other hand, results in an earlier onset of the senescence phase of leaf development. These findings sug-
gest that leaf developmental programming is broadly responsive to a range of source strength conditions.
This programming includes alterations in the patterns of gene expression that extend beyond photosyn-
thetic gene expression, inasmuch as the abundances of chloroplast rRNA, chloroplast DNA, and total cell
protein are all affected by source strength. On the other hand, source strength does not affect all elements
of tobacco leaf development, as is evident from the similarity in the expansion rates of leaves from wild-
type and antisense plants and leaves exposed to elevated CO 2 [28].
The mechanism by which source strength is sensed is likely to be complex. One hypothesis is that
hexokinase acts as a sugar sensor, initiating a signal transduction pathway that modulates the expression
of various nuclear genes (reviewed in Refs. 4, 6, 14, and 20). Chloroplast genes for subunits of chloro-
plast multimeric protein complexes (e.g., LS and SS of Rubisco) are also expressed coordinately in re-
sponse to alterations in source strength during leaf development (very likely at the transcriptional level),
and thus sugar sensing must involve regulatory communication between the nucleus and the plastid.
These regulatory circuits are poorly defined (reviewed in Ref. 40). One further complication is that many
other factors, e.g., hormones and light, influence the progression and duration of leaf development. Com-
ponents of signal transduction pathways for all of these factors probably interact and share elements in
common [15,51].
Our studies have shown that carbohydrates are able to regulate leaf developmental programming in a
predictable manner, consistent with the idea of feedback inhibition of photosynthesis (“sink regulation”
hypothesis). We suggest that in some cases a threshold source strength is sensed and that this regulates a
developmental switch, for instance, a phase transition in shoot morphogenesis [41] or the onset of the
senescence phase of leaf development [28]. In other cases, source strength is able to modulate the duration
of development responses once they have commenced, e.g., the senescence phase of leaf development [27].
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