Phytohormones during tuber and storage root formation

Understanding the molecular mechanism of tuber and storage root (SR) development is crucial for designing effective strategies to enhance tuber and SR crops' yield. In the past, numerous molecular factors have been shown to directly or indirectly converge at phytohormones and influence crucial physiological changes happening in the stolon (potato), or adventitious roots transitioning to SRs (sweetpotato). In general, the potato tuber development pathway has been widely studied for the functions of phytohormone-related genes, whereas similar studies in sweetpotato and other SR crops are just emerging. However, a comprehensive understanding of the hormonal network underlying both tuber and SR development is limited. In this review, the updated information regarding the changes of endogenous phytohormone levels, functions of phytohormone metabolism, transport and signalling pathway-associated genes during tuber or SR development are summarized. The expression patterns of different phytohormones (auxin, cytokinin, gibberellin, abscisic acid, ethylene, salicylic acid, jasmonic acid, brassinosteroid and strigolactone) and their pathway components during development of potato tubers and sweetpotato SRs are also highlighted. The regulation of key phytohormone pathway genes during tuber development (or SR developmental stages) by various epigenetic mechanisms, such as DNA methylation, histone modifications, and small non-coding RNAs, especially microRNAs and phased short-interfering RNAs are discussed.