Pollen-Specific SKP1-Like Proteins are Components of Functional SCF Complexes and Essential for Lily Pollen Tube Elongation

The ubiquitinproteasome pathway mediates protein degradation and is involved in diverse aspects of plant development and differentiation, including pollen tube elongation and self-incompatibility. We characterized three lily (Lilium longiflorum) SKP1-like genes, LSK1LSK3, that are specifically expressed in late pollen developmental stages and the elongating pollen tube. The encoded peptide sequences reveal that LSK1LSK3 share high identity with Arabidopsis ASK1 and contain a putative N-terminal CUL1- and a C-terminal F-box-interacting domain. Yeast two-hybrid and in vitro affinity binding assays revealed that the LSKs associate with lily CULLIN1. In addition, the LSK genes can functionally complement the yeast skp1 deletion mutant YDR328C. To investigate their biological functions in pollen tube elongation, an in vivo approach for green fluorescent protein (GFP)-tagged dominant-negative LSK1LSK3 was developed. Microprojectile bombardment with N-terminally truncated LSK1LSK3 (LSK1LSK3GFP) significantly retarded pollen tube elongation in both in vitro germination and in vivo self- and cross-pollination after 12h incubation. Interestingly, elongation of pollen tubes harboring overexpressed LSK2GFP and LSK3GFP was substantially inhibited within the self-pollinated styles. The elongation of most LSK2GFP-transformed pollen tubes could germinate only on the stigmatic surface of self style and showed statistically significant growth arrest as compared with control pollen tubes. Lily exhibits typical gametophytic self-incompatibility via an unknown mechanism, but LSK2 and LSK3 may be involved in this complex machinery. These results suggest critical roles for LSK1LSK3 in regulating fundamental pollen tube elongation in vitro and in vivo and that the 26S proteasome-mediated protein pathway plays an important role in pollen tube elongation.