Photodamage and repair of Photosystem II

Plants are remarkable organisms in their ability to absorb light energy and convert it into chemical energy. Unlike animals, however, plants cannot move away from unfavourable environmental conditions and may not be able to process all the excitation energy to which they are exposed. This excess energy will lead to the generation of active oxygen species and membrane protein damage if it is not dissipated effectively. Such photodamage appears to be mainly confined to the D1 protein in the core Photosystem II reaction centre complex and inactivation can be rapidly detected following minutes of exposure to high light energies.

Fortunately, a damage-repair cycle (Figure 5.9) is present in which the PS II reaction centre is disassembled, a new D1 protein is synthesised and the complex reassembled and activity is restored (Melis, 1999). The precise mechanism of photodamage remains unclear. Perhaps an over-reduction of the quinone acceptor is the cause, although the presence of two P -carotene molecules in the reaction centre appears to be crucial to the process. Without these molecules, the breakdown of protein D1 is favoured. While the precise role of these pigments is uncertain, it is clear that the inhibition of carotenoid biosynthesis by

Functioning PS II

High light XA/V/XA/Vi flux density ^W^W

Functioning PS II

High light XA/V/XA/Vi flux density ^W^W

D1

D2

V

Damaged and inactivated PS II

Disassembly of PS II

Damaged and inactivated PS II

Disassembly of PS II

Synthesis of D1

Reassembly of PS II

Functioning PS II Figure 5.9 Damage-repair cycle in Photosystem II.

lycopene cyclase or phytoene desaturase inhibitors leads initially to an inhibition of pho-tosynthetic electron flow and later to pigment bleaching (Fedtke et al., 2001).

Herbicidal action as a consequence of binding to the D1 protein is now well known. Activity is effective and well characterised both in crops and weeds. One disadvantage, however, is the relatively high application rates required for phytotoxicity, typically 2-4 kg ha-1 with atrazine. Duff and colleagues (Fabbri et al., 2005; Duff et al., 2007) at Monsanto have suggested that the inhibition of D1 protein biosynthesis could be a better target for herbicide action at gram rather than kilogram doses. They have discovered that a carboxy-terminal processing protease, CtpA, is a low-abundance thylakoid enzyme that catalyses the conversion of pre-D1 into the active form by cleaving the 9-C terminal residues. They expressed a recombinant form of the enzyme in Escherichia coli (rCtpA), purified it and used it in a high throughput screen for CtpA inhibitors. Lead compounds were studied in vitro and most were shown to be competitive inhibitors with Ki values in the range of 2-50 ^M. These authors conclude that CtpA inhibitors could become a new generation of effective herbicides.

5.5 Structures and uses of Photosystem II inhibitors

Observations made in recent decades have shown that the quinone-herbicide binding domain in PS II is highly conserved throughout the plant kingdom. Since this domain involves the interaction of many amino acids in the D1 protein, it is not too surprising that a large number of chemically dissimilar molecules have been found to bind at this site, thus preventing QB reduction. Many have become highly successful herbicides and so detailed quantitative structure-activity relationships have been attempted with the PS II inhibitors. Trebst et al. (1984) have suggested that the ureas and triazines, now classified in the 'serine' family, all have a lipophilic group in close association with an sp2 hybrid and an essential positive charge (Figure 5.10). Examples of herbicide classes in the 'serine' family are shown in Figure 5.11.

Trebst and colleagues (1984) considered that the essential features of the phenol-type 'histidine' herbicides are those shown in Figure 5.12. Examples of herbicides of this type are shown in Figure 5.13.

A few major examples of PS II herbicides in commercial use today are presented in Figure 5.14. It is noteworthy that about 50% of all the pesticides cited in The Pesticide Manual (Tomlin, 2000) are inhibitors of photosynthetic electron transport, although recent legislation referred to in Chapter 2 may reduce this value in years to come.

5.6 Interference with electron flow at Photosystem I

The bipyridinium compounds paraquat and diquat are well known as potent, total herbicides with a contact action. Paraquat was known in the 1930s as methyl viologen and used

Figure 5.10 Detail of the environment surrounding the lipophilic group in the 'serine' family of herbicides.
Figure 5.11 Examples of herbicide classes in the serine family.
Figure 5.12 Essential features of the phenol-type ' histidine' herbicides (after Trebst et al., 1984).
Figure 5.13 Structures of the ' histidine' family of herbicides: the hydroxybenzonitriles bromoxynil and the nitrophenol dinoseb.

as an oxidation-reduction indicator, but it was not until the mid-1950s that its herbicidal properties were discovered. Paraquat and diquat, with redox potentials of -0.446 V and -0.349 V respectively, exist as dications which can accept an electron from one of the iron-sulphur proteins near ferredoxin (-0.420V) on the stromal side of PS I to form a stable free radical.

