PRESSED FLOWER works downstream of ASYMMETRIC LEAVES 2 to affect sepal flatness in Arabidopsis

The development of flattened organs such as leaves and sepals is essential for proper plant function. While much research has focused on leaf flatness, little is known about how sepals achieve flat organ morphology. Previous study has shown that in Arabidopsis an ASYMMETRIC LEAVES 2 (AS2) gene mutation as2-7D causes ectopic AS2 expression on the abaxial sepal epidermis, which leads to growth discoordination between the two sides of sepals, resulting in outgrowth formation on abaxial sepal epidermis and sepal flatness disruption. Here we report that the PRESSED FLOWER (PRS) works downstream of AS2 in affecting sepal flatness. Genetic analysis showed that PRS mutations suppressed the outgrowth formation on the abaxial sepal epidermis in as2-7D mutant. Through tracking the PRS expression dynamics at a cellular resolution throughout the early developmental stages in WT and as2-7D sepals, we found that on the abaxial epidermis of as2-7D sepals, ectopic AS2 expression up-regulated PRS expression, leading to the epidermal outgrowth initiation. AS2 affected PRS activity on multiple levels: AS2 activated PRS expression through direct binding to PRS promoter region; AS2 also physically interacted with PRS. Our study highlights the complex interplay between AS2 and PRS in modulating sepal flatness.