Handbook of Plant and Crop Physiology

It has been discovered that most inhibitors belonging to this group act in competition for the binding
site of QBlocated in D1; therefore they block the reoxidation of QA, inhibiting electron flow [10–12].
Protein D1 is encoded by the gene psbA, which has been sequenced in cyanobacteria, algae, and plants
and has been confirmed to be conserved in most cases [13].
There are several families of herbicides that act in a similar way:

Ureas

Triazines

Uracils

Phenolics

Among these families, the first three consist of mobile and the last of immobile herbicides.

1. Mobile Herbicides

Herbicides derived from urea and triazine have behaviors as well as action sites in common; therefore, a
superfamily of herbicides including ureas and triazines together can be considered. Within this group, the
two most important herbicides are N -(3,4-dichlorophenyl)-N,N-dimethylurea (DCMU; diuron) (without
doubt the most used and studied herbicide) and atrazine, both acting to displace QBfrom its binding site
[10–12]. The herbicides belonging to these three families tend to act in a noncompetitive way [14]. It is
believed that ureas and triazines have two binding sites, one of them in common [15,16].

2. Immobile Herbicides

Within the phenol-type herbicides, dinitrocresol (DNOC), bromoxynil, and ioxinil are the most used. It is
believed that these compounds interact in the same region of PSII as ureas and triazines, although their
mechanisms of action are different because the phenolic herbicides have a special binding site and act in
a competitive way [17,18]. Interestingly, mutants showing resistance to atrazine and diuron often exhibit
increased sensitivity to phenolic herbicides [6]. These compounds have a pK of about 4; their binding to
the thylakoid membrane is extremely slow at physiological pH but can be markedly accelerated by a
lower pH [19].
Electron flow can also be inhibited by analogues of benzoquinone, the best known of which is DB-
MIB (2,5-dibromo-3-methyl-6-isopropylbenzoquinone) which prevents the oxidation of plastoquinone
[20,21].

776 DE LA ROSA

Figure 2 Schematic model of the PSII core complex. Proteins D1 and D2 bind the redox-active prostetic
groups that are involved in electron flow. CP43 and CP47 are proteins that bind chlorophyll molecules for light
absorption. A is the site of action of herbicides binding quinone.