Dwarfs, physiological

Some biochemical functions defined by the Arabidopsis dwarf mutants were later confirmed by heterologous expression of genes and by in vivo conversion of postulated substrates [17-20]. As part of these physiological and biochemical studies, tomato cell suspension cultures have also been established to investigate intermediates and enzymes of brassinosteroid biosynthesis and metabolism [21-23]. Enzyme activities from partially purified protein extracts were first detected in this model system [24]. 

As many as 135 GAs had been identified from many plant species by July 2007. It is now considered that all plant species produce GAs in nature, although the number of plant species from which GAs have been conclusively identified is still limited. Some mutants lacking the GA biosynthetic enzyme(s) cannot produce physiologically active GAs and show a dwarf phenotype. The plants from which the identification of GAs has been reported and the information on plant GAs are reviewed in the literature225 and are available on the website http //www.plant-hormones.info/occurrence of gas in plants.htm. The publications reporting the characterization of GAs after GA118 are cited in the literature.231-234... 

Most of the dwarf mutants in maize are non-GA responders (Figs. 1-5) the physiological/biochemical basis for their reduced growth has yet to be defined. For instance, they could be receptor mutants to either known or unknown classes of plant hormones they could be mutants blocking steps in the biosynthesis of, as yet. 

Four non-allelic, simple recessive, non-GA-responding dwarf mutants of maize are shown in Figs. 1-4. Nothing is known about the physiology of these mutants. Dominant Dwarf (D8 although a non-GA-responding dwarf, is considered to be a GA mutant (see above) it is shown in Fig. 5. 

The growth/temperature relationships in GA-sensitive dwarfs of Oryza and Zea are modified towards higher temperatures for growth cessation [17]. Similar studies on Rht i have revealed no differences in growth/temperature relationships, indicating that the dwarfism has a different physiological basis from that pertaining in maize and rice. 

Low temperatures are involved in quite different processes. On the one hand they can lift barriers imposed by inhibitors and thus render the embryo capable of germinating. This particular temperature effect should actually have been included in the previous section. On the other hand germination itself can, in many cases, still take place. However, the subsequent development of the seedling is inhibited if it is not subject to lower temperatures. So-called physiological dwarfs then develop. Considered in more detail, the growth of both the epicotyl and the hypocotyl or simply the epicotyl can be inhibited. Accordingly, one speaks of either hypocotyl and/or epicotyl dormancy which must be broken by lower temperatures. In this case dormancy is really the wrong word since it is actually retarded development. 

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