The CESA1 component of cellulose synthase is phosphorylated at sites clustered in two hypervariable regions of the protein. Mutations of the phosphorylated residues to Ala (A) or Glu (E) alter anisotropic cell expansion and cellulose synthesis in rapidly expanding roots and hypocotyls. Expression of T166E, S686E, or S688E mutants of CESA1 fully rescued the temperature sensitive cesA1-1 allele (rsw1) at a restrictive temperature whereas mutations to A at these positions caused defects in anisotropic cell expansion. However, mutations to E at residues surrounding T166 (i.e., S162, T165, and S167) caused opposite effects. Live-cell imaging of fluorescently labeled CESA showed close correlations between tissue or cell morphology and patterns of bidirectional motility of CESA complexes in the plasma membrane. In the WT, CESA complexes moved at similar velocities in both directions along microtubule tracks. By contrast, the rate of movement of CESA particles was directionally asymmetric in mutant lines that exhibited abnormal tissue or cell expansion, and the asymmetry was removed upon depolymerizing microtubules with oryzalin. This suggests that phosphorylation of CESA differentially affects a polar interaction with microtubules that may regulate the length or quantity of a subset of cellulose microfibrils and that this, in turn, alters microfibril structure in the primary cell wall resulting in or contributing to the observed defect in anisotropic cell expansion.Arabidopsis | cell wall | CESA protein | microtubule | regulation I n higher plants, cellulose is a composite polymer of long unbranched β-1,4-linked glucan chains that are hydrogen-bonded to form microfibrils of approximately 36 chains (1). The microfibrils are the primary load-bearing component of cell walls (2, 3), and defects in cellulose synthesis or organization (4) result in decreased anisotropy of cell wall expansion and an inability to achieve differentiated cell shape (2-4).Cellulose is synthesized by plasma-membrane complexes that have been visualized by freeze-fracture EM as hexagonal rosettes 25 to 30 nm in diameter (5). The complexes are thought to be composed of three functional types of structurally similar CESA proteins that are believed to be the catalytic subunits. For primary wall cellulose synthesis, CESA1 and CESA3 appear to be absolutely required whereas CESA2, CESA5, CESA6, and CESA9 are at least partially redundant (6). Dynamic visualization of CESA complexes in growing Arabidopsis cells showed that labeled (YFP:: CESA6) synthetic complexes appeared as discrete particles at the plasma membrane that move bidirectionally along linear paths coincident with underlying cortical microtubules (7). These observations, together with previous (8) and subsequent studies (9-11), suggest that cortical microtubule organization plays a role in guiding the orientation of cellulose microfibril deposition.All CESA proteins are predicted to have two transmembrane helices in the N-terminal region and six in the C-terminal region (Fig. 1). Between the tw...

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Suc transporters (SUTs) play a key role in the allocation and partitioning of photosynthetically fixed carbon in plants. While a function could be assigned to many members of the SUT family, almost no information is available on their regulation. Here, the transcriptional regulation of SUTs in response to various environmental stimuli in the leaves of five dicots (Arabidopsis Extensive data on expression of SUTs in relation to changes of environmental conditions were obtained through a global analysis of 168 transcriptomics data sets. Results were validated by quantitative PCR measurements and extended by the measurement of photosynthesis rate and phloem sugar content to draw insight on the correlation of SUT expression and sugar export from leaves. For the apoplasmic phloem loaders, a clear difference in transcriptional regulation in response to different environmental stimuli was observed. The consistent patterns of SUT expression under abiotic stress indicates which types of SUTs are involved in the regulation of leaf sugar status and in stress signaling. Furthermore, it is shown that down-regulation of phloem loading is likely to be caused by transcriptional regulation of SUTs, while up-regulation depends on post-transcriptional regulation. In poplar, expression of PtaSUT4 was found to consistently respond to environmental stimuli, suggesting a significant role in the regulation of sugar export from leaves in this passive symplasmic phloem loader.

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