Young Seedling Stripe1 (YSS1) was characterized as an important regulator of plastid-encoded plastid RNA polymerase (PEP) activity essential for chloroplast development at rice seedling stage. Chloroplast development is coordinately regulated by plastid- and nuclear-encoding genes. Although a few regulators have been reported to be involved in chloroplast development， new factors remain to be identified， given the complexity of this process. Here， we report the characterization of a temperature-sensitive young seedling stripe1 (yss1) rice mutant， which develops striated leaves at the seedling stage， particularly in leaf 3， but produces wild-type leaves in leaf 5 and onwards. The chlorotic leaves have decreased chlorophyll (Chls) accumulation and impaired chloroplast structure. Positional cloning combined with sequencing demonstrated that aberrant splicing of the 8th intron in YSS1 gene， due to a single nucleotide deletion around splicing donor site， leads to decreased expression of YSS1 and accumulation of an 8th intron-retained yss1 transcript. Furthermore， complementation test revealed that downregulation of YSS1 but not accumulation of yss1 transcript confers yss1 mutant phenotype. YSS1 encodes a chloroplast nucleoid-localized protein belonging to the DUF3727 superfamily. Expression analysis showed that YSS1 gene is more expressed in newly expanded leaves， and distinctly up-regulated as temperatures increase and by light stimulus. PEP- and nuclear-encoded phage-type RNA polymerase (NEP)-dependent genes are separately down-regulated and up-regulated in yss1 mutant， indicating that PEP activity may be impaired. Furthermore， levels of chloroplast proteins are mostly reduced in yss1 seedlings. Together， our findings identify YSS1 as a novel regulator of PEP activity essential for chloroplast development at rice seedling stage.