29/27 13.7 ±3.8 0.93±0.05 39.9±5.2 21+1a 6 + 09a

29/27 13.7 ±3.8 0.93±0.05 39.9±5.2 21+1a 6 + 09a

a In these cases potatoes grown under the 31/29°C treatment were used; there was no statistical difference between the two cultivars. Vmax: in the enzyme assay the substrates UDP glucose and fructose-6-P were saturating; V|im: the concentrations of both substrates were reduced to about one-third, i.e. they were limiting in relation to the amount of enzyme used. Sucrose-synthase: sucrose + UDP<->UDP glucose+fructose; sucrose-phosphate-synthase: UDP glucose + fructose-6-P<->sucrose-6-P +UDP; AGPase = ADP glucose pyrophosphorylase: glucose-1-P + ATP<->ADP-glucose + PPr; ADP glucose starch; UDPase = UDP glucose pyrophosphorylase: glucose-1-P + UTP<->UDP-glucose + PP.; UDP glucose ->• ->• sucrose

From the selection of data presented in Tables 1, 2 and 3 the following can be derived:

1. Biomass production (growth) is inhibited at temperatures above the optimum. This inhibition was significantly less in the heat-tolerant than in the heat-sensitive variety.

2. Tuber formation was almost completely suppressed in both varieties.

3. Heat-stressed plants exhibited greater growth length than the controls.

4. The total levels of carbohydrate in the leaves of the heat-stressed plants were drastically reduced, primarily due to the starch contents. The comparatively low levels of soluble carbohydrates were not (glucose) or only slightly (sucrose) affected.

5. The sucrose-phosphate synthase activity (SPS) in the leaves was significantly in creased at elevated temperature, whereby there was no difference in the responses of the two cultivars.

6. In the rudimentary tubers of the high-temperature plants the activities of both the enzymes involved in starch metabolism sucrose synthase (active phloem unloading: sucrose+UTP^->UDP glucose+fructo-se+PO and ADP glucose-pyrophosphory-lase (supplies the substrate for starch synthase: glucose-1P+ATP ^->ADP gluco-se+PPO were clearly decreased, while UDP glucose-pyrophosphorylase (sucrose synthesis) showed no noticeable differences in activity. SPS activity of the tubers was also reduced in the high-temperature plants. Thus neither starch nor sucrose synthesis takes place in these plants: the tubers have no sink strength.

The total redirection of protein synthesis in favour of HSPs occurs, in principle, only a few K above the optimal temperature of the organism. In other words: Many organisms already live near the upper temperature limit at which the stress reaction occurs, i.e. where activation of the heat shock genes starts. For example, E. coli has a optimum temperature of 37 °C versus 42 °C for the heat shock reaction; baking yeast: 23/

37 °C; human beings: 37/42-45 °C; Euglena: 25/ 37 °C; tomato: 25/37 °C.

At present ten families of HSPs are known, which are ordered according to their molecular weight (Box 1.3.3). They occur in all compartments of cells.

Most HSPs are constitutively expressed proteins, so-called molecular chaperones ("governesses"), and are present under "normal" condi-

Overview of the heat shock protein families (After Nover and Hdhfeld 1« )6)

HSP family

Cellular localisation


^^ Group of

Cytoplasm/nucleus ER/Golgi

Mitochondria (m)^B

Kj organisms

Was this article helpful?

0 0

Post a comment