The isolation of apparently homoplastidic mutants induced by a nuclear recessive gene in Arabidopsis thaliana

The nuclear recessive gene, chm1, of Arabidopsis thaliana is a imitator that induces a variety of plastid alterations giving rise to mixed cells and variegated leaves. The variegation is maternally transmitted but chm1 is transmitted in a Mendelian fashion (Rédei 1973; Rédei and Plurad 1973). In order to characterize the different types of plastid alterations induced by chm1, isolating homoplastidic lines, each apparently containing one type of mutant plastid in its cells, was essential since such characterization cannot be carried out on mixed cells. We have used two genetic approaches to isolate several apparently homoplastidic mutant lines by the removal of the mutator from the genetic background, and the maternal transmission of the mutant plastids. The rapidity of obtaining homoplastidic lines in the absence of chm1 indicated a non-stochastic sorting-out of plastids in mixed cells. That each of the chm1-free homoplastidic mutant lines was apparently homoplastidic for one type of mutant plastids was confirmed by electron microscopic observations. Here we report, for the first time, the production of different homoplastidic lines in the absence of the nuclear-mutator gene. Such genetically-stable homogeneous material should be a useful tool for studying the molecular mechanism(s) by which chm1 induces a variety of heritable plastid alterations.

The guanine-hypoxanthine permease GhxP of Erwinia amylovora facilitates the influx of the toxic guanine derivative 6-thioguanine

AIM

Erwinia amylovora is the causal agent of fire blight, a devastating disease of apples and pears. This study determines whether the E. amylovora guanine-hypoxanthine transporter (EaGhxP) is required for virulence and if it can import the E. amylovora produced toxic analogue 6-thioguanine (6TG) into cells.

METHODS AND RESULTS

Characterization of EaGhxP in guanine transport deficient Escherichia coli reveals that it can transport guanine, hypoxanthine and the toxic analogues 8-azaguanine (8AG) and 6TG. Similarly, EaGhxP transports 8AG and 6TG into E. amylovora cells. EaGhxP has a high affinity for 6TG with a K_i of 3·7 µmol l^-1 . An E. amylovora ⊿ghxP::Cam^r strain shows resistance to growth on 8AG and 6TG. Although EaGhxP is expressed during active disease propagation, it is not necessary for virulence as determined on immature apple and pear assays.

CONCLUSIONS

EaGhxP is not required for virulence, but it does import 6TG into E. amylovora cells.

SIGNIFICANCE AND IMPACT OF THE STUDY

As part of the disease establishment process, E. amylovora synthesizes and exports a toxic guanine derivative 6TG. Our results are counter intuitive and show that EaGhxP, an influx transporter, can move 6TG into cells raising questions regarding the role of 6TG in disease establishment.