Water uptake and distribution in germinating tobacco seeds investigated in vivo by nuclear magnetic resonance imaging.

The regulation of water uptake of germinating tobacco (Nicotiana tabacum) seeds was studied spatially and temporally by in vivo (1)H-nuclear magnetic resonance (NMR) microimaging and (1)H-magic angle spinning NMR spectroscopy. These nondestructive state-of-the-art methods showed that water distribution in the water uptake phases II and III is inhomogeneous. The micropylar seed end is the major entry point of water. The micropylar endosperm and the radicle show the highest hydration. Germination of tobacco follows a distinct pattern of events: rupture of the testa is followed by rupture of the endosperm. Abscisic acid (ABA) specifically inhibits endosperm rupture and phase III water uptake, but does not alter the spatial and temporal pattern of phase I and II water uptake. Testa rupture was associated with an increase in water uptake due to initial embryo elongation, which was not inhibited by ABA. Overexpression of beta-1,3-glucanase in the seed-covering layers of transgenic tobacco seeds did not alter the moisture sorption isotherms or the spatial pattern of water uptake during imbibition, but partially reverted the ABA inhibition of phase III water uptake and of endosperm rupture. In vivo (13)C-magic angle spinning NMR spectroscopy showed that seed oil mobilization is not inhibited by ABA. ABA therefore does not inhibit germination by preventing oil mobilization or by decreasing the water-holding capacity of the micropylar endosperm and the radicle. Our results support the proposal that different seed tissues and organs hydrate at different extents and that the micropylar endosperm region of tobacco acts as a water reservoir for the embryo.

Distinct ultraviolet-signaling pathways in bean leaves. DNA damage is associated with beta-1,3-glucanase gene induction, but not with flavonoid formation.

The enzyme beta-1,3-glucanase (betaGlu) was found to be strongly induced by ultraviolet (UV-B; 280-320 nm) radiation in primary leaves of French bean (Phaseolus vulgaris). This was demonstrated on the level of gene transcription, protein synthesis, and enzyme activity and was due to the expression of bean class I betaGlu (betaGlu I). In contrast to other proteins of the family of pathogenesis-related proteins, the induction of betaGlu I by UV correlated with the formation of photoreversible DNA damage, i.e. pyrimidine dimer formation. In conditions that allowed photorepair of this damage, betaGlu I induction was blocked. Therefore, UV-induced DNA damage seems to constitute a primary signal in the pathway leading to the induction of the betaGlu I gene(s). The induction was a local response because in partly irradiated leaves betaGlu I was selectively found in leaf parts exposed to UV. Although short wavelength UV (lambda < 295 nm) was most efficient in betaGlu I induction, longer wavelength UV (lambda > 295 nm) as present in natural radiation was still effective. In contrast to UV induction of betaGlu I, the induction of flavonoids in bean leaves was optimally triggered by much more moderate fluences from the UV wavelength range no longer effective in betaGlu I induction. UV induction of the flavonoid pathway shows no correlation with DNA damage and thus should be mediated via a different signal transduction pathway.