Molecular mechanisms regulating storage root formation in plants

Systems biology approaches are important and successful in understanding complex biological processes through molecular mechanisms involving the interaction of large numbers of genes. However, there are significant limitations in many of these methods (Slovak et al. 2016). For example, the mechanisms of the Si-mediated protection against metal deficiency remain poorly understood. Recently, it has been proposed that Si may act by an interaction with this biometal in the root apoplast contributing to its movement through the plant, mitigating Zn deficiency symptoms (Pascual et al. 2016). Plant adaptation to limited phosphate availability comprises a wide range of responses to internal phosphate sources and to enhance phosphate acquisition. In Arabidopsis, root growth modulation correlates with an altered expression of cell wall modifying enzymes and changes in the pectin network of the phosphate-deprived root tip, indicating that pectins are involved in iron binding and phosphate mobilization (Hoehenwarter et al. 2016). In sweet potato, storage roots develop from adventitious roots present in stem cuttings that serve as propagation material. Nodal position has a significant effect on the developmental status and number of root primordia inside the stem. Environmental conditions affect adventitious roots initiation, development, and capacity to form storage roots (Ma et al. 2015). 

Turnips (Brassica rapa subsp. rapa) represent one of the morphotypes that form tubers and can be used to study the genetics underlying storage organ formation. The enlarged turnip tuber consists of both hypocotyl and root tissue, but the proportion of the two tissues differs between accessions. The ratio of sucrose to fructose and glucose differed among accessions. Vernalization resulted in reduced flowering time and smaller tubers for the Asian turnips (Zhang et al. 2014). The maintenance of the symbiotic characteristics of the incorporated bacterial strains was important in the formation of nodules in the soybean seedlings. A larger number of nodules formed in soybean seedlings from seeds inoculated with rhizobia demonstrated that there is a great alternative to the usual protector inoculants because of its unprecedented capacity to control the release of bacteria (Damasceno et al. 2013). Intraspecific variability in root colonization, extraradical growth pattern, and survival after cold storage of Lactarius deliciosus isolates was determined in pure culture conditions using Pinus pinaster as a host plant, indicating tolerance to cold water storage of L. deliciosus was isolate dependent (Parlade et al. 2011).

Physicochemical stability and biological activity of Withania somnifera root aqueous extract were affected by storage conditions. Temperature and humidity are important for storage conditions and shelf life of ashwagandha formulations (Patil et al. 2010). In response to suboptimal temperatures, plants increase root growth, build-up carbohydrates, and display typical morphological and anatomical changes. For carrot, suboptimal temperature promoted reserve structures, rather than the increase in carbohydrate concentration typical of most temperate annual species and woody perennials (Gonzalez et al. 2009). In this review, we overview transcriptional regulation of storage root formation, proteomic regulation of storage root formation, ethylene regulation of storage root formation, auxin regulation of storage root formation, gene expression regulation of storage root formation, metabolism regulation of storage root formation (Fig. 1). This review describes the basic regulatory principles of storage root formation in plants and will be valuable to research work in storage root growth and development regulation at the molecular level.