Longer roots in summertime weather
Often warm weather and drought come as a pair. Although there is a reasonable amount known of how the above ground parts of plants deal with this, we know less about how roots deal with the combination of heat and drought. To change this Dutch researchers decide to study this in more detail.
The first thing the researchers did was confirming that the roots indeed respond to a combination of heat and drought. For this the researchers grew tale cress seedlings in soil under different conditions. They compared seedling growing at 20 and 28C. And compared well watered and less well watered seedlings. When the seedlings had enough water then the roots of seedlings growing at 20 and 28 C were of roughly equal length. But were the seedlings less well watered, then the roots of those growing at 28C were longer. There was indeed also a root growth reaction in response to the combination of heat and drought.
Subsequently the researchers started to decipher the genes of the heat-drought protocol. Firstly, they studied the effect of the absence of the water stress regulation gene SnRK2. Roots of plants without SnRK2 were responding less to the combination of heat and drought than those of plants that did have SnRK2. Suggesting that SnRK2 is one of the heat-drought protocol regulatory genes.
COP1 reduction allows the root length to increase
Next on the list were the heat response genes. The effect of the absence of those genes was studied. For one of them COP1 the researchers observed that the plants reacted differently in response to heat and drought. They had even longer roots than plants with COP1.
Following this the researchers studied the influence of the combination of heat and drought on the activation of COP1. They did this by coupling a gene for a fluorescent protein to the COP1-gene on switch. This results in that the gene for the fluorescent protein is turned on when the COP1-gene is turned on. They observed the fluorescent protein mostly in the root hairs. But only under normal conditions. By a combination of heat and drought the protein was hardly visible. Also when the plants were missing SnRK2, then fluorescent protein was hardly visible. This appears to be a little contradictory as on one hand as result of the heat-drought protocol COP1 reduction allows the root length to increase. But in the case of SnRK2 free plants, this increase of root length does not occur even though there is hardly any COP1 present.
SnRK2 is required to keep a stable amount of HY5 available
In order to get more insight into this the researchers studied the effect of a known COP1 regulator, HY5. While studying the heat and drought effect on HY5 it was observed that when the plant only experiences heat, the in root hairs available HY5 was degraded quickly. But experiences the plant both heat and drought than the HY5 in root hairs stabilizes. However, in plants without COP1 HY5 is also stable when the plants only experience heat. But is SnRK1 absent, then the researcher noticed hardly any HY5 in the root hairs.
This all together suggests that SnRK2 is required to keep a stable amount of HY5 available. Only in that case can the heat-drought protocol regulator COP1 influence the amount of HY5 during a combination of heat and drought. The amount of COP1 reduces by heat and drought, allowing the amount of HY5 to increase. And only the increase of HY5 enables root hair increase. But this all depends on the starting values of HY5, is HY5 absent, then COP1 has nothing to regulate.
Literature
Scott Hayes, Cheuk Ka Leong, Wenyan Zhang, Marthe Lamain, Jasper Lamers, Thijs de Zeeuw, Francel Verstappen, Andreas Hiltbrunner, Christa Testerink (2023) Warm temperature and mild water stress cooperatively promote root elongation. bioRxiv 2023.11.30.569400; doi: https://doi.org/10.1101/2023.11.30.569400
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