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. 2005 Dec;17(12):3219-26.
doi: 10.1105/tpc.105.037473.

Pectin methylesterases and pectin dynamics in pollen tubes

Maurice Bosch  1 , Peter K Hepler
Affiliations
Free PMC article

Pectin methylesterases and pectin dynamics in pollen tubes

Maurice Bosch et al. Plant Cell. 2005 Dec.
Free PMC article
No abstract available

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Figures

Figure 1.
Figure 1.
Analysis of PME Isoforms Encoded in the Arabidopsis Genome. (A) The number of isoforms with and without a pro-region as well as the number of isoforms of which the pI of the predicted PME domain and pro-region falls within a certain pH interval are depicted. (B) The total relative expression level of the PME isoforms in a certain organ or cell type. The contribution of acidic, neutral, and basic PME isoforms is depicted. Numbers above the bar segments indicate the number of isoforms that contribute to the expression of the segment. Total PME activity is extremely high in pollen when compared with the other organs/cell types. (C) Organ and cell-type specificity of PME isoforms. Each isoform was analyzed for its expression in several organs and cell types. The number of isoforms specifically expressed in one organ/cell type or combination of organs/cell types is depicted. Pollen expresses its own specific subset of isoforms. By comparison, other organs/cell types share isoforms as is indicated by the high number of isoforms expressed in leaf, seedling, and silique. Si, silique; S, seedling; P, pollen; L, leaf. Only present calls with expression levels >200 are included in the analysis. Expression data of the PME isoforms are taken from Pina et al. (2005).
Figure 2.
Figure 2.
Regulation of PME Activity in Pollen Tubes. (A) Example of the distribution of relatively deesterified pectins and esterified pectins in N. tabacum pollen tubes by means of JIM5 and JIM7, respectively, immunolabeling. Notice the sudden transition from apical esterified pectin epitopes to deesterified pectin epitopes. (B) Potential feedback mechanism for the regulation of apical PME activity in pollen tubes that could contribute to an oscillatory growth pattern. PG, polygalacturonase; PL, pectate lyase. (C) Regulation of PME activity by its pro-region, PMEIs, and oscillating ion fluxes. (1) The pro-region functions as an intramolecular inhibitor of PME activity, preventing the premature deesterification of pectins prior to secretion. (2) Expression of only the PME domain can lead to premature, intravesicular, demethylesterification of pectins, causing inhibition of tube growth. (3) It remains to be seen if PMEIs can inhibit premature deesterification of PME isoforms lacking a pro-region. (4) Pectins are secreted in a highly methylesterified state. (5) Removal of the pro-region by cleavage of pro-PMEs is necessary for the PME domain to become enzymatically active. (6) The active PME domain catalyzes the demethylesterification of pectins. (7) Ca2+ binds cooperatively to the free carboxyl groups, which causes rigidification of the cell wall. (8) PMEIs are able to inhibit the PME activity in the cell wall by forming a 1:1 reversible complex. (9) The oscillatory influx of H+ and Ca2+ ions, which maintain the apical acidic domain and the tip-focused Cai2+ gradient, respectively, through the apical cell wall might directly influence the PME activity, the conditions necessary for cleavage of pro-PME, and the formation of the pH-dependent PMEI-PME complex. (10) The same is true for the oscillatory efflux of H+ ions maintaining the alkaline band at the base of the clear zone.
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References

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