PLANT PHYSIOLOGY , Vol 113, Issue 4 1113-1124, Copyright © 1997 by American Society of Plant Biologists
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DEVELOPMENT AND GROWTH REGULATION |
Restriction of Chlorophyll Synthesis Due to Expression of Glutamate 1-Semialdehyde Aminotransferase Antisense RNA Does Not Reduce the Light-Harvesting Antenna Size in Tobacco
H. Hartel, E. Kruse and B. Grimm
Institut fur Biologie, Lehrstuhl fur Pflanzenphysiologie, Humboldt-Universitat zu Berlin, Unter den Linden 6, Sitz: Philippstrasse 13, D-10099 Berlin, Germany (H.H.)
The formation of 5-aminolevulinate is a key regulatory step in tetrapyrrole
biosynthesis. In higher plants, glutamate 1-semialdehyde aminotransferase
(GSA-AT) catalyzes the last step in the sequential conversion of glutamate
to 5-aminolevulinate. Antisense RNA synthesis for GSA-AT leads to reduced
GSA-AT protein levels in tobacco (Nicotiana tabacum L.) plants. We have
used these transgenic plants for studying the significance of chlorophyll
(Chl) availability for assembly of the light-harvesting apparatus. To avoid
interfering photoinhibitory stress, plants were cultivated under a low
photon flux density of 70 [mu]mol photons m-2 s-1. Decreased GSA-AT
expression does not seem to suppress other enzymic steps in the Chl
pathway, indicating that reduced Chl content in transgenic plants (down to
12% of the wild-type level) is a consequence of reduced GSA-AT activity.
Chl deficiency correlated with a drastic reduction in the number of
photosystem I and photosystem II reaction centers and their surrounding
antenna on a leaf area basis. Different lines of evidence from the
transgenic plants indicate that complete assembly of light-harvesting
pigment-protein complexes is given preference over synthesis of new
reaction center/core complexes, resulting in fully assembled photosynthetic
units with no reduction in antenna size. Photosynthetic oxygen evolution
rates and in vivo Chl fluorescence showed that GSA-AT antisense plants are
photochemically competent. Thus, we suggest that under the growth
conditions chosen during this study, plants tend to maintain their
light-harvesting antenna size even under limited Chl supply.
