Juvenile-adult phase transition of vegetative traits is not affected in the root deficient mutant rtcs
--Hochholdinger, F; Hetz, W and Feix, G

Vegetative development of maize can be divided into a juvenile and an adult phase. Each phase is characterized by specific traits that appear in two distinct forms during development (Lawson and Poethig, TIG 11:263-268, 1995). Juvenile traits always appear in basal parts of the maize plant, since the polar growth of the apical shoot meristem separates the juvenile and adult phase spatially as well as temporally. The transition between the phases is however gradual. The most obvious phase specific markers are presence or absence of epicuticular waxes and epidermal macrohairs on the leaf blade. Beside some further leaf and shoot related traits, the presence of "adventitious roots" (crown roots) is a distinct juvenile trait (Moose and Sisco, Plant Cell 6:1343-1355, 1994).

The mechanism of the juvenile adult transition is unknown, however it is assumed that the root stock exerts an influence on the transition. This allusion was now tested with help of the root deficient mutant rtcs recently isolated by us (Hetz et al., MNL 66:45, 1992).

rtcs is characterized by a drastically reduced root system, lacking also the "adventitious" roots considered as a juvenile vegetative trait. Instead of the complex root system of a wild type plant, consisting of a primary root, lateral seminal-, crown- and brace roots, the mutant rtcs displays only a primary root, which is nevertheless sufficient to produce a fertile plant. The rtcs plants were now used for a study of occurrence and timing of the juvenile-adult phase transition by examining in comparison to wild type siblings the phase specific markers of the leaf epicuticular wax formation (examined by toluidin blue staining) and the formation of macrohairs. Surprisingly we could not detect any significant difference between rtcs and wild type plants in the expression of these traits (Table 1). Also the total number of leaves in wild type and rtcs showed no significant difference.

 Table 1. Epicuticular wax and hairs in wildtype and rtcs plants.
 

Trait	wild type (14)	rtcs (11)
Total number of leaves	13.9±0.6	12.5±0.8
First leaf with hairs	5.6±1.2	6.1±0.8
First leaf partially lacking wax	7.1±1.0	7.3±1.1

Each value is the average ± two standard errors. The averages for rtcs mutants are paired with the averages for their wild type siblings. The number of plants of each type is given in parantheses.

At the time of the juvenile-adult phase transition (between leaf 5 and 7 in our case) the wild type had formed primary-, lateral seminal- and crown roots whereas rtcs displayed only the primary root. So far we have not made a quantitative determination of the total size of the root system of wild type and rtcs plants at the time of the juvenile - adult transition by determining the root length and weight of all roots present at this time. The results of such experiments might give more detailed insights into the question, to what extent factors produced by the root system in general or more specifically by particular roots promote the juvenile phase or inhibit adult development. In this case phase change might be sensitive to the size of the root system as discussed in Lawson and Poethig (1995). 

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