Deciduous trees in the northern hemisphere shed their broad leaves to avoid the weight of ice and snow, which could damage branches. Evergreens have narrow needle shaped leaves that are more durable to winter conditions than the broad leaves on deciduous trees and can take being frozen. The abscission process is coupled with the senescence process to recapture vital nutrients in the leaves prior to shedding. Recapturing nitrogen contained in the pigment chlorophyll results in the color change that is observed in fall.
Across the globe, plants also shed their leaves in response to drought. Shedding leaves during drought allows trees to limit their total water load, allowing their survival. Typically, plants will reclaim nitrogen from leaves shed as protection against drought similarly as to when they are shed before winter. Localized infections on a plant can also be shed to limit the spread of the disease as a defense mechanism.
Plants can abscise their leaves because they have specialized tissue regions at their base of the leaving organ that can separate once an intricate and sophisticated signaling pathway is activated. These regions, called abscission zones, are visible by eye on careful examination and have densely packed cells. A cross section shows how abscission zone cells are small than surrounding cells. However, after abscission is activated, the cells expand. After the separation plane is formed the abscission zone cells collapse forming a protective layer. Here, this abscission zone in an Arabidopsis leaf has been activated by drought and its cells go through an expansion and collapse as the separation plane is formed.
At the molecular level, the abscission signaling pathway has two molecular signal amplifiers that allow the abscission signaling switch to be fully turned on rapidly. First there is a MAPK kinase cascade to amplify the signal and second the entire pathway is organized in a positive feedback loop. The pathway also features an extracellular peptide signal perceived by receptor-like kinase since a spatial gradient is as part of the regulatory mechanism. Finally, the abscission signaling circuity is terminated by a circuit breaker-like mechanism similar to the electrical circuit breaker invented by Thomas Edison in 1879. The initiating peptide signal is secreted from the leaving side of the abscission zone so that once the leaf separates the initiating signal is also separated from the plant. This ensures that the violent cellular processes associated with abscission are well localized to the intended area and do not cause harm to other parts of the plant.
I hope the next time you see leaves, fruits, or other parts of the plant falling, perhaps you will remember nature’s ingenuity building that system.