Sequential patterns of intramural digestion of galactoxyloglucan in tamarind seedlings

The structure and breakdown of galactoxyloglucan (GXG)-rich cells was studied from cotyledons of Tamarindus indicus L. The depletion of GXG was followed at different levels: quantitative, histochemical and cytochemical. At the ultrastructural level two probes were used: one general for polysaccharides (periodic acid - thiocarbohydrazide - silver proteinate test), the other specific for the terminal galactosyl residues of GXG (β-galactosidase-gold complex). They were complemented by water-extraction of the GXG and analysis of the constituting monosaccharides by gas chromatography. Despite their collenchymateous aspect and the chemical similarity of the reserve GXG with the structural xyloglucan of growing walls, the thickened storage walls are not interpretable as being an hypertrophied primary wall. The tamarind cells produce an original type of wall construction in which GXGs are sequestered in a sort of homomolecular bulk. There is no evidence for intussusception of the molecules within a network of cellulose. The bulk of GXG is sandwiched between two thin layers: the outer is comparable to a regular primary wall, the inner behaves like a barrier during GXG withdrawal. Temporal and spatial patterns of GXG-mobilisation lead to the definition of a sequence of stages of cell activities (premobilising, mobilising, postmobilising). They are synchronized with the growth of the seedling axis, the duration and characteristics of the stages being subordinated to the location of the cells within the organ. Cell lysis is initiated in close relationship with intramural cavities. The development of digestion pockets results in a highly digested wall. The barrier prevents any engulfing of the cytoplasm in the wall clefts and creates an increasing free space. The attack front of digestion is always sharp. During all steps, the monosaccharide composition remains stable. At the end of GXG depletion, the storage wall is withdrawn and cells are rendered in a parenchyma-like state. The breakdown is not a complete wall collapse but an original controlled and limited wall-thinning. The data lead to the speculation that the hydrolytic activities result from a complementation between precursors relinquished by the cytoplasm and factors already present in the storage wall.