Root and tuber crops have been an important part of human nutrition since the early days of humanity, providing us with essential carbohydrates, proteins, and vitamins. Today, they are especially important in tropical and subtropical regions of the world, where they help to feed an ever-growing population. Early induction and storage organ size are important agricultural traits, as they determine yield over time. During potato tuberization, environmental and metabolic status are sensed, ensuring proper timing of tuberization mediated by phloem-mobile signals. Coordinated cellular restructuring and expansion growth, as well as controlled storage metabolism in the tuber, are executed. This review summarizes our current understanding of potato tuber development and highlights similarities and differences to important tuberous root crop species like sweetpotato and cassava. Finally, we point out knowledge gaps that need to be filled before a complete picture of storage organ development can emerge. Expected final online publication date for the Annual Review of Plant Biology, Volume 72 is May 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

The controlled distribution of sugars between assimilate-exporting source tissues and sugar-consuming sink tissues is a key element for plant growth and development. Monosaccharide transporters of the SUGAR TRANSPORT PROTEIN (STP) family contribute to the uptake of sugars into sink cells. Here, we report on the characterization of STP7, STP8, and STP12, three previously uncharacterized members of this family in Arabidopsis (). Heterologous expression in yeast () revealed that STP8 and STP12 catalyze the high-affinity proton-dependent uptake of glucose and also accept galactose and mannose. STP12 additionally transports xylose. and are highly expressed in reproductive organs, where their protein products might contribute to sugar uptake into the pollen tube and the embryo sac. and T-DNA insertion lines developed normally, which may point toward functional redundancy with other STPs. In contrast to all other STPs, STP7 does not transport hexoses but is specific for the pentoses l-arabinose and d-xylose. promoter-reporter gene plants showed an expression of especially in tissues with high cell wall turnover, indicating that STP7 might contribute to the uptake and recycling of cell wall sugars. Uptake analyses with radioactive l-arabinose revealed that 11 other STPs are able to transport l-arabinose with high affinity. Hence, functional redundancy might explain the missing-mutant phenotype of two T-DNA insertion lines. Together, these data complete the characterization of the STP family and present the STPs as new l-arabinose transporters for potential biotechnological applications.

Cassava storage roots are among the most important root crops worldwide and represent one of the most consumed staple foods in Sub-Saharan Africa. The vegetatively propagated tropical shrub can form many starchy tuberous roots from its stem. These storage roots are formed through the activation of secondary root growth processes. However, the underlying genetic regulation of storage root development is largely unknown. Here we report on distinct structural and transcriptional changes occurring during the early phases of storage root development. A pronounced increase in auxin-related transcripts and the transcriptional activation of secondary growth factors, as well as a decrease in gibberellin-related transcripts was observed during the early stages of secondary root growth. This was accompanied by increased cell wall biosynthesis, increased most notably during the initial xylem expansion within the root vasculature. Starch storage metabolism was activated only after the formation of the vascular cambium. The formation of non-lignified xylem parenchyma cells and the activation of starch storage metabolism coincided with increased expression of the KNOX/BEL genes KNAT1, PENNYWISE and POUND-FOOLISH, indicating their importance for proper xylem parenchyma function.

SUMMARYCassava's storage roots represent one of the most important sources of nutritional carbohydrates worldwide. Particularly, smallholder farmers in sub‐Saharan Africa depend on this crop plant, where resilient and yield‐improved varieties are of vital importance to support steadily increasing populations. Aided by a growing understanding of the plant's metabolism and physiology, targeted improvement concepts already led to visible gains in recent years. To expand our knowledge and to contribute to these successes, we investigated storage roots of eight cassava genotypes with differential dry matter content from three successive field trials for their proteomic and metabolic profiles. At large, the metabolic focus in storage roots transitioned from cellular growth processes toward carbohydrate and nitrogen storage with increasing dry matter content. This is reflected in higher abundance of proteins related to nucleotide synthesis, protein turnover, and vacuolar energization in low starch genotypes, while proteins involved in sugar conversion and glycolysis were more prevalent in high dry matter genotypes. This shift in metabolic orientation was underlined by a clear transition from oxidative‐ to substrate‐level phosphorylation in high dry matter genotypes. Our analyses highlight metabolic patterns that are consistently and quantitatively associated with high dry matter accumulation in cassava storage roots, providing fundamental understanding of cassava's metabolism as well as a data resource for targeted genetic improvement.