B. Hormone Responses and Mutants
As was the case with Volvox, many mutants have been isolated that affect the moss developmental path-
way. Because the moss can, unlike Volvox, be propagated from single, nonreproductive cells, mutants
that block the normal reproductive pathway can be readily maintained. Calmutants, for example, are
unable to undergo the first differentiation from chloronema to caulonema. For some isolates with this
phenotype, the defect can be overcome by the addition of auxins or cytokinins to the medium, show-
ing that these substances are involved in the differentiation. In these cases the lesions are most likely
in genes involved in the biosynthesis of these compounds. Other calisolates, however, are not re-
sponsive to the addition of hormones and thus are likely to be more directly related to the develop-
mental response itself. Similarly, budmutants are unable to form buds, and gadmutants form buds
that do not differentiate into gametophores. Some, but not all, budmutants can be rescued by the ex-
ogenous addition of cytokinins. The existence of separate cytokinin-responsive calandbudmutants
suggests that the cytokinin response threshold is different for the two types of differentiations: in bud
mutants of this type, some cytokinin must be present for caulonemal cells to form, but it apparently is
not enough to trigger the development of buds. A similar two-tiered response to cytokinin has been
found for induction of caulonemal branching and of bud formation in the moss Funaria hygrometrica
[50].
Reutter et al. [51] transformed various Physcomitrellamutants with a construct containing the ipt
gene from Agrobacterium, resulting in the endogenous production of excess cytokinin. The bacterial gene
was able to rescue a budmutant and another cytokinin-related mutant with defective plastid division,
but not a gadmutant, suggesting that cytokinins are needed for bud formation but that other triggers are
involved in further gametophore development. The results were rather different from those when cy-
tokinins were added to the medium, demonstrating a difference between exogenously added and endoge-
nously produced plant hormones.
Finally, a third class of hormone-response mutants are given the name ove. These mutants overpro-
duce buds as a direct consequence of supernormal levels of cytokinin. Somewhat surprisingly, mutations
in at least three separate loci give this phenotype, indicating that cytokinin concentrations must be very
carefully controlled by the moss [52].
C. Light and Gravity Responses and Mutants
Tropisms in a moss protonema occur in the apical cells only; i.e., they involve a change in the direction
of growth of a single cell (Figure 6). This makes the moss particularly attractive for the study of the in-
tracellular signaling events involved in tropisms because the effects of treatments or manipulations that
affect an individual cell can be directly monitored via time-lapse video microscopy. Similarly, the estab-
lishment of cell polarity in response to light can be readily measured in regenerating moss protoplasts
[53].
As to phototropism, chloronemata, caulonemata, and gametophores all show phototropic and po-
larotropic responses, and the different cell types vary in their reactions to different wavelengths and light
intensities [54,55]. In Physcomitrella, mutants in at least three loci have been described that have lost pho-
totropism in gametophores (ptrmutants). Caulonemal cells are similarly affected in these mutants, but
the chloronemal response is more complex (reviewed in Ref. 56). These mutations do not alter the pho-
tomorphic effects on the normal developmental pathways (Figure 5), so they are apparently not involved
in light perception per se. Phytochrome is clearly involved in most of these responses: in Ceratodon,ptr
mutants with both normal and altered phytochromes have been characterized [57,58]. The variation in the
tropisms of the different cell types in both wild-type and mutant strains makes it clear that no simple con-
trols of phototropism operate in mosses, and more work is needed to determine the mechanisms, both ge-
netic and biochemical, involved.
Caulonemal cells and gametophores also show a negative gravitropic response, but this can be ob-
served only in darkness or infrared light because the phototropic response will override it [59,60]. As is
true of the phototropic response, it is only the apical cell of a caulonemal filament that responds to grav-
ity (Figure 6), and there is increasingly good evidence that migrating amyloplasts interacting with cy-
toskeletal components are the statoliths [61,62]. Several mutants with altered or reduced caulonemal
gravitropism (gtrmutants) have been isolated [59]. Some of these show no gravitropism whatsoever, and,
812 WALLACE