Tuber Development - The Potato: Botany, Production and Uses - page 53

Fig. 4.4. Stolon growth and tuber set in the StGA2ox1 overexpression (a) and StGA2ox1 suppression
clone (b). The presence of small tuber incipients in the overexpression clone is indicated with white
arrows, as well as the slight elongated tuber shape in the suppression clone (Kloosterman et al., 2007).

Fig. 4.5. Schematic overview of proposed hormonal influences on potato tuber formation and stolon
branching. (a) GA 20 has a higher mobility than GA 1 and can be transported down to the stolon, where it is
converted into bioactive GA 1 . During tuber formation, GA 1 levels in the stolon decrease, while auxin is
synthesized in the tuber and transported basally to inhibit stolon axillary outgrowth. Strigolactones
(discussed below) are produced in the roots and transported to the stolon, which can promote axillary
outgrowth. (b) Under noninducing conditions, GA levels in the stolon are high, promoting longitudinal
growth. During tuber initiation, GA levels decrease rapidly while indole- 3- acetic acid (IAA) content
increases. This shift is likely to result in reorientation of the plane of cell division, promoting stolon swelling
and subsequent tuber growth. The concentration and localization of cytokinins (CKs) and abscisic acid
(ABA) during early tuber development is not clear, but both hormones may be involved in establishing
sink strength, impacting sucrose import.

promoting tuber formation. In line with these
findings is the observation that when StGA20ox1
is overexpressed, transgenic plants are taller
and show delayed tuber formation under indu-
cing conditions as more GA 20 becomes available

for transport towards the stolon, where it can
be converted into bioactive GAs (Carrera et al.,
2000).
Several GA metabolism genes have been
shown to be under the control of feedback or