Sucrose promotes branch-thorn occurrence of Lycium ruthenicum through dual effects of energy and signal.

Lycium ruthenicum is an important ecoeconomic thorny shrub. In this study, the L. ruthenicum plants of a clone showed two types of 'fewer leaves without thorn' and 'more leaves with thorns' under the same condition after transplanting. Microscopic observation revealed that the apical buds of the thornless (Thless) and thorny (Thorny) branches should be selected as materials for further study. RNA-Seq analysis showed that the KEGG pathway of starch and sucrose metabolism and differentially expressed genes of sugar transport protein 13 (SUT13), sucrose synthase (SUS), trehalose-phosphate phosphatase (TPP) and trehalose-phosphate synthase (TPS) were significantly up-regulated in Thorny. The results of qRT-PCR confirmed the accuracy and credibility of the RNA-Seq. The content of sucrose in Thorny was significantly higher than that in Thless, but the content of trehalose-6-phosphate (T6P) was opposite. Leaf-clipping treatments reduced sucrose content and inhibited the occurrence/development of branch-thorns; exogenous sucrose of 16 g l-1 significantly promoted the occurrence and growth of branch-thorns, and the promotion effects were significantly higher than those treated with non-metabolizable sucrose analogs (isomaltolose and melitose). These findings suggested that sucrose might play a dual role of energy and signal in the occurrence of branch-thorns. Higher sucrose supply in apical buds from more leaves promoted the occurrence of branch-thorns via a lower content of T6P and higher expression levels of SUS, TPP and TPS, whereas fewer leaves inhibited the occurrence. The molecular hypothesis model of the leaf number/sucrose supply regulating the occurrence of branch-thorns in L. ruthenicum was established in the study, which provides foundation for breeding both Thless L. ruthenicum and Thless types of other species.