Plasticity of root growth and respiratory activity: Root responses to above-ground senescence, fruit removal or partial root pruning in soybean.

This work focuses on the alterations in soybean root growth and activity during whole plant senescence and the contribution of roots to source-sink relations during plant development. The experiments were designed to analyze the activity of roots in relation to: a) whole plant senescence, b) total pod removal and c) root pruning (15, 25 and 50% of DW) during seed growth stages. Roots can grow until an advanced R5 stage and their specific activity decreases along the reproductive development but whole root activity declines from R6. However root respiration is maintained at a basal level until R8. Depodded plants showed a large increase of root dry matter (about 470%) and a large increase of AOX protein. Root pruning treatments showed a proportional increase of specific root respiration in 25 and 50% treatments but no differences of whole root respiration and dry matter partitioning at R7. These results indicate that roots are under the control of the requirements of above ground organs until final stages of seed growth but, after this, roots may survive independently for some time. This suggests that roots do not suffer a senescence-like process as leaves do. Also, plants have a high capacity to buffer changes in root biomass production and specific root activity under pod removal or partial root pruning.

Qualitative and quantitative modifications of root mitochondria during senescence of above-ground parts of Arabidopis thaliana.

This work studied modifications experienced by root mitochondria during whole plant senescence or under light deprivation, using Arabidopsis thaliana plants with YFP tagged to mitochondria. During post-bolting development, root respiratory activity started to decline after aboveground organs (i.e., rosette leaves) had senesced. This suggests that carbohydrate starvation may induce root senescence. Similarly, darkening the whole plant induced a decrease in respiration of roots. This was partially due to a decrease in the number of total mitochondria (YFP-labelled mitochondria) and most probably to a decrease in the quantity of mitochondria with a developed inner membrane potential (ΔΨm, i.e., Mitotracker red- labelled mitochondria). Also, the lower amount of mitochondria with ΔΨm compared to YFP-labelled mitochondria at 10d of whole darkened plant, suggests the presence of mitochondria in a "standby state". The experiments also suggest that small mitochondria made the main contribution to the respiratory activity that was lost during root senescence. Sugar supplementation partially restored the respiration of mitochondria after 10d of whole plant dark treatment. These results suggest that root senescence is triggered by carbohydrate starvation, with loss of ΔΨm mitochondria and changes in mitochondrial size distribution.

Impact of brassinosteroids and ethylene on ascorbic acid accumulation in tomato leaves.

Plant steroid hormones brassinosteroids (BRs) and the gaseous hormone ethylene (ET) alter the ascorbic acid-glutathione (AA-GSH) levels in tomato (Solanum lycopersicum L.) plants. The interaction of these hormones in regulating antioxidant metabolism is however unknown. The combined use of genetics (BR-mutants) and chemical application (BR/ET-related chemicals) shows that BRs and ET signalling pathways interact, to regulate leaf AA content and synthesis. BR-deficient (d(x)) leaves display low total AA but BR-accumulating (35S:D) leaves show normal total AA content. Leaves with either BR levels lower or higher than wild type plants showed a higher oxidised AA redox state. The activity of L-galactono-1,4-lactone dehydrogenase (L-GalLDH), the mitochondrial enzyme that catalyses the last step in AA synthesis is lower in d(x) and higher in 35S:D plants. BR-deficient mutants show higher ET production but it is restored to normal levels when BR content is increased in 35S:D plants. Suppression of ET signalling using 1-methylcyclopropene in d(x) and 35S:D plants restored leaf AA content and L-GalLDH activity, to the values observed in wild type. The suppression of ET action in d(x) and 35S:D leaves leads to the respective decreasing and increasing respiration, indicating an opposite response compared to AA synthesis. This inverse relationship is lacking in ET suppressed d(x) plants in response to external BRs. The modifications in the in vivo activity of L-GalLDH activity do not correlate with changes in the level of the enzyme. Taken together, these data suggest that ET suppresses and BRs promote AA synthesis and accumulation.