Is the Suppressor-Mutator Element Controlled by a Basic Developmental Regulatory Mechanism?

We report the results of genetic studies on derivatives of two different alleles of the maize a locus with an insertion of the Suppressor-mutator (Spm) transposable element in which the element is inactive, but can be reactivated readily. We present evidence that the mechanism that determines whether the element is in an active or inactive phase has two genetically distinguishable components. One determines whether or not the element is genetically active (the phase setting) and the other determines the stability of the setting in development, its heritability, and its phase in the next generation (the phase program). We show that the element's phase can be reset in a reproducible pattern during plant development. We also show that the Spm element can be reprogrammed to undergo a subsequent phase change without a concomitant phase change. The capacity to reset and reprogram the Spm element is differentially expressed within the plant in a pattern that is correlated with the developmental fate of apical and lateral meristems, suggesting the involvement of a basic developmental determination mechanism.     

The Heritable Activation of Cryptic Suppressor-Mutator Elements by an Active Element

A weakly active maize Suppressor-mutator (Spm-omega) element is able to heritably activate cryptic Spm elements in the maize genome. The spontaneous activation frequency, which is 1-5 x 10(-5) in the present genetic background, increases by about 100-fold in the presence of an Spm-omega and remains an order of magnitude above the background level a generation after removal of the activating Spm-omega. Sectorial somatic reactivation of cryptic elements can be detected phenotypically in kernels. Selection of such kernels constitutes an efficient selection for plants with reactivated Spm elements. Analysis of the reactivation process reveals that it is gradual and proceeds through genetically metastable intermediates that exhibit different patterns of element expression during plant development. Newly reactivated elements tend to return to an inactive form. However, the probability that an element will remain in a heritably active state increases when the element is maintained in the presence of an active Spm element for several generations.     

The viviparous8 mutation delays vegetative phase change and accelerates the rate of seedling growth in maize

Post-embryonic shoot development in plants can be divided into a juvenile vegetative, an adult vegetative, and a reproductive phase, which are expressed in different domains on the shoot axis. The number and position of the phytomers in each phase are determined by the time at which a plant begins and ceases making phytomers of a particular phase and the rate at which phytomers are made during that phase. The viviparous8 (vp8) mutation of maize increases the number of juvenile vegetative phytomers and decreases the number of adult vegetative phytomers by affecting both of these processes. vp8 increases the number of juvenile vegetative phytomers by increasing the rate of leaf initiation early in shoot development and delaying the juvenile-to-adult transition (vegetative maturation). It reduces the number of adult phytomers because the delay in vegetative maturation is not matched by a corresponding delay in flowering time; vp8 plants produce a tassel at the same time as wild-type plants. Thus, Vp8 normally controls the production of a factor that functions both to repress the rate of growth early in shoot development and to promote vegetative maturation, but which has no major role in floral induction. vp8 dramatically enhances the phenotypes of the dwarf and Teopod mutants and requires a functional Glossy15 gene to prolong the expression of juvenile epidermal traits. Evidence suggesting that vp8 does not affect phase change by reducing the level of abscisic acid is discussed.     

Mutations, epimutations, and the developmental programming of the maize Suppressor-mutator transposable element

Information about the structure, function and regulation of the maize Suppressor-mutator (Spm) transposable element has emerged from the genetic and molecular characterization of both deletion mutations and an unconventional type of reversible genetic change (epimutation). The element is subject to an epigenetic mechanism that can either stably inactivate it or specify one of a variety of heritable programs of differential element expression in development. The essay explores the relationship between the Spm element's epigenetic developmental programming mechanism and the determinative events central to plant development and differentiation.     

A COMPARATIVE DEVELOPMENTAL STUDY OF THE MUTANT SIDESHOOTLESS AND NORMAL TOMATO PLANTS

'Sideshootless,’ a mutant strain of tomato which does not produce axillary buds during vegetative growth, was compared with normally branching plants in order to study the nature of development particularly with regard to axillary buds. Sectioned material revealed no indication of axillary bud initiation in the sideshootless plant at any time during the vegetative phase of growth. In the normal plants, buds were noted to arise in the axil of the fifth youngest leaf. The buds take their origin in tissue which is in direct continuity with the apical meristem. The bud primordia later become set apart from the apex as vacuolation takes place in the surrounding tissue. At the time of floral initiation, the mutant and normal strains behave similarly. Axillary buds appear in the axils of the 2 leaves immediately below the floral apex. One of the buds elongates to overtop the existing plant axis; the other develops as a typical sidebranch. The inflorescence is pushed aside in the process. This pattern is repeated with each inflorescence; thus an axis composed of several superimposed laterals results.