Polyamine metabolism and biosynthetic gene expression in Arabidopsis thaliana under salt stress.

In the present study we analysed polyamine metabolism in Arabidopsis thaliana (ecotype Columbia) rosette leaves collected at vegetative and reproductive stages from plants germinated and grown under increasing salt stress (0-75 mM NaCl) conditions. The expression level of the different isoforms of polyamine biosynthetic enzymes was analysed by reverse transcriptase-polymerase chain reaction (RT-PCR) and the polyamine biosynthetic enzyme activities were determined both in supernatant and pellet fractions. Free and perchloric acid (PCA)-conjugated (soluble and insoluble) polyamines, were measured. At vegetative stage, plants were able to adapt up to 50 mM NaCl, showing a significant growth inhibition only at 75 mM NaCl. At this growth stage and NaCl concentration there was an up-regulation of spermine biosynthesis. At reproductive stage, plants were able to flower up to 50 mM NaCl, even if with a delay of 7 days. On the contrary, at 75 mM NaCl two different phenotypes were isolated: 75/01 (salt sensitive) and 75/02 (salt tolerant). The sensitive plants (75/01) showed a severely stressed phenotype, compared to the tolerant ones (75/02), and the polyamine metabolism was up-regulated, with the increase of free putrescine and spermine.

Pigment content and leaf plastid ultrastructure in the tomato mutant lutescent-2.

The non-lethal tomato mutant «lutescent-2¼ shows an early yellowing of normal developed leaves. Its ripe fruits display a yellow colouring, red pigment synthesis being delayed by up to two weeks. Typical pigment synthesis, related to leaf maturation, does not occur in mutant leaves. Both the concentration of chl a and chl b start to decrease very quickly at the end of leaf expansion. Early yellowing of «1-2¼ leaves appears to be related to the reduced car(470) content, which leads to chlorophyll photooxidation. Structural evidence of a deficiency in car(470) content in young «1- 2¼ plastids is given by a reduction of stroma thylakoids as well as by a limited grana stacking. The altered balance between the two pigment classes determined an active, even if incomplete, conversion of chloroplasts to chromoplast-like organelles.