The molecular memory of plants

Heat stress procures the capacity for remembering in plant cells

How a plant grows depends on its genetic blueprint on the one hand, and on how the environment influences molecular and physiological processes on the other. Of particular importance are the so-called meristems, those divisible tissues that contain the plant stem cells. The shoot apical meristem is crucial for the growth of the organs above ground. Researchers led by Bernd Mueller-Roeber at the Max Planck Institute of Molecular Plant Physiology and the University of Potsdam have investigated how the shoot apical meristem responds to heat stress and found that it develops a stress memory. Because of this memory capacity, it is able to respond better to another subsequent stress and even survive lethal stress events.

The left part of the photo shows Arabidopsis seedlings that were initially treated with a moderate heat stress (Priming) followed by high temperatures (44°C). The right part of the figure shows seedlings that were exposed to high temperatures without priming. The experiments make it clear that molecular heat stress (HS) memory allows plants to survive an otherwise lethal high-temperature stress (44°C) when they are subject to moderate temperature stress (priming) days before.

 

Many external factors determine the growth of a plant. Light, nutrient and water supply, temperature, and the availability of carbon dioxide. In particular, sudden environmental stress, such as heat, has a negative effect on the plant and its growth. If plant cells are able not only to respond to one stress but also to remember it, they are enabled to respond better to a second, subsequent stress and even survive an otherwise lethal stress. However, little is known about the memory capacity of plant cells.

Having a stress memory is an extremely important adaptation to fluctuating environmental conditions. One warm day is very likely to be followed by other warm or even hot days. But how do plants do this without having a brain? "In general, individual cells are able to respond to an acute stress. This happens at different levels. For example, metabolism can be altered or corresponding genes can be switched on or off, resulting in the synthesis or reduction of crucial proteins in the cell," explains Justyna Jadwiga Olas, first author of the study.

The shoot apex meristem has its own heat memory

The shoot apical meristem with its stem cells generates its own strong heat stress memory at the transcriptional level, independent of other neighboring organs. "Because of its function, the meristem is a particularly important tissue that must remain functional for plant growth to be maintained. We have now shown that genes of carbohydrate metabolism, protein folding and meristem maintenance serve as important components of heat stress memory in the shoot apex," Bernd Mueller-Roeber summarizes.

The meristem itself has no chloroplasts in which photosynthesis and thus sugar production take place. Accordingly, it is dependent on the sugar supply of the surrounding leaves. In the studies, it became clear that sugar availability is an important factor for the temperature memory of the meristem.

In a comprehensive gene expression analysis, the researchers were able to identify responsible genes of the heat stress memory. These encode an aldolase (FBA6), an enzyme of carbohydrate metabolism, which is thus responsible for energy supply, a heat shock protein (HSP17.6A), which protects other proteins from destruction under heat, and two stem cell regulators (CLAVATA 1 and 3). "Compared to other organs, such as leaves, we showed that there is independent, tissue-specific regulation in the meristem. Both the components and the rates of the individual reactions differ greatly from the reactions in other organs," describes Justyna Jadwiga Olas.

Heat stress limits growth and yield

The shoot apical meristem is primable by a prior moderate heat stress and displays a distinct transcriptional thermomemory which is crucially dependent on sugar availability and primary carbon metabolism gene FBA6 (left panel). Thermomemory is controlled by HSFA2 that is required for the initial transcriptional activation of memory genes (right panel).
 

The fact that plants build up a memory through an initial mild stress is particularly important from an ecological point of view. A later severe stress, which could even lead to death, can thus be mitigated. Above all, protection of the meristem is important so that the plant can continue to grow and form new leaves afterwards, especially when dead organs need to be replaced. The renewal ability of stem cells makes this possible.

"It also became clear that because of heat memory, a second, more severe stress only delays plant growth, but does not inhibit it in the long term. Growth inhibition during a heat period makes absolute sense and is necessary for survival, as this also prevents flower formation during this time and thus counteracts a possible loss of yield," Mueller-Roeber explains. The ability of a plant to form a memory and thus adapt to upcoming stress conditions is therefore of great interest, especially for agriculture, in order to be able to breed more climate-resistant plants in the future. A more precise understanding of the underlying mechanisms could form the basis for future variety development.

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