The old die to protect the young
Flooding gives plants a big blow. Still, if the flooding doesn’t take too long, plants can survive due to a number of survival techniques. One of these is the shedding of old leaves. Now Dutch researchers show in a new study what happens after flooding.
Flooding is a shock for plants. Suddenly they are in a low oxygen state and can’t no longer photosynthesize. This initiates a number of different processes, like the restriction of stored energy use. You might think that resubmerging after a flood is all good news, but it actually gives plants a new shock. Suddenly oxygen flows back in, and due to the damaged roots plants have a drought like experience, even if the soil they are standing in is still quite moist.
If a plant survives flooding depends on a number of things, one of them the plant age. Older plants tend to die faster than younger ones. This same observation can be made for leaves. While the younger leaves recover, the older leaves die after resubmerging from the flood. The researchers wanted to find out why.
Dehydrated
The first thing they did was finding out which genes were active, during the flood and the recovering phase afterwards. They looked at both old and young leaves. Finding that while both leaves activate a flooding response, old leaves activated this response faster. In addition, young leaves were quick to turn those flood responding genes off again when the flood is gone. In old leaves, in contrast, the flood response kept going after resubmerging.
One of the visible differences between young and old leaves is that old leaves after flooding is that the older leaves look like to dehydrate faster. After confirming that these older leaves indeed loose water faster, the question rose what caused it. To control dehydration, plants use the hormone ABA to regulate stomata opening. Old leaves, in contrast to what was expected, did show a good activation of the ABA regulation. And there was no difference either in ABA production. However, young leaves of plants that could not respond to ABA showed a similar dehydration response after flooding as the older leaves. Suggesting that older leaves had become less sensitive to ABA, preventing to adequately prevent stomata opening during a drought.
Misfolding proteins
Another thing the researchers noticed when they looked at which genes where active during flooding and recovery, was the activity of genes that respond to a build up of misfolded proteins. Those genes were especially active in young leaves. This made the researchers wonder if they experienced a greater build up of misfolded proteins than older leaves. But that appeared not to be the case. Both young and old leaves got similar increased levels of misfolded protein build up. However, while the young leaves activated processes to deal with this, the older leaves did not. Resulting in greater damage in older leaves.
This all point towards why older leaves are quicker to die and might point to directions that breeders can look into to make crops more flooding resilient. At the same time I think it is important to also remember another fact. The dying of those older leaves after flooding occurs in a controlled matter, similar to leaves dying in autumn. This allows the recovering plants to reallocate resources to young and developing leaves and roots. Giving the plant time to grow new healthy leaves and restart the photosynthesis.
Literature
Tom Rankenberg, Maria Angelica Sanclemente, Hongtao Zhang, Hendrika ACF Leeggangers, Omar Orozco-Granados, Muthanna Biddanda Devaiah, Janet Rong Chao, Hans van Veen, Frederica L Theodoulou, Rashmi Sasidharan, Distinct processes contributing to post-submergence recovery determine leaf age-specific flood resilience in Arabidopsis, Plant Physiology, 2026;, kiag317, https://doi.org/10.1093/plphys/kiag317
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