Root transcript profiling of two Rorippa species reveals gene clusters associated with extreme submergence tolerance.

Complete submergence represses photosynthesis and aerobic respiration, causing rapid mortality in most terrestrial plants. However, some plants have evolved traits allowing them to survive prolonged flooding, such as species of the genus Rorippa, close relatives of Arabidopsis (Arabidopsis thaliana). We studied plant survival, changes in carbohydrate and metabolite concentrations, and transcriptome responses to submergence of two species, Rorippa sylvestris and Rorippa amphibia. We exploited the close relationship between Rorippa species and the model species Arabidopsis by using Arabidopsis GeneChip microarrays for whole-genome transcript profiling of roots of young plants exposed to a 24-h submergence treatment or air. A probe mask was used based on hybridization of genomic DNA of both species to the arrays, so that weak probe signals due to Rorippa species/Arabidopsis mismatches were removed. Furthermore, we compared Rorippa species microarray results with those obtained for roots of submerged Arabidopsis plants. Both Rorippa species could tolerate deep submergence, with R. sylvestris surviving much longer than R. amphibia. Submergence resulted in the induction of genes involved in glycolysis and fermentation and the repression of many energy-consuming pathways, similar to the low-oxygen and submergence response of Arabidopsis and rice (Oryza sativa). The qualitative responses of both Rorippa species to submergence appeared roughly similar but differed quantitatively. Notably, glycolysis and fermentation genes and a gene encoding sucrose synthase were more strongly induced in the less tolerant R. amphibia than in R. sylvestris. A comparison with Arabidopsis microarray studies on submerged roots revealed some interesting differences and potential tolerance-related genes in Rorippa species.

Wait or escape? Contrasting submergence tolerance strategies of Rorippa amphibia, Rorippa sylvestris and their hybrid.

BACKGROUND AND AIMS: Differential responses of closely related species to submergence can provide insight into the evolution and mechanisms of submergence tolerance. Several traits of two wetland species from habitats with contrasting flooding regimes, Rorippa amphibia and Rorippa sylvestris, as well as F(1) hybrid Rorippa × anceps were analysed to unravel mechanisms underlying submergence tolerance. METHODS: In the first submergence experiment (lasting 20 d) we analysed biomass, stem elongation and carbohydrate content. In the second submergence experiment (lasting 3 months) we analysed survival and the effect of re-establishment of air contact on biomass and carbohydrate content. In a separate experiment we analysed expression of two carbohydrate catabolism genes, ADH1 and SUS1, upon re-establishment of air contact following submergence. KEY RESULTS: All plants had low mortality even after 3 months of submergence. Rorippa sylvestris was characterized by 100 % survival and higher carbohydrate levels coupled with lower ADH1 gene expression as well as reduced growth compared with R. amphibia. Rorippa amphibia and the hybrid elongated their stems but this did not pay-off in higher survival when plants remained submerged. Only R. amphibia and the hybrid benefited in terms of increased biomass and carbohydrate accumulation upon re-establishing air contact. CONCLUSIONS: Results demonstrate contrasting 'escape' and 'quiescence' strategies between Rorippa species. Being a close relative of arabidopsis, Rorippa is an excellent model for future studies on the molecular mechanism(s) controlling these strategies.

Functional significance of shade-induced leaf senescence in dense canopies: an experimental test using transgenic tobacco.

Canopy photosynthesis models have predicted an optimal leaf area index (LAI; leaf area per unit surface area) and leaf nitrogen distribution at which whole-plant carbon gain per unit N is maximized. In this study we experimentally tested these models, using transgenic P(SAG12)-IPT tobacco (SAG; Nicotiana tabacum L.) plants with delayed leaf senescence and therefore a greater LAI and more uniform N distribution than the wild type (WT). In a competition experiment, the increased density of surrounding WT plants caused a greater reduction in dry mass of mature SAG target plants than in that of WT target plants, indicating negative effects of delayed leaf senescence on performance at high canopy density. Vegetative SAG plants achieved a lower calculated daily carbon gain than competing WT plants because the former retained leaves with a negative carbon gain in the shaded, lower part of the canopy. Sensitivity analyses showed that the carbon gain of SAG plants would increase if these lower leaves were shed and the N reallocated from these leaves were used to form additional leaf area at the canopy top. This strategy, which is adopted by the WT, is most advantageous because it results in the shading of competing neighbors.

