Genetic elements controlling starch biosynthesis and secondary growth in root and tuber crops

Root and tuber crops (RTCs) are characterized by having underground organs as their main storage tissues, which they use for nutrient absorption. Simultaneous root thickening via secondary growth and nutrient accumulation are the two main indicators of yield and quality in RTCs. Given the crucial role of RTCs, specifically their high content of starch contributing to meeting the energy requirements of humans worldwide, being an important component of animal feed, having various industrial applications, and being used for biofuel production, researchers have made tremendous efforts toward studying hormonal signal transduction, transcription factor activity, and the expression of genes that regulate secondary growth and starch biosynthesis. These studies have focused primarily on a few RTCs, such as potatoes and sweet potatoes, neglecting important members that exhibit different developmental fates. A holistic and thorough understanding of the molecular mechanisms of secondary growth and starch biosynthesis in these crops is essential to address the ongoing climate change, rapidly increasing food demand, and various industrial requirements for starch-based materials. This paper reviews recent findings regarding hormone signaling, the role of transcription factors in secondary growth and starch biosynthesis, as well as gene expression during these two processes. The review emphasizes the necessity for further exploring these topics and proposes genes for which the expression requires elucidation. Thus, this article paves the way toward conducting focused research and obtaining data that can be adopted by RTC breeders and used as biotechnological tools to enhance the yield and qualities of starch in RTCs.