Induction of resistance to Helminthosporium maydis , race T in Texas maize

In order to obtain resistant plants, two kinds of mutagenesis were carried out on seeds of the French line F7-T: (1) exposure to gamma rays (15,000 R), and (2) soaking in a 2.5 g/l ethyl methyl sulfonate solution (EMS).

The plants from the treated seeds are called M1 generation; crosses with a restorer line (F71) produce a male fertile progeny called M2 giving M3 by selfing. In order to screen for resistance to HmT, M3 seedlings are checked either with the cultured filtrate medium or by direct inoculation with conidia. Tests revealed resistant plants from 42 M2 progenies (6.7%) after treatment and from only one M2 progeny after gamma ray irradiation (see for details Cassini et al., Ann. Amélior. Plantes, 1977, 27:753-766).

In all cases one M2 plant carrying the resistant trait appeared to segregate resistant and susceptible plants in its M3 progeny. This resistant trait appeared to be transmitted to M4 progenies for only 15 families:
 
Mutagenic Treatments No. Families from M1 Plants M2 Selfed Plants Families Producing Nonsusceptible Plants in M3 or M4 Families Entirely Nonsusceptible in Field in M5
gamma rays 670 2,330 1 1
EMS 692 2,167 14 6
Total 1,362 4,497 15 7

M5 plants were grown in fields with climatic conditions favorable to the fungus; this test revealed homogeneous resistant lines in seven distinct families which were named A type families. Other families, with less striking resistance characteristics, kept segregating in M5 and constituted B-type families.

A-type families have the following characteristics (fig. 1): (1) A majority of nonsusceptible plants appear in M3, (2) No male-sterile individuals in some of the M3 offspring, (3) M2 plants, grown from spare seeds, appear as nonsensitive to leaf scarification tests, (4) from the M4 on, it is possible to obtain lines that are both resistant and male-fertile and which do not segregate for these characters; resistance and male-fertility can be maintained either by selfing or by crossing with a Texas maintainer inbred, (5) by crossing with Texas male-sterile inbred, as female, susceptible offspring are obtained, either all male fertile, or all male sterile, or segregating fertile/sterile.

Figure 1.

From these results, it can be concluded that: (i) induced HmT resistance and male fertility are not pollen transmissible, (ii) in these families, there are male fertile plants recessive for restorer genes; (iii) as a consequence, male fertility and resistance to HmT, simultaneously reverted by a mutagenic treatment, are due to cytoplasmic modification(s) in the Texas material.

"Male-sterile" (ms) plants, appearing in M3 or later on, are of various types: aborted tassel, non-opened anthers, tassel seed types, etc. These types of male sterility are not maintained in crosses with Texas maintainers. However, one can find in subsequent generations, after selfing, these male sterile types again (fig. 1). These phenomena are strongly analogous to these described by Gengenbach et al. (P.N.A.S., 1977, 74:5113-5117).

B-type families are characterized as follows (fig. 2): (1) the M3 fertile/ sterile segregation is apparently normal, (2) individuals with intermediate susceptibility appear in M3; this character can be detected by tests on germinating seeds, on leaves (scarification test) and on isolated mitochondria (A. Bervillé, 1978, in Plant Mitochondria, ed. by G. Ducet and C. Lance, 427-434), (3) M2 plants were generally susceptible to HmT, (4) after the M3 generation, intermediate susceptibilities and sometimes intermediate male fertility are found, but no strict correlation seems to exist at this level, between such types of resistance and fertility. It is possible that minor modifications of nucleus-cytoplasm interactions are involved. These modifications could set up an intermediate balance which seems to be hard to stabilize.

Figure 2.

In conclusion, these results, together with those obtained by Gengenbach et al. (1977), after in vitro culture of maize calluses, prove that Texas cytoplasmic hereditary characteristics can be modified. If this can be done for the Texas system, then there is no reason to believe that it cannot be performed for other cytoplasmic systems.

A. Cornu, R. Cassini and A. Bervillé


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