This radical is re-oxidised by molecular oxygen to produce active oxygen species, and the paraquat is then free to accept another electron from near ferredoxin (Figure 5.15).

In this way a futile cycle operates, whereby the dication is constantly regenerated in the light and active oxygen species kill the plant (see section 5.8).

Although many molecules have been tested, only paraquat and diquat have found commercial use. They appear to possess unique properties for bipyridinium-type action, namely:

(a) they are readily soluble in water and are stable at physiological pH ranges;

(b) the herbicidal dication can be tightly bound by plants, organic matter or soil clay minerals, and so is rapidly inactivated and unavailable to plants;

(c) they have suitable negative oxidation-reduction potentials to accept a single electron from PS I;

Figure 5.14 A selection of herbicides that inhibit photosystem II electron flow. UREAS USES

Diuron

Diuron

General weed control in non-crop areas.

Selective, pre-emergent control of annual weeds in alfalfa, maize, cotton, pineapple, sugar cane and sorghum. Soil active and residual. Readily absorbed by roots and leaves and rapidly translocated.

Isoproturon

Isoproturon

Control of annual grasses and broadleaf weeds in barley, rye and wheat. Soil active and residual. Can leach to groundwater.

Chlorotoluron

Chlorotoluron

As isoproturon.

Linuron

Linuron

Selective, pre-emergent control of annual weeds in potatoes, carrots, peas, beans, cotton, maize, soybean and winter wheat. Soil active and residual.

Figure 5.14 (Continued)

TRIAZINES Atrazine

TRIAZINES Atrazine

Simazine
TRIAZINONES Metribuzin

USES

Selective pre- and post-emergent control of annual weeds in maize, sorghum, sugar cane, raspberries and roses. Residual, can persist in soils at high soil pH. Can leach to groundwater.

Total weed control in non-crop areas. Pre-emergent or early post-emergent control of annual weeds in beans and maize, fruit bushes, canes and trees.

Pre-emergent control of annual weeds in cotton, peas and carrots. Post-emergent weed control in many vegetable crops and sunflowers.

Pre-emergent control of blackgrass and broadleaf weed seedlings in winter wheat, barley, sugar cane and sunflowers.

Pre- or post-emergent control of annual weeds in lucerne, potatoes, soybean, tomatoes, asparagus and sugar beet. Readily absorbed by roots and leaves and rapidly translocated.

Pre- or post-emergent control of annual weeds in sugar beet and fodder beet.

(Continued)

Figure 5.14 (Continued)

URACILS Lenacil

Terbacil
ANILIDES Propanil CI

PHENYLCARBAMATES Phenmedipham

Post-emergent selective control of most broadleaf weeds in sugar beet. Readily absorbed by foliage but poorly translocated.

MISCELLANEOUS Bentazone

MISCELLANEOUS Bentazone

USES

Pre-emergent control of annual weeds in sugar beet and fodder beet. Readily absorbed by roots and leaves and rapidly translocated.

Soil applied for weed control in sugar cane, strawberry, peach, citrus and apple.

Post-emergent control of barnyard grass, sedges and some broadleaf weeds in rice, maize and wheat. Contact action.

Controls annual weeds in carrots, celery, strawberries, tomato and soft fruit.

Post-emergent selective control of most broadleaf weeds in sugar beet. Readily absorbed by foliage but poorly translocated.

Post-emergent selective control of broadleaf weeds in wheat, barley, maize, rice, peas, beans and soybean. Absorbed by leaves and rapidly transported in the transpiration stream.

Figure 5.15 How paraquat and diquat act at Photosystem I to give rise to active oxygen species.

(d) the reduced dication is stable and readily oxidised by molecular oxygen; and

(e) the molecule is always available to accept further electrons from PS I.

On a more cautionary note, the bipyridinium herbicides are toxic to animals including mammals, as well as plants. Consequently, their use is now banned in some countries and becoming increasingly limited in others. Although the manufacturers have added bright colourants and powerful emetics to the formulation, intelligent and careful use of these molecules is called for.

There have been many attempts to develop new herbicides based on this site of action, of which a few are noted here. Itoh and Iwaki ( 1989) have proposed a herbicide-binding site in PS I, designated the Qv or phylloquinone-binding site. They found that phylloquinone (Ax) could be reversibly extracted from PS I with diethyl ether, leaving the photochemical charge separation almost intact. Reconstitution experiments identified potential inhibitors at this site, which appears to be more hydrophobic than equivalent quinone sites in PS II, and diuron and atrazine, for example, bind weakly to it.

A recent report by Smith and colleagues (2005) demonstrated impressive in vitro activity of indolizine-5,8-diones, a novel class of quinone-(ike compounds, though volatility and photoinstability conspired to reduce foliar persistence, and so these compounds have not been developed commercially.

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