When NaCl knocks on plant’s door

Plant & zo

The science of plants and more

When NaCl knocks on plant’s door

When a plant root comes across a patch of soil with a higher amount of salt, one of the first things it does is sending up a wave of Ca²⁺ signal up to the shoot. Depending of the size of the plant this Ca²⁺ wave can reach the top within minutes. There it will regulate gene expression and protein activity to prepare the shoot for the osmotic and salt stress. For example activating the expression of genes that encode Na⁺ channels that will either exclude Na+ from the cell or compartmentalise it.

As plants don’t have a nervous system like animals do, signals between distant part of the plant need to travel from cell to cell, in general this is a relatively slow process. However some signals travel faster than others. Ca²⁺ are versatile second messengers, and in contrast to some other signalling molecules, Ca²⁺ has been shown to be able to travel quickly from cell to cell. In general the Ca²⁺ concentration in the cytoplasm is relatively low, however cells have high levels of Ca²⁺ stored in storage compartments. Upon a stimulus this stored Ca²⁺ will be released to increase the cytoplasmic Ca²⁺ levels which in turn will activate a set of proteins that sent a signal out of the cell that is picked up by neighbouring cells, setting off the whole signalling cascade again.

So what will happen when the root comes across a patch of high saline soil. The first thing it will notice that there is an increase in Na⁺ ions. These Na⁺ ions will bind to a negatively charged membrane lipid called glycosyl inositol phosphorylceramide or GIPC for short. The binding of Na⁺ to GIPC then activates an GIPC-associated Ca²⁺ channel. Resulting in an influx of Ca²⁺ ions into the cytosol. One of the things these newly entered Ca²⁺ ions will trigger is the signalling cascade triggering the Ca²⁺ wave towards distal parts of the plant, telling the rest of the plant to prepare for an influx of Na⁺ and Cl^– ions.

The best model to date indicates that to create this Ca²⁺ wave the plant makes use of H₂O₂ which can feely diffuse between cells. Upon entering the cell Ca²⁺ is detected by CPKs and CBL-CIPKs which in turn phosphorylate a plasma membrane bound RBOH NADPH oxidase. The phosphorylation of RBOH results in its activation and the production of H₂O₂, which will then diffuse to the neighbouring cells. There it will trigger Ca²⁺ release from the vacuole by either directly or indirectly activating the vacuolar Two-Pore Channel 1 or TPC1. This newly released Ca²⁺ will again result in the activation of RBOH. Resulting in a self-sustaining cell-to cell-propagating long-distance Ca²⁺ wave.

ca2+ wave corrected — Model of the Ca²⁺ wave signaling cascade, showing that it keeps going once it started.

While researchers are still trying to get the details right and verify this model beyond any doubt, it fits nicely with our current evidence. Part of the difficulty is that plants have many CPKs, CBL-CIPKs and RBOHs which in addition forming the Ca²⁺ wave are involved in many Ca²⁺ and ROS signalling pathways. making deciphering their interactions and effects complex and time consuming.