Polyamines (PAs) are low molecular weight aliphatic nitrogenous bases containing two or more amino groups. They are produced by organisms during metabolism and are present in almost all cells. Because they play important roles in diverse plant growth and developmental processes and in environmental stress responses, they are considered as a new kind of plant biostimulant. With the development of molecular biotechnology techniques, there is increasing evidence that PAs, whether applied exogenously or produced endogenously via genetic engineering, can positively affect plant growth, productivity, and stress tolerance. However, it is still not fully understood how PAs regulate plant growth and stress responses. In this review, we attempt to cover these information gaps and provide a comprehensive and critical assessment of the published literature on the relationships between PAs and plant flowering, embryo development, senescence, and responses to several (mainly abiotic) stresses. The aim of this review is to summarize how PAs improve plants' productivity, and to provide a basis for future research on the mechanism of action of PAs in plant growth and development. Future perspectives for PA research are also suggested.

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Aims: To increase the L-isoleucine production in Corynebacterium glutamicum by overexpressing the global regulator Lrp and the two-component export system BrnFE. Methods and Results:The brnFE operon and the lrp gene were cloned into the shuttle vector pDXW-8 individually or in combination. The constructed plasmids were transformed into an L-isoleucine-producing strain C. glutamicum JHI3-156, and the L-isoleucine production in these different strains was analysed and compared. More L-isoleucine was produced when only Lrp was expressed than when only BrnFE was expressed. Significant increase in L-isoleucine production was observed when Lrp and BrnFE were expressed in combination. Compared to the control strain, L-isoleucine production in JHI3-156/pDXW-8-lrp-brnFE increased 63% in flask cultivation, and the specific yield of L-isoleucine increased 72% in fed-batch fermentation. Conclusions: Both Lrp and BrnFE are important to enhance the L-isoleucine production in C. glutamicum. Significance and Impact of the Study: The results provide useful information to enhance L-isoleucine or other branched-chain amino acid production in C. glutamicum.