Canopy light gradient perception by cytokinin.

We have recently identified cytokinin as an important xylem-carried signal involved in the photosynthetic acclimation of plants to light gradients in dense canopies. Lower leaves become shaded in a dense canopy and consequently have reduced transpiration rates. our measurements have shown that this results in a reduced delivery of cytokinins carried in the transpiration stream to shaded leaves, as compared to light-exposed leaves. Cytokinins are involved in the regulation of photosynthetic acclimation to the light gradient by stimulating the expression of photosynthetic enzymes in light-exposed leaves. In shaded leaves, the low delivery rate of cytokinin leads to reduced photosynthetic capacity and ultimately senescence. We show evidence for this role of cytokinin, as part of a complex of signaling pathways where other regulatory mechanisms are also involved. A model is presented depicting the regulation of photosynthetic acclimation by cytokinin delivery to leaves dependent on the irradiance they receive.

Cytokinin import rate as a signal for photosynthetic acclimation to canopy light gradients.

Plants growing in dense canopies are exposed to vertical light gradients and show photosynthetic acclimation at the whole-plant level, resulting in efficient photosynthetic carbon gain. We studied the role of cytokinins transported through the transpiration stream as one of probably multiple signals for photosynthetic acclimation to light gradients using both tobacco (Nicotiana tabacum) and Arabidopsis (Arabidopsis thaliana). We show that substantial variation in leaf transpiration parallels the light gradient in tobacco canopies and experimental reduction of the transpiration rate of a leaf, independent of light, is sufficient to reduce photosynthetic capacity in both species, as well as transcript levels of the small subunit of Rubisco (rbcS) gene in Arabidopsis. Mass spectrometric analysis of xylem sap collected from intact, transpiring tobacco plants revealed that shaded leaves import less cytokinin than leaves exposed to high light. In Arabidopsis, reduced transpiration rate of a leaf in the light is associated with lower cytokinin concentrations, including the bioactive trans-zeatin and trans-zeatin riboside, as well as reduced expression of the cytokinin-responsive genes ARR7 and ARR16. External application of cytokinin to shaded leaves rescued multiple shade effects, including rbcS transcript levels in both species, as did locally induced cytokinin overproduction in transgenic tobacco plants. From these data, we conclude that light gradients over the foliage of a plant result in reduced cytokinin activity in shaded leaves as a consequence of reduced import through the xylem and that cytokinin is involved in the regulation of whole-plant photosynthetic acclimation to light gradients in canopies.

Members of the aquaporin family in the developing pea seed coat include representatives of the PIP, TIP, and NIP subfamilies.

Water and nutrients required by developing seeds are mainly supplied by the phloem and have to be released from a maternal parenchyma tissue before being utilized by the filial tissues of embryo and endosperm. To identify aquaporins that could be involved in this process four full-length cDNAs were cloned and sequenced from a cDNA library of developing seed coats of pea (Pisum sativum L.). The cDNA of PsPIP1-1 appeared to be identical to that of clone 7a/TRG-31, a turgor-responsive gene cloned previously from pea roots. PsPIP1-1, PsPIP2-1, and PsTIP1-1, or their possible close homologues, were also expressed in cotyledons of developing and germinating seeds, and in roots and shoots of seedlings, but transcripts of PsNIP-1 were only detected in the seed coat. In mature dry seeds, high hybridization signals were observed with the probe for PsPIP1-1, but transcripts of PsPIP2-1, PsTIP1-1, and PsNIP-1 were not detected. Functional characterization after heterologous expression in Xenopus oocytes showed that PsPIP2-1 and PsTIP1-1 are aquaporins whereas PsNIP-1 is an aquaglyceroporin. PsNIP-1, like several other NIPs, contains a tryptophan residue corresponding with Trp-48 in GlpF (the glycerol facilitator of Escherichia coli) that borders the selectivity filter in the permeation channel. It is suggested that PsPIP1-1 and/or its possible close homologues could play a role in water absorption during seed imbibition, and that PsPIP2-1, possibly together with PsPIP1-1, could be involved in the release of phloem water from the seed coat symplast, which is intimately connected with the release of nutrients for the embryo.