terial genes. As such, it would represent a relic of the prokaryotic nature of the chloroplast and its en-
dosymbiont origins.
IV. LEAF DEVELOPMENT IN THE RUBISCO ANTISENSE MUTANTS
The preceding studies on the Rubisco antisense mutants were conducted on plants growing in tissue cul-
ture medium supplemented with sucrose. Under these conditions, the antisense plants grew at a similar rate
and were morphologically similar to wild-type plants. However, exogenously supplied sugars can result
in altered patterns of growth and development [14]. Therefore, to study leaf development in the antisense
mutants, we grew the plants on soil in the greenhouse [13,27,41]. Under these conditions, the antisense
plants are impaired in their ability to fix carbon and to produce carbohydrates suitable for export [45,46].
A. Whole Plant Development
As a background for the leaf development experiments, we examined whole plant development in anti-
sense plants with up to 80% reductions in Rubisco holoenzyme content [41]. We found that an early,
slow-growth phase of shoot morphogenesis is markedly prolonged in the mutant plants (Figure 3). Leaf
emergence is retarded during this phase, and a higher than normal number of (very small) leaves are pro-
duced. Following this phase, the wild-type and mutant plants have similar fast-growth phases in terms of
leaf emergence rates and numbers, internode distances, leaf sizes, and leaf dry weights. Plant height, to-
tal leaf areas, and shoot dry weights are similar at flowering. Collectively, these data suggest that source
strength regulates the duration of an early phase of tobacco shoot development and the transition to a later
phase. This phase change may occur in response to the attainment of a threshold source strength, which
is delayed in the mutant plants.
B. Canopy Leaf Development
Jiang and Rodermel [13] examined photosynthesis and photosynthetic gene expression in the antisense
plants as a function of leaf nodal position on the plant. All of the leaves on antisense and wild-type plants
120 RODERMEL ET AL.Figure 2 Models of mechanisms of control of Rubisco subunit accumulation by SS and LS abundance.
Enhanced SS protein levels, either directly or indirectly, positively influence the recruitment of ribosomes onto
rbcL mRNAs (positive regulation). Alternatively, reduced SS levels may inhibit ribosome recruitment, perhaps
via negative feedback by excess LS or LS breakdown products (negative regulation, dashed line extending from
LS protein). As demonstrated in conditions of LS limitation, as in rbcL mutants [40], LS concentrations influ-
ence SS protein stability (positive regulation). (Adapted from Ref. 44.)