Cotyledon opening during seedling deetiolation is determined by ABA-mediated splicing regulation.

During seedling deetiolation, plants adjust their development to expose photosynthetic tissues to sunlight, enabling the transition from heterotrophic to autotrophic growth. While various plant hormones are known to influence this process, the role of abscisic acid (ABA) remains unclear. Here, we reveal that ABA plays a major role in controlling the dynamics of cotyledon aperture during seedling deetiolation. In the dark, ABA accumulates in the cotyledons to effectively repress their opening. However, light exposure reverses this effect, allowing the cotyledons to open. Our findings indicate that ABA-mediated regulation of cotyledon dynamics is accompanied by genome-wide rearrangements in both transcriptional and splicing patterns. We demonstrate that ABA-dependent adjustments of cotyledon and splicing dynamics in response to light depend on the positive role of two splicing factors, RS40 and RS41. Moreover, we identify transcriptional and posttranscriptional mechanisms that control the activity of these proteins. Altogether, this work sheds light on the interplay between light and ABA, highlighting cotyledon opening as a new developmental outcome, and identifying alternative splicing as the underlying layer of gene regulation.