Mutagenesis—A Potential Approach for Crop Improvement - Crop Improvement: New Approaches and Modern Techniques - page 171

Nevertheless, such an approach cannot replace a gene recombination breeding pro-
gram that focuses on such characters of the root system as the dynamics of soil
penetration; seminal, adventitious, and secondary root numbers; total root length;
weight, number, and distribution of root hairs; and many physiological characters
that directly influence plant productivity. Phosphorus uptake in barley can be almost
doubled by increasing root hair density and length (Gahoonia and Nielsen 1997 ).
The availability of phosphorous, zinc and other nutrients in poor soils as well as
water and nitrogen nutrition depend also on mycorrhiza associations (Barker et al.
1998 ). Mycorrhizal fungi transfer assimilated carbon between tobacco plants (Mull-
er and Dulieu 1998 ). These examples indicate an urgent need for further develop-
ment of selection methods and for studies of the inheritance of characters related
to the root system structure, function, and their linkage with other plant characters.
Mutational analysis of selected root characters in breadth and depth would be the
most desired approach, especially since a high frequency of induced mutations has
been observed in relation to the root characters. More than 3.3 % of progenies of
barley M 1 plants have indicated mutation in root system characters after combined
seed treatment with sodium azide (NaN 3 ) and N-methyl-N-nitroso-urea (MNH) ac-
cording to the mutagenic treatment method described by Szarejko and Maluszynski
( 1980 ). Mutant lines selected in M 3 generation indicated mutations related to root
hairs, number and length of seminal roots, rootlessness, and abnormal root tip de-
velopment.
Root mutants, described in maize, were obtained after mutagenic treatment with
EMS and mutator MU (Feix et al. 1997 ). Mutants with unusual gravitropism be-
havior, aberrant lateral root formation, premature root degradation, and with lack of
crown and brace roots were described in mutated generations. The genetic analysis
of mutants indicated that the formation of the various root types and classes is con-
trolled by different genes. Mutational analysis has been demonstrated as a powerful
tool to dissect signaling pathways for plant defense responses (Dangl et al. 1996 ;
Yang et al. 1997 ). There are also several examples of the use of mutational analysis
to define the physical size, organization, and the sequence complexity of the major
cluster of pathogenesis-related genes or the fine gene structure, e.g. downy mildew
resistance genes in lettuce (Anderson et al. 1996 ) and for the Mlo locus for powdery
mildew resistance in barley (Buschges et al. 1997 ).
Induced mutations in rice, especially for semi-dwarfness and earliness, are
most often used to demonstrate the fastest way to obtain these characters in geno-
types where crosses can modify particular characters such as adaptability, aroma,
taste as well as requirements of local markets. More recently, mutation techniques
have also been used to generate mutants with particular requirements related to
quality characters where a rapid selection method is available. Very useful mu-
tants have been obtained for fatty acid composition in rapeseed (Kott 1995 ), canola
(Wong and Swanson 1991 ), flax (Dribnenki et al. 1996 ), soybean (Schnebly et al.
1995 ), cuphea (Knapp and Tagliani 1991 ), camelina (Vollmann et al. 1997 ), for
grain quality in rice (Kumamaru et al. 1997 ) and for amylose-free starch in potato
(Leij et al. 1991 ).