Drought Disrupts Auxin Localization in Abscission Zone and Modifies Cell Wall Structure Leading to Flower Separation in Yellow Lupine

Drought causes the excessive abscission of flowers in yellow lupine, leading to yield loss and serious economic consequences in agriculture. The structure that determines the time of flower shedding is the abscission zone (AZ). Its functioning depends on the undisturbed auxin movement from the flower to the stem. However, little is known about the mechanism guiding cell–cell adhesion directly in an AZ under water deficit. Therefore, here, we seek a fuller understanding of drought-dependent reactions and check the hypothesis that water limitation in soil disturbs the natural auxin balance within the AZ and, in this way, modifies the cell wall structure, leading to flower separation. Our strategy combined microscopic, biochemical, and chromatography approaches. We show that drought affects indole-3-acetic acid (IAA) distribution and evokes cellular changes, indicating AZ activation and flower abortion. Drought action was manifested by the accumulation of proline in the AZ. Moreover, cell wall-related modifications in response to drought are associated with reorganization of methylated homogalacturonans (HG) in the AZ, and upregulation of pectin methylesterase (PME) and polygalacturonase (PG)—enzymes responsible for pectin remodeling. Another symptom of stress action is the accumulation of hemicelluloses. Our data provide new insights into cell wall remodeling events during drought-induced flower abscission, which is relevant to control plant production.