Sink to Source Transition of Populus Leaves

Development of the Populus leaf is presented as a model system to illustrate the sequence of events that occur during the sink to source transition. A Populus leaf is served by three leaf traces, each of which consists of an original procambial trace bundle that differentiates acropetally and continuously from more mature procambium in the stem and a complement of subsidiary bundles that differentiates bidirectionally from a leaf basal meristem. During development these subsidiary bundles maintain continuity through the meristematic region of the node. The basipetally developing subsidiary bunles form phloem bridges that serve to integrate adjacent leaf traces of the stem vasculature. Distal to the node the acropetally developing bundles from all three leaf traces are reoriented in a precise and orderly sequence to form tiers of petiolar bundles. These tiers of bundles extend into the midrib where bundles diverge at intervals as the major lateral veins. The dorsal-most tier of bundles extends to the lamina tip and each successive tier of bundles contributes to lateral veins situated more proximally in the lamina. Although the midrib and the major vein system differentiate acropetally in the lamina, they mature basipetally. Maturation of the mesophyll and other lamina tissues also mature basipetally. As a consequence of the basi-petal maturation process, the lamina tip matures very early and begins exporting photosynthates while the lamina base is still importing from other leaves. The transition of a leaf from sink to source status must therefore be considered as a progression of structural and functional events that occur in synchrony.     

Modification of cell proliferation patterns alters leaf vein architecture in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis>

Formation of leaf vascular pattern requires regulation of a number of cellular processes, including cell proliferation. To assess the role of cell proliferation during vein order formation, leaf development in genetic lines exhibiting aberrant cell proliferation patterns due to altered expression patterns of ANT and ICK1 genes was analyzed. Modification of cell proliferation patterns alters the number of higher order veins and the number of minor tertiary veins remodeled as intersecondary veins in Arabidopsis rosette leaves. Minor vein complexity, as indicated by branch point and freely ending veinlet number, is highly correlated with a decrease or increase in cell proliferation. Observations of procambial strand formation in modified cell proliferation pattern lines showed that vein pattern is specified early in leaf development and that formation of freely ending veinlets is temporally correlated with the expansion of ground meristem when cell proliferation is terminated prematurely. Taken together, our observations indicate that: (1) genes that modulate cell proliferation play a key role in regulating the meristematic competence of ground meristem cells to form procambium and vein pattern during leaf development, and (2) ANT is a crucial part of this regulation.     

Cytomixis in shoot apex of Norway spruce [<Emphasis Type="Italic">Picea abies</Emphasis> (L.) Karst.]

Atypical cell walls and nuclei were observed in the apex of Norway spruce shoots from late April to early May on the material collected from a few grafts of a clone of Norway spruce growing on an experimental area. Images of ultrastructure attest to cytomixis. The phenomenon of cytomixis has previously been described in various plant material, both in the meiotic and mitotic cells, but this is the first report of cytomixis in gymnosperms.     

Benzyladenine and low temperature promote phase transition from juvenile to vegetative adult in bulblets of Lilium?×?formolongi ‘White Aga’ cultured in vitro

Scales excised from lily bulblets were cultured on MS medium supplemented with 0.044 or 4.4 μM BA in the dark for 180 days. The culture period was divided into stage 1 (day 0–30), stage 2 (day 31–90) and stage 3 (day 91–180). The scales were cultured at 25°C in stage 1, 25°C or 8°C in stage 2, and 25°C in stage 3. When the scales were cultured on medium with 4.4 μM BA at 25°C for 180 days, bulblets with and without an elongated stem were produced. The percentage of bulblets with elongated stems greatly increased when the scales had been cultured at 8°C in stage 2. On medium with 0.044 μM BA, only bulblets without elongated stems were produced. The diameter of shoot primodia significantly enlarged in bulblets produced on medium with 4.4 μM BA at 8°C in stage 2 and no such enlargement occurred under the other conditions. Nearly square parenchyma cells were observed in the non-elongated shoot primodia in the former bulblets but not in the latter. These cells changed into longitudinally rectangular ones in the internode of elongated stems. Procambium was arranged almost parallel to the shoot axis in the stem of bulblets in the medium with 4.4 μM BA, but not in the medium with 0.044 μM BA.     

Cell differentiation in the longitudinal veins and formation of commissural veins in rice (Oryza sativa) and maize (Zea mays)

Vascular development is a central theme in plant science. However, little is known about the mechanism of vascular development in monocotyledons (compared with dicotyledons). Therefore, we investigated sequential processes of differentiation into various different vascular cells by carrying out detailed observations using serial sections of the bases of developing leaves of rice and maize. The developmental process of the longitudinal vascular bundles was divided into six stages in rice and five stages in maize. The initiation of differentiation into procambial progenitor cells forming the commissural vein arose in a circular layer cell that was adjacent to both a metaxylem vessel and one or a few phloem cells in stage V longitudinal vascular bundles. In most cases the differentiation of ground meristem cells into procambial progenitor cells extended in one direction, toward the next longitudinal vascular bundle, and subsequent periclinal divisions and further differentiation produced a vessel element, two companion cells and a sieve element to form a commissural vein. These results suggest the presence of an intercellular signal(s) that induces differentiation of the circular layer cell and the ground meristem cells into procambial progenitor cells, forming a commissural vein sequentially.