Altered sucrose metabolism and plant growth in transgenic Populus tomentosa with altered sucrose synthase PtSS3.

Improvement of wood quality is an important focus of forest genetics and breeding research. Sucrose synthase (SS) catalyzes the reaction of sucrose and uridine diphosphate into uridine diphosphate glucose and fructose. It is a key enzyme involved in cell wall formation during secondary growth by providing the UDP-Glucose substrate for cellulose biosynthesis. In this study, we isolated the single-copy gene PtSS3 from the SS gene family of Populus tomentosa and analyzed its structure. To identify its function in secondary growth, we generated 19 transgenic lines of P. tomentosa using PtSS3 overexpression (OE) and artificial microRNA (amiRNA) constructs. We also performed comprehensive analyses of the transgenic P. tomentosa plants, including phenotypic analyses, quantitative real-time PCR, enzyme activity assays and sugar metabolism. We found significantly higher PtSS3 enzyme activity, fructose, and glucose levels and significantly lower sucrose levels in the stems and leaves of OE-PtSS3 plants. The opposite trend was observed in the amiRNA-PtSS3 lines. Gene expression analyses showed that PtSS3 transcript levels in stems and leaves were up-regulated in the OE-PtSS3 lines and down-regulated in the amiRNA-PtSS3 lines, and the OE-PtSS3 plants grew taller than the wild-type and amiRNA-PtSS3 plants. These findings indicate that PtSS3 plays an important role in sucrose metabolism and growth of trees.