Assimilatory Starch for Growth in Arabidopsis thaliana Wild-Type
125
In the present study, three types of Arabidopsis mutants were used to
have three levels of assimilatory starch under various conditions of light and
nutrition. The performance of a wild type (WT) was compared with the
growth response of (1) a mutant deficient in phosphogluco-mutase in the
chloroplast (PGM mutant) (Caspar et al. 1986), (2) a mutant showing
only 7% of the wild-type activity of ADP glucose phosphorylase (7%
ADPGPPase mutant) in which starch production is reduced by 70%, and (3)
a progeny of a cross between wild-type and 7% ADPGPPase mutant with a
55% ADP glucose phosphorylase activity and a 20% reduction in starch
formation (55% ADPGPPase mutant). Using these mutants, we investigated
whether the formation of assimilatory starch decreases or increases relative
growth rates.
6.3 The Diurnal Starch Turnover
During the course of a day, the experimental plants showed large variations
in assimilatory starch formation and turnover (Fig. 6.2). In the wild type,
starch concentration increased at high nitrogen supply (6mM NH4N0 3 )
from about 70 !lmol g -1 fw in the morning to about 200 !lmol g -1 fw in the
evening. At low N supply (0.1 mM NH4N03), the daily increase of starch
was similar, but it occurred from a higher base line starch level, i.e.,
starch concentrations in the morning were 200 !lmol g -1 fw as compared to
70 !lmol g-1 fw at high N supply (Fig. 6.2A). The daily increase in starch
concentration decreased from 130 !lmol g-l fw at high light (photon flux
density, PFD = 600!lmolm-2s-1) to about 50!lmolg-1fw in low light
(PFD = 80 !lmol m- 2 S-l) (Fig. 6.2D).
In contrast to the wild type, the starch turnover in the 55% ADPGPPase
mutant at low light was lower than in the wild type (Fig. 6.2E), and the 7%
ADPGPPase mutant also had a reduced starch turnover at high light (Fig.
6.2B). The PGM mutant lacked starch formation, but showed an increase of
soluble carbohydrates during the course of a day, which increased with
nutrition (Fig. 6.2C and F).
Starch accumulation is highly dependent on leaf age (Fig. 6.3). If leaf
weight is taken as indicator for leaf age in the rosette of vegetative Arabidopsis, we observe that starch concentrations were larger in leaves of low
weight (young leaves) than in leaves of high weight (old leaves1. The
variation of evening and morning concentrations is large, because different
leaves were collected from different plants for each data point. Therefore, it
is not clear if the daily increase in starch is smaller or larger in young than in
old leaves. However, the high starch level in young leaves may be an
indication that sink leaves accumulate starch not only from their own photosynthesis, but also from carbohydrates which are imported from source
leaves via the phloem.
125
In the present study, three types of Arabidopsis mutants were used to
have three levels of assimilatory starch under various conditions of light and
nutrition. The performance of a wild type (WT) was compared with the
growth response of (1) a mutant deficient in phosphogluco-mutase in the
chloroplast (PGM mutant) (Caspar et al. 1986), (2) a mutant showing
only 7% of the wild-type activity of ADP glucose phosphorylase (7%
ADPGPPase mutant) in which starch production is reduced by 70%, and (3)
a progeny of a cross between wild-type and 7% ADPGPPase mutant with a
55% ADP glucose phosphorylase activity and a 20% reduction in starch
formation (55% ADPGPPase mutant). Using these mutants, we investigated
whether the formation of assimilatory starch decreases or increases relative
growth rates.
6.3 The Diurnal Starch Turnover
During the course of a day, the experimental plants showed large variations
in assimilatory starch formation and turnover (Fig. 6.2). In the wild type,
starch concentration increased at high nitrogen supply (6mM NH4N0 3 )
from about 70 !lmol g -1 fw in the morning to about 200 !lmol g -1 fw in the
evening. At low N supply (0.1 mM NH4N03), the daily increase of starch
was similar, but it occurred from a higher base line starch level, i.e.,
starch concentrations in the morning were 200 !lmol g -1 fw as compared to
70 !lmol g-1 fw at high N supply (Fig. 6.2A). The daily increase in starch
concentration decreased from 130 !lmol g-l fw at high light (photon flux
density, PFD = 600!lmolm-2s-1) to about 50!lmolg-1fw in low light
(PFD = 80 !lmol m- 2 S-l) (Fig. 6.2D).
In contrast to the wild type, the starch turnover in the 55% ADPGPPase
mutant at low light was lower than in the wild type (Fig. 6.2E), and the 7%
ADPGPPase mutant also had a reduced starch turnover at high light (Fig.
6.2B). The PGM mutant lacked starch formation, but showed an increase of
soluble carbohydrates during the course of a day, which increased with
nutrition (Fig. 6.2C and F).
Starch accumulation is highly dependent on leaf age (Fig. 6.3). If leaf
weight is taken as indicator for leaf age in the rosette of vegetative Arabidopsis, we observe that starch concentrations were larger in leaves of low
weight (young leaves) than in leaves of high weight (old leaves1. The
variation of evening and morning concentrations is large, because different
leaves were collected from different plants for each data point. Therefore, it
is not clear if the daily increase in starch is smaller or larger in young than in
old leaves. However, the high starch level in young leaves may be an
indication that sink leaves accumulate starch not only from their own photosynthesis, but also from carbohydrates which are imported from source
leaves via the phloem.