Vitamins in Fleshy Fruits - Fruit Ripening: Physiology, Signalling and Genomics - page 154

Ethylene

GR

?

C 2 H 4 C 2 H 4

Cu Cu

ER

Ethylene receptors

Ubi

E 3

ETR1, -2, -4-7

ETR3

Autocatalytic
ethylene

CTR

CTR1, -3, -4

Cytosol

EIN2

Yang cycle

P

EIN3

Ubi

EBF

ERE

Nucleus

ERFs

AP2a

ERF6

GCC GCC

GCC

Ripening-related genes

Autocatalytic
ethylene

Aroma/volatile

Carotenoids

Sugar metabolism

Cell-wall metabolism

Ripening responses

Plate 6. Ethylene perception and signal transduction pathway in tomato. Initially, ethylene is perceived by ethylene
receptors (ETR1-7). The Green-Ripe (GR) protein, which is yet to be characterized, causes a reduction in ethylene
sensitivity. The turnover of the receptor proteins is regulated via the ubiquitin-mediated protein degradation pathway.
These receptor proteins interact with downstream negative regulators, i.e. constitutive triple response (CTR) proteins.
In the presence of ethylene, CTR proteins stay inactive. This results in activation of the positive regulator, i.e.
Ethylene Insensitive 2 (EIN2). The signal is transduced to nuclear proteins EIL1-4, which recognize ethylene-
response elements (EREs) in the promoter of senescence and ripening-related genes including ethylene-response
factor (ERF) genes. The transactivation potential of EIN3/EIN3-like (EIL) proteins is regulated in a phosphorylation-
dependent manner, while turnover of these proteins is maintained by two EIN3-binding F-box (EBF) proteins via
the 26S proteasome-mediated protein degradation pathway. ERF proteins further bind to the GCC box in the
promoter of ethylene-responsive genes and regulate ripening responses. Solid brown arrows indicate responses of
the recently identified ERFs, AP2a and ERF6, while black dotted arrows indicate responses of the other members
of the ERF family and their role in various aspects of ripening in tomato.