Isolation and gene expression analysis of a papain-type cysteine protease in thermogenic skunk cabbage (Symplocarpus renifolius)

Molecular characterization of SrSTP14, a sugar transporter from thermogenic skunk cabbage, and its possible role in developing pollen

In floral thermogenesis, sugars play an important role not only as energy providers but also as growth and development facilitators. Yet, the mechanisms underlying sugar translocation and transport in thermogenic plants remain to be studied. Asian skunk cabbage (Symplocarpus renifolius) is a species that can produce durable and intense heat in its reproductive organ, the spadix. Significant morphological and developmental changes in the stamen are well-characterized in this plant. In this study, we focused on the sugar transporters (STPs), SrSTP1 and SrSTP14, whose genes were identified by RNA-seq as the upregulated STPs during thermogenesis. Real-time PCR confirmed that mRNA expression of both STP genes was increased from the pre-thermogenic to the thermogenic stage in the spadix, where it is predominantly expressed in the stamen. SrSTP1 and SrSTP14 complemented the growth defects of a hexose transporter-deficient yeast strain, EBY4000, on media containing 0.02, 0.2, and 2% (w/v) glucose and galactose. Using a recently developed transient expression system in skunk cabbage leaf protoplasts, we revealed that SrSTP1 and SrSTP14-GFP fusion proteins were mainly localized to the plasma membrane. To dig further into the functional analysis of SrSTPs, tissue-specific localization of SrSTPs was investigated by in situ hybridization. Using probes for SrSTP14, mRNA expression was observed in the microspores within the developing anther at the thermogenic female stage. These results indicate that SrSTP1 and SrSTP14 transport hexoses (e.g., glucose and galactose) at the plasma membrane and suggest that SrSTP14 may play a role in pollen development through the uptake of hexoses into pollen precursor cells.

Characterization of two PEBP genes, SrFT and SrMFT, in thermogenic skunk cabbage (Symplocarpus renifolius)

Floral thermogenesis has been found in dozens of primitive seed plants and the reproductive organs in these plants produce heat during anthesis. Thus, characterization of the molecular mechanisms underlying flowering is required to fully understand the role of thermogenesis, but this aspect of thermogenic plant development is largely unknown. In this study, extensive database searches and cloning experiments suggest that thermogenic skunk cabbage (Symplocarpus renifolius), which is a member of the family Araceae, possesses two genes encoding phosphatidyl ethanolamine-binding proteins (PEBP), FLOWERING LOCUS T (SrFT) and MOTHER OF FT AND TFL1 (SrMFT). Functional analyses of SrFT and SrMFT in Arabidopsis indicate that SrFT promotes flowering, whereas SrMFT does not. In S. renifolius, the stage- and tissue-specific expression of SrFT was more evident than that of SrMFT. SrFT was highly expressed in flowers and leaves and was mainly localized in fibrovascular tissues. In addition, microarray analysis revealed that, within floral tissues, SrFT was co-regulated with the genes associated with cellular respiration and mitochondrial function, including ALTERNATIVE OXIDASE gene proposed to play a major role in floral thermogenesis. Taken together, these data suggest that, among the PEBP genes, SrFT plays a role in flowering and floral development in the thermogenic skunk cabbage.