at minimum, deciphering the hormonal and genetic pathways in the process gives new ways to try and affect other parts of the plant and the use of growth hormones like auxins to control the body shape and structural integrity of plants. When trying to mitigate the impact of climate change on global agriculture, every bit of information helps and any piece may turn out to play a bigger role than expected.
Photo CCs: Проводящий пучок Pteridium aquilinum from Wikimedia Commons