Maize Lls1 (lethal leaf-spot 1) gene and its Arabidopsis orthologue AtAcd1 have been suggested to encode pheide a oxygenase (PaO), a key enzyme catalyzing chlorophyll breakdown. To further elucidate the molecular mechanism that regulates chlorophyll catabolism during soybean leaf senescence, a soybean Lls1 homolog was cloned and designated as GmLls1 (GenBank Accession No. DQ154009). Database searches using the deduced protein sequence revealed that it was highly ho mologous to Lls1 genes or Lis1 orthologues in Arabidopsis, maize, cowpea and tomato. Structural analysis of the predicted GmLLS1 protein revealed typical Rieske [2Fe-2S] and mononuclear iron-binding domains as well as the C-terminus CxxC motif that were conserved in and featured PaO homologues. RT-PCR results showed that the transcription of GmLls1 was up-regulated in all the three tested senescence systems: the natural leaf senescence process, the dark-induced primary leaf senescence and senescing cotyledons. We have previously described the involvement of an LRR receptor-like kinase (RLK), RLPK2 in regulation of soybean leaf senescence. Here we report that the expression of GmLls1 gene was dramatically down-regulated by the RNAi-mediated suppression of rlpk2 and, as expected, greatly up regulated by the CaMV 35S promoter derived overexpression of this RLK gene.These results suggested that the expression of GmLls1 was controlled by the RLPK2-mediated senescence signaling pathway. The observation that the detached rlpk2-RNAi transgenic leaves exhibited light-dependent necrotic lesions, which featured the Ils1 mutation, further supported this hypothesis.