Analysis of the rice SHORT-ROOT5 gene revealed functional diversification of plant neutral/alkaline invertase family

Neutral/alkaline invertase (Inv-N) catalyzes the hydrolysis of sucrose to produce glucose and fructose. Although the rice genome contains eight Inv-N-like genes, their functions in plant development are unknown. We previously described a rice mutant, srt5, which exhibits extremely stunted post-embryonic root growth that is rescuable by metabolizable sugars. In the present study, we performed a functional analysis of the SRT5 gene. We showed that SRT5 encodes a putative cytosolic Inv-N that cleaves sucrose at pH 7.0 and 8.0. An SRT5-GFP fusion protein was localized to the cytosol. Sucrose levels in srt5 root cells were elevated, consistent with a crucial role for SRT5 in cytosolic sucrose cleavage at early root developmental stages. SRT5 was found to be ubiquitously expressed in various plant organs, whereas the other seven rice Inv-Ns were differentially expressed, suggesting that the functions of these latter proteins are distinct from that of SRT5. In support of this view, molecular evolutionary analysis revealed that all of the most closely related paralogs of SRT5 (OsNIN5, OsNIN6, and OsNIN7) have multiple non-conservative amino acid substitutions not found in SRT5. These results suggest that SRT5 is the key isoform of Inv-Ns required for carbon and energy supply during early root development.