Instant Notes: Plant Biology

under gravity and respond by allowing a calcium gradient to be set up. It may

also be that stretch activated channels in the ER respond to moving amylo-

plasts.

Transmission of the signal and growth response

Auxinis suggested to transmit the signal from the statocytes to the growth

zone. In roots, high levels of auxin on the lower side inhibit growth, while lower

levels on the upper side permit it; in shoots, higher levels on the lower side

stimulate growth while lower levels on the upper side inhibit it. Experiments

using plants with promoter-reporter gene constructs (Fig. 1; see also Topic E2)

G2 – Tropisms 89

Tobacco seedling expressing

SAUR–GUSgene construct:

No curvature Curvature

EqualGUS expression

on both sides of stem

• implies equal
  distribution of auxin

More GUS expression

on shaded side of stem

• implies more auxin
  on shaded side

Dark Unilateral

light

SAUR GUS

Auxin-responsive

promoter

Reporter

gene

(a)

(b) (c)

Time (h) Curvature (o) % GUS expression on the dark side

0050

12 30 65

8 + auxin transport inhibitor 0 50

Fig. 1. Use of tobacco seedlings transformed with a construct of SAUR(small auxin up-

regulated) promoter (which responds to auxin) and a β-glucuronidase(GUS)gene to detect

altered auxin concentrations in phototropism (a). The transgenic plants created were either

kept in darkness (b) or exposed to unilateral light (c) and the concentration of auxin measured

by estimating the amount of purple-colored product formed when the seedlings were soaked

in the substrate for GUS. As the GUSgene is regulated by the SAURpromoter, its expression

is proportional to the concentration of auxin; so high auxin ultimately results in a more intense

purple coloration.