Auxin signaling and vascular cambium formation enables storage metabolism in cassava tuberous roots

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.