Metabolic Engineering of Plants by Manipulating Polyamine Transport and Biosynthesis
Transport is an essential component of the regulation of polyamines, but to date only one family of Polyamine Uptake Transporters (PUTs) have been characterized in plants, and their impact on polyamine regulation has not been defined. Here we show that knockout mutants of put5 in A. thaliana, promote early flowering and result in plants with smaller leaves, thinner stems, and fewer flowers. In contrast, heterologous expression of the rice gene OsPUT1 in A. thaliana using the Put5 promoter at 22°C produced plants with larger leaves, a two-week delay of flowering and more flowers and siliques. Similar effect on leaf size, flowering time and number of siliques also were observed in transgenic plants with constitutive expression of OsPUT1 or OsPUT3. The delay of flowering was associated with significantly higher levels of spermidine and spermidine conjugates in the leaves prior to flowering. These experiments outline the first genetic evidence for the control of flowering by polyamines. How polyamine levels control the timing of flowering at a molecular level is not yet known, but this delay of flowering has been demonstrated to be upstream of the stimulation of flowering by the gibberellin and temperature sensitive response pathways.
It has been assumed that there exists a single cytosolic pathway for the synthesis of putrescine in A. thaliana. Here we show that A. thaliana and Glycine max, have a chloroplastlocalized putrescine biosynthetic pathway. This pathway comprises of arginine decarboxylase and an agmatinase to synthesize putrescine from arginine. Analysis of expression data suggests that it is the major route of putrescine synthesis in response to stress signals.
Since compartmentation of polyamines has been demonstrated to play an essential role in polyamine homeostasis, the identification of other types of polyamine transporters is a critical knowledge gap. We show here that PDR11 is an important long-distance transporter of polyamines in plants and that OCT5 functions as a vacuolar transporter for polyamines. Taken, these findings will accelerate interest in manipulating polyamine metabolism to generate more stress responsive crop plants.