| Summary: | ALBINO3 (ALB3) protein functions in the insertion and assembly of thylakoid membrane protein complexes and plays a critical role for chloroplast development in <i>Arabidopsis</i>. However, the biological function of ALB3 homologs in rice, OsALB3, remains elusive. Here, we identified a rice mutant, <i>yellow leaf and lethal1</i> (<i>yll1</i>), that displayed yellow leaves and died at the seedling stage. The content of chlorophyll in <i>yll1</i>, compared with wild type, was significantly decreased. Transmission electron microscopy observation shows that the chloroplast of <i>yll1</i> lacks thylakoid membranes. The causal mutation, which is located in <i>OsALB3</i>, was isolated by Mutmap+ combined with a simple mutation filtering process. Knockout of <i>OsALB3</i> leads to yellow leaves and seedling lethality, mimicking the phenotype of <i>yll1</i>. <i>OsALB3</i> is widely expressed and OsALB3 is chloroplast-localized. Moreover, the content of light-harvesting chlorophyll-binding proteins in <i>yll1</i> is reduced. Together, our study demonstrated the essential role of OsALB3 in chloroplast development and provided clues to the possible conserved molecular function of ALB3 in rice.
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