We have analyzed the maize leaf transcriptome using Illumina sequencing. We mapped more than 120 million reads to define gene structure and alternative splicing events and to quantify transcript abundance along a leaf developmental gradient and in mature bundle sheath and mesophyll cells. We detected differential mRNA processing events for most maize genes. We found that 64% and 21% of genes were differentially expressed along the developmental gradient and between bundle sheath and mesophyll cells, respectively. We implemented Gbrowse, an electronic fluorescent pictograph browser, and created a two-cell biochemical pathway viewer to visualize datasets. Cluster analysis of the data revealed a dynamic transcriptome, with transcripts for primary cell wall and basic cellular metabolism at the leaf base transitioning to transcripts for secondary cell wall biosynthesis and C(4) photosynthetic development toward the tip. This dataset will serve as the foundation for a systems biology approach to the understanding of photosynthetic development.

Mechanisms of phloem loading in the minor veins of leaves are known for only a few species. We propose that there are a limited number of loading strategies for the primary photoassimilates, sucrose and sugar alcohols. These strategies can be predicted based on thermodynamic and anatomical considerations and identified by autoradiography of veins following uptake of 14 C-labeled compounds, analysis of leaf solute composition and concentrations, and plasmodesmatal counting. Experiments on 45 dicotyledonous species identified the predicted loading patterns. Over 50-fold differences in concentrations of sucrose and sugar alcohols in leaves were measured. The cumulative concentrations of transport compounds in leaves correlated with loading mechanisms, a previously unrecognized association. Comparisons of solute concentrations and osmotic potentials of whole leaves suggest that sucrose and sugar alcohols are more concentrated in the cytosol than in the vacuoles of mesophyll cells, thus increasing the driving force for passive loading in species that employ this strategy. Passive loading is more widespread than previously thought, especially in trees. The results indicate that plants have exploited all thermodynamically feasible and structurally compatible loading strategies and that these strategies can be identified with straightforward protocols.plasmodesmata ͉ polymer trap ͉ raffinose ͉ stachyose ͉ sucrose P hloem loading is the starting point for export of carbohydrates and other nutrients from leaves (1-4). To date, mechanisms of sucrose loading have been established for a relatively small number of species. Less is known about sugar alcohols, which in some plants are present in phloem sap at higher concentrations than sucrose (5-7).In many species, loading involves an apoplastic step, driven by plasma-membrane transporters and energized by the proton motive force (8). In other plants, loading is symplastic, driven by a downhill concentration gradient from the mesophyll to the phloem and requiring high plasmodesmatal densities (2, 3). In some plants that load symplastically, the process is driven by diffusion alone and is passive (9, 10). In other species that load via the symplast, sucrose from the mesophyll diffuses into specialized companion cells (CCs) in the minor veins (11), known as intermediary cells, and is converted to raffinose and stachyose. These raffinose family oligosaccharides (RFOs) are larger than sucrose and are apparently unable to diffuse back to the mesophyll through the intermediary cell plasmodesmata. The RFOs accumulate in the phloem, a process known as polymer trapping (12), to a combined concentration that is similar to that of sucrose in apoplastic loaders (13,14). Thus, there are 3 recognized strategies of sucrose loading: Apoplastic and symplastic with or without polymer trapping.It is reasonable to assume that the anatomical features associated with sucrose loading in a given sieve element-companion cell (SE-CC) complex constrain available strategies of sugar alcohol loading. If s...

Enhancing the output of Rubisco, an enzyme that converts atmospheric CO 2 into energy-rich molecules, could improve photo-synthetic efficiency, and therefore crop yield, in plants. Maize is a C4 grass, which uses four-carbon compounds to carry CO 2 into an interior compartment; subsequent release of CO 2 increases its local concentration and favors efficient activity of Rubisco. Rice, however, is a C3 grass and lacks this pathway. Wang et al. compared transcripts and metabolites in developing maize and rice plants as a step toward understanding the biochemical and anatomical bases of C4 photosynthesis. Furthermore, Lin et al. transplanted Rubisco from a cyanobacterium, which also relies on a CO 2-concentrating apparatus, into tobacco (a C3 plant) chloro-plasts.-GJC Nat.

The phloem transports nutrients, defensive compounds, and informational signals throughout vascular plants. Sampling the complex components of mobile phloem sap is difficult because of the damage incurred when the pressurized sieve tubes are breached. In this review we discuss sampling methods, the artifacts that can be introduced by different sampling procedures, the intricate pathways by which materials enter and exit the phloem, and the major types of compounds transported. Loading and unloading patterns are largely determined by the conductivity and number of plasmodesmata and the position-dependent function of solute-specific, plasma membrane transport proteins. Recent evidence indicates that mobile proteins and RNA are part of the plant's long-distance communication signaling system. Evidence also exists for the directed transport and sorting of macromolecules as they pass through plasmodesmata. A future challenge is to dissect the molecular and cellular aspects of long-distance macromolecular trafficking in the signal transduction pathways of the whole plant.

No abstract