Like human beings, plants need to control water uptakeÂ to survive. Unlike humans, they cannot simply move to a dryer place when water levels rise, or go to a water sourceÂ when they areÂ in danger ofÂ dehydration. Instead, they need to cope while standing still. Plants take up water mostly via transporters in theirÂ roots. TheseÂ transporters are called aquaporins (aqua = water, porin = protein) and only let water pass through. Aquaporins areÂ located in the membranesÂ surrounding theÂ root cells and thus allow water to pass through the cell layers to the interior of the root. When the water reaches the central cylinder itÂ starts its travel upwards to the leaves.
The transporters in the root can be opened, closed or removed from the cell membraneÂ depending on the amount of water in the environment. When there is very little water available, the plant needs a lot of water transporters. In case of flooding the opposite, as few transporters as possible, is not always advantageous. When the aboveground parts of the plant are also under water, little water uptake is advantageous, so the plant will indeed want to restrict root water permeability. However, when the aboveground parts of the plant are not under water, water uptake is still required, since water is lost to evaporation. Thus, the aboveground environment determines the root water permeability that leads to greatest fitness. It is thereforeÂ reasonable to suppose that plants in locations with different kinds of recurrent floodsÂ have adapted by different responses to root flooding.
In theirÂ recent paper,Â ShahzadÂ and co-authors describe their search forÂ genetic factors that are involved in the regulation of root water permeability. In other words, they wanted to find parts of the genome, the heritable ‘code’ that each organism contains, thatÂ regulate this process. To find these parts, theyÂ performed a method called ‘QTL mapping’ in the model plantÂ Arabidopsis thalianaÂ (thale cress). First, the researchers took two populations of thale cress that differed in root water permeability. They then crossed these populations, resulting in offspring with pieces of the genome from each of theÂ parents. The root water permeability of this offspring can beÂ intermediate between the two parentsÂ or something more extreme, that is, close to that of either of the parents. Next, the genetic code ofÂ all the offspring with an extreme phenotype is checked. This offspring will have either parent’s DNA at random at most sites of the genome. However, the piece of the genome that is correlated with root water permeability will come from the parent whose root water permeability was inherited. In this way, Shahzad and co-authors found a region in the genome that explained 15% of the observed differences inÂ root water permeability. This region contained the code ofÂ several genes. They thenÂ changed the code of each these genesÂ one by one to check which gene contributed to root water permeability. This led to the discovery of aÂ gene that negatively affectsÂ root hydraulics, which they namedÂ Hydraulic Conductivity of Root 1Â (HCR1). The activity of this gene is influenced by potassium and oxygen levels, thus integrating diverse signals in the soil.
This brings us back to the strategy decision of plants with flooded roots. To study the effect of HCR1Â on a plant’s response to flooding, the researchers studied a wild-type cultivarÂ of thale cress and plants from the same cultivar with a mutated, that is, nonfunctional,Â HCR1Â gene. When only the roots of these plants are flooded, the mutant grows more and has a higher water content in the aboveground parts of the plant than the wild-type plant. This confirms their hypothesis that a decrease in root water permeability is not always advantageous when roots are flooded. Apparently, inÂ the wild-type plants the decrease in root water permeability by HCR1Â results inÂ less water uptakeÂ whileÂ the aboveground parts of the plant loose water by evaporation, resulting in a dehydrated plant.Â In contrast, in plants recovering from submergence the wild-type plants outperform the mutants. This shows thatÂ HCR1 increases plant growth and shoot water content in plants recovering from submergence.
Thus, the researchers discovered a gene involved in root water permeability regulation:Â HCR1. In further experiments they show that this gene differentially affectsÂ a plant’s fitness inÂ different flooding conditions in one specific cultivar. The researchers subsequently discover that cultivarsÂ of thale cress fromÂ different locations across the world differ in their code ofÂ HCR1.Â These populations also differ in root water permeability, suggesting that changes in the code ofÂ HCR1Â are a way in which plants have adapted to different flooding conditions.Â Future work will have to show whether the correlation between differences inÂ HCR1Â and root water permeability in the different cultivarsÂ is indeedÂ causal. In addition, it remains to be elucidated howÂ HCR1Â decreasesÂ root water permeability in the first place.
Shahzad, Z. et al. (2016) A Potassium-Dependent Oxygen Sensing Pathway Regulates Plant Root Hydraulics. Cell 167, 87â€“98.e14. DOI:Â http://dx.doi.org/10.1016/j.cell.2016.08.068
Disclaimer: blog posts in the category ‘journal club’ are not intended to cover the whole paper discussed. Instead, I discuss the parts that IÂ think areÂ most interesting for a general public. I try my utmost to prevent any mistakes in these blogs, I apologize in advance for any mistakesÂ that I make anyway.