Characterization of des17

--M. Galbiati and G. Gavazzi

This mutant, recognizable at an early stage of germination, was isolated in the progeny of a Mu line, kindly provided by Dr. Robertson, DS, crossed to homozygous TB-8Lc males (from Dr. Beckett, JB). It is easily detected on the basis of reduced height, partial suppression of primary root growth, failure of the coleoptile to thoroughly enclose the emerging seedling and contorted leaf morphology. Segregation ratios, as determined by selfing +/des17 heterozygotes, show a significant shortage of recessives over the expected one-quarter (116/754; C2 = 37.18 P < 0.001). If germinations are performed after separating seeds originating from different sectors of the heterozygous ear, the segregation ratios observed (see Table 1) are different and best explained by assuming active expression of des17 in the male gametophyte.

Table 1. Mutant segregation in the apical, central and basal region of selfed +/des17 ears.
 
Cod. No. of selfed ear Region No. seedlings Mutants (%)
6291 (9) apical 152 15
  central 158 9
  basal 141 10
6291 (11) apical 77 16
  central 78 10
  basal 68 7

Mutant seedlings grown in pots die within the first 60 days. The first attempts to repair the abnormal phenotype by growing homozygous embryos on media supplemented with the main plant growth regulators gave negative results, while some degree of phenotypic repair was observed by growth on mineral Murashige and Skoog (MS) media enriched with casein hydrolisate (200 mg/l), yeast extract and yeast hydrolisate (40mg/l each). When the organic components were added separately to the mineral M and S medium the results reported in Table 2 were obtained.

Table 2. Effect of the addition of different organic components to the basic (MS) medium on the growth of homozygous des17 mutants as estimated after 7 days of culture. Ht and LA refer to average seedling height and primary root length in cm respectively. Each determination is the average of 5-10 single observations.
 
Organic component Ht LA
none 7.3 1.0
casein hydrolisate 7.8 2.1
yeast extract 6.7 2.9
yeast hydrolisate 11.0 5.2

We are presently testing the effect of single nitrogen bases on growth of detached mutant roots and embryos with the intent of correlating the des17 phenotype with a specific metabolic requirement. The positive response of the mutant to yeast hydrolisate allows us to attempt its rescue. We succeeded in growing homozygous des17 plants up to maturity by continuous spraying with yeast hydrolisate solution but failed to reproduce them because of their failure to develop a tassel.


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