Effect of chromosome arm dosage on the activity of isolated maize chloroplasts

Mutants in algae and flowering plants have proven to be valuable tools for the study of chloroplast structure, function, and assembly. The disadvantage of working with most higher plant photosynthetic mutants is that they are often pigment deficient, possess fragile plastids that are difficult to isolate, and are often seedling lethal. For these reasons we are currently studying the effect of segmental aneuploidy on the activity and structure of the chloroplast. This system provides the opportunity to study genetic alterations in photosynthesis in green, non-lethal material. Additionally, one might expect that changes of arm dosage might be correlated with increases as well as decreases in chloroplast activity. Support for the general hypothesis that aneuploidy may influence enzymatic properties comes from the work of Birchler (MGCNL 51:13-17), who found that increases in the activities of certain soluble enzymes could be correlated with changes in the number of doses of the long arm of chromosome 1.

Hypoploid and hyperploid material was generated by crossing quarter sterile (simple translocations) or semisterile (compound translocations) plants from highly backcrossed B-A translocation bearing stocks onto arm testers. Euploid control material was generated by crossing fertile sibs or the recurrent inbred onto the same tester lines. In order to test whether the markers used to identify dosage had an independent effect on photosynthesis, some tester material was selfed to recover the marker. Suitable substitutes were made in cases of failure or when insufficient material was available. Dosage series (1 vs. 2 vs. 3 doses) for several different chromosome arms were generated in this way; the preliminary nature of this report precludes a complete listing.

We were initially interested in studying those arms carrying hcf loci (see reports of Leto and Miles) known to be essential for normal photosynthetic activity. Preliminary observations for chromosomes 1L (location of several hcf loci), chromosome 1S (location of hcf*-3) and chromosome 3S (location of hcf*-19YG) are given below:


Although chromosome arms 1S and 3S are known to carry loci important in the assembly and function of photosystem II, there appear to be no effects of arm dosage on photosystemII-dependent electron transport. Seedlings hypoploid for the long arm of chromosome 1 (carrying several hcf loci) show a small decrease in photosystem II activity but a significant decrease in whole chain electron transport relative to euploid control. This suggests that the major alteration in electron transport in these hypoploids probably involves either photosystem I itself or a portion of the electron transport chain connecting the two photosystems. Measurement of P-700, the reaction center pigment of photosystem I, indicates a 33%-50% decrease in P-700 on a per chlorophyll basis in hypoploid material relative to euploid control.

Preliminary electrophoretic data show an increased staining intensity of a lamellar polypeptide with an apparent molecular weight of 45kD in material hyperploid for the long arm of chromosome 1 relative to euploid control. The staining intensity of a diffuse band with an apparent molecular weight of 27.5kD is reduced relative to euploid control in material either hyperploid or hyperploid for the long arm of chromosome 9. The significance of these changes is under investigation.

Kenneth Leto, Jack Beckett and Charles Arntzen

Please Note: Notes submitted to the Maize Genetics Cooperation Newsletter may be cited only with consent of the authors.

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