The Stress Signal: Abscisic Acid Function

If a plant detects low water levels in the soil, then it must prevent water loss through the leaves. If ABA triggers the exit of potassium ions from guard cells, then water follows by osmosis, causing the stomata to close and conserve moisture.

If a seed were to sprout during a freezing winter, then the young plant would die. If ABA levels remain high in the seed, then they inhibit the growth signals of gibberellins, effectively maintaining dormancy until spring.

If the concentration of potassium (K+) inside the guard cells drops, then the water potential inside the cell increases. If water then leaves the cell, then the cells go limp and the pore closes.

If ABA is absent, then PP2C binds to and inhibits SnRK2 kinases. If ABA is present and binds to its receptor, then the receptor blocks PP2C, allowing SnRK2 to activate stress-response genes.

If the plant's water balance is threatened by dry soil, salt drawing water out, or freezing conditions, then the plant produces ABA as a universal "stress alarm" to shut down non-essential growth.

If ABA is derived from carotenoid precursors, and if carotenoids are found in chloroplasts and other plastids, then almost every part of the plant (roots, stems, leaves) is capable of synthesizing it.

If ABA is the "brake" that stops growth in a developing seed, then a lack of ABA means the brake is missing. If the brake is missing, then the seed continues to grow into a seedling prematurely on the parent plant.

If gibberellins are the "gas pedal" that causes cells to elongate and seeds to wake up, and if ABA is the "brake" that prevents these actions, then the two hormones are functional antagonists.

If ABA triggers a signaling cascade, then calcium levels rise, causing chloride and potassium to leave the cell. If these solutes leave, then the cell loses water and shrinks, closing the stomatal opening.

If a plant is under stress, then ABA generally inhibits cell division and elongation to save energy. While it might reallocate resources, its primary role is to act as a growth inhibitor, not a promoter.

If ABA is water-soluble and acts as a germination inhibitor, then it acts as a chemical rain gauge. If a heavy rain occurs, then the ABA is leached out of the seed, allowing gibberellins to take over and start growth.

If the roots are the first to "feel" the soil drying out, then they must send a message to the leaves. If ABA is that message, then it travels upward through the water-conducting tissue known as the xylem.

If a stress signal needs to create new proteins like dehydrins, then it must turn on specific genes. If the ABA signaling pathway activates TFs, then those proteins can land on DNA and start the process of transcription.

If early scientists found a chemical that appeared to cause leaves to drop (abscise), then they named it accordingly; however, we now know that ethylene is the main hormone for leaf drop, while ABA's role is mostly stress and dormancy.

If the cell needs to process an ABA signal, then it uses a receptor to detect the hormone, a phosphatase to act as a switch, and a kinase to relay the message. ATP synthase and hemoglobin are not part of this specific signaling pathway.

If ABA is a sesquiterpene, then it is built from the breakdown of larger yellow and orange pigments. If those pigments are carotenoids (like beta-carotene), then they are the required starting material.

If the roots sense dry soil before the leaves lose their internal water, then the ABA signal provides an early warning. If the stomata close early, then the plant prevents a "hydraulic failure" or total dehydration.

If a perennial plant needs to protect its delicate growing tips from frost, then it must seal them. If ABA triggers the development of tough bud scales and stops the growth of the apical meristem, then the plant is prepared for winter.

If a seed must survive being completely dried out, then it needs protective proteins and sugars to stabilize its membranes. If metabolic activity drops to near zero, then the seed can survive in a "suspended" state for years.

If the "water-saving" signal cannot be received, then the stomata will stay open forever (a "wilty" mutant). If the plant constantly "bleeds" water vapor regardless of the environment, then it will quickly dehydrate and perish.