Identifying Differentially Expressed Genes in Pollen from Self-Incompatible Wuzishatangju and Self-Compatible Shatangju Mandarins

Self-incompatibility (SI) is one of the important factors that can result in seedless fruit in Citrus. However, the molecular mechanism of SI in Citrus is not yet clear. In this study, two suppression subtractive hybridization (SSH) libraries (forward, F and reverse, R) were constructed to isolate differentially expressed genes in pollen from Wuzishatangju (SI) and Shatangju (self-compatibility, SC) mandarins. Four hundred and sixty-eight differentially expressed cDNA clones from 2077 positive clones were sequenced and identified. Differentially expressed ESTs are possibly involved in the SI reaction of Wuzishatangju by regulating pollen development, kinase activity, ubiquitin pathway, pollen-pistil interaction, and calcium ion binding. Twenty five SI candidate genes were obtained, six of which displayed specific expression patterns in various organs and stages after self-and cross-pollination. The expression level of the F-box gene (H304) and S1 (F78) in the pollen of Wuzishatangju was 5-fold higher than that in Shatangju pollen. The F-box gene, S1, UBE2, UBE3, RNaseHII, and PCP were obviously up-regulated in pistils at 3 d after self-pollination of Wuzishatangju, approximately 3-, 2-, 10-, 5-, 5-, and 2-fold higher, respectively than that at the same stage after cross-pollination of Wuzishatangju x Shatangju pistils. The potential involvement of these genes in the pollen SI reaction of Wuzishatangju is discussed.