During the transition from seed to seedling, emerging embryos strategically balance available resources between building up defenses against environmental threats and initiating the developmental program that promotes the switch to autotrophy. We present evidence of a critical role for the phenylalanine (Phe) biosynthetic activity of AROGENATE DEHYDRATASE3 (ADT3) in coordinating reactive oxygen species (ROS) homeostasis and cotyledon development in etiolated Arabidopsis (Arabidopsis thaliana) seedlings. We show that ADT3 is expressed in the cotyledon and shoot apical meristem, mainly in the cytosol, and that the epidermis of adt3 cotyledons contains higher levels of ROS Genome-wide proteomics of the adt3 mutant revealed a general down-regulation of plastidic proteins and ROS-scavenging enzymes, corroborating the hypothesis that the ADT3 supply of Phe is required to control ROS concentration and distribution to protect cellular components. In addition, loss of ADT3 disrupts cotyledon epidermal patterning by affecting the number and expansion of pavement cells and stomata cell fate specification; we also observed severe alterations in mesophyll cells, which lack oil bodies and normal plastids. Interestingly, up-regulation of the pathway leading to cuticle production is accompanied by an abnormal cuticle structure and/or deposition in the adt3 mutant. Such impairment results in an increase in cell permeability and provides a link to understand the cell defects in the adt3 cotyledon epidermis. We suggest an additional role of Phe in supplying nutrients to the young seedling.
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