Isolation and characterization of an isocitrate lyase gene from senescent leaves of sweet potato (Ipomoea batatas cv. Tainong 57)

Isocitrate lyase and malate synthase are two key enzymes of the glyoxylate cycle, and are associated with lipid degradation and gluconeogenesis in microorganisms, plants, nematodes, and animals. Although genes of isocitrate lyase have been cloned from seedling cotyledons of several plants, none were isolated from the senescent leaves. In this study, sweet potato yellow leaves with disassembled chloroplast thylakoids and remarkable osmiophilic globule accumulation in stroma were used as material. Using both P.-b. subtractive hybridization and RACE PCR, a 2.1 kb full-length splCL cDNA was cloned. The open reading frame contained 1728 nucleotides (576 amino acids) and exhibited more than 75 % sequence identity with plant cotyledon isocitrate lyases of tomato, upland cotton, cucumber, castor bean, rape, soybean, and loblolly pine. The splCL-encoded protein contained the Leu-169, Lys-170, Pro-171 (LKP) and Thr-210, Lys-211, Lys-212 (TKK) motifs, with a reported substrate binding domain function, as well as a putative peroxisomal targeting signal (PTS) Ala-Arg-Met (ARM) tripeptide at the C-terminus. Its mRNA accumulated exclusively in the senescing and completely yellow leaves, but not in the green leaves, roots, or stems. Hence, the sweet potato splCL is the first isocitrate lyase isolated from senescent leaves. The data may provide molecular evidence to support the notion that glyoxylate cycle, a metabolic pathway utilized in cotyledons for postgerminative growth of oilseeds, is also involved in lipid degradation and gluconeogenesis of senescent leaves.