Reductions in maize root-tip elongation by salt and osmotic stress do not correlate with apoplastic O2*- levels.

BACKGROUND AND AIMS: Experimental evidence in the literature suggests that O(2)(*-) produced in the elongation zone of roots and leaves by plasma membrane NADPH oxidase activity is required for growth. This study explores whether growth changes along the root tip induced by hyperosmotic treatments in Zea mays are associated with the distribution of apoplastic O(2)(*-). METHODS: Stress treatments were imposed using 150 mm NaCl or 300 mM sorbitol. Root elongation rates and the spatial distribution of growth rates in the root tip were measured. Apoplastic O(2)(*-) was determined using nitro blue tetrazolium, and H(2)O(2) was determined using 2', 7'-dichlorofluorescin. KEY RESULTS: In non-stressed plants, the distribution of accelerating growth and highest O(2)(*-) levels coincided along the root tip. Salt and osmotic stress of the same intensity had similar inhibitory effects on root elongation, but O(2)(*-) levels increased in sorbitol-treated roots and decreased in NaCl-treated roots. CONCLUSIONS: The lack of association between apoplastic O(2)(*-) levels and root growth inhibition under hyper-osmotic stress leads us to hypothesize that under those conditions the role of apoplastic O(2)(*-) may be to participate in signalling processes, that convey information on the nature of the substrate that the growing root is exploring.

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.

Leaf expansion in grasses under salt stress.

Restriction of leaf growth is among the earliest visible effects of many stress conditions, including salinity. Because leaves determine radiation interception and are the main photosynthetic organs, salinity effects on leaf expansion and function are directly related to yield constraints under saline conditions. The expanding zone of leaf blades spans from the meristem to the region in which cells reach their final length. Kinematic methods are used to describe cell division and cell expansion activities. Analyses of this type have indicated that the reduction in leaf expansion by salinity may be exerted through effects on both cell division and expansion. In turn, the components of vacuole-driven cell expansion may be differentially affected by salinity, and examination of salinity effects on osmotic and mechanical constraints to cell expansion have gradually led to the identification of the gene products involved in such control. The study of how reactive oxygen species affect cell expansion is an emerging topic in the study of salinity's regulation of leaf growth.