Jun-Ho Ha,^1,4 Hyo-Jun Lee,^1,4 Jae-Hoon Jung,² and Chung-Mo Park<sup>1,3,5,*

1</sup>Department of Chemistry, Seoul National University, Seoul 151-742, Korea
²Sainsbury Laboratory, University of Cambridge, Cambridge CB2 1LR, UK
³Plant Genomics and Breeding Institute, Seoul National University, Seoul 151-742, Korea
⁴Co-first author
⁵Lead Contact

^*Correspondence : Chung-Mo Park

Summary
Chlorophyll biosynthesis enables autotrophic development of developing seedlings. Upon light exposure, the chlorophyll precursor protochlorophyllide produces reactive oxygen species (ROS). Developing seedlings acquire photosynthetic competence through the action of protochlorophyllide oxidoreductases (PORs) that convert protochlorophyllide to chlorophyllide, reducing ROS production that would otherwise induce cellular damage and chlorophyll bleaching. Here, we show that FCA mediates the thermostabilization of PORs to trigger the conversion of protochlorophyllide to chlorophyllide in developing seedlings. FCA also facilitates the thermal induction of POR genes through histone acetylation that promotes the accessibility of RNA polymerases to the gene promoters. The combined action of FCA maintains PORs at warm temperatures, shifting the chlorophyll-ROS balance toward autotrophic development. We propose that the FCA-mediated thermal adaptation of autotrophic development allows developing seedlings to cope with the heat-absorbing soil surface layer under natural conditions. The thermal adaptive mechanism would provide a potential basis for studying crop performance at warm temperatures.