Ji Ye Rhee

Transgenic Arabidopsis and tobacco plants overexpressing an aquaporin respond differently to various abiotic stresses

Despite the high isoform multiplicity of aquaporins in plants, with 35 homologues including 13 plasma membrane intrinsic proteins (PIPs) in Arabidosis thaliana, the individual and integrated functions of aquaporins under various physiological conditions remain unclear. To better understand aquaporin functions in plants under various stress conditions, we examined transgenic Arabidopsis and tobacco plants that constitutively overexpress Arabidopsis PIP1;4 or PIP2;5 under various abiotic stress conditions. No significant differences in growth rates and water transport were found between the transgenic and wild-type plants when grown under favorable growth conditions. The transgenic plants overexpressing PIP1;4 or PIP2;5 displayed a rapid water loss under dehydration stress, which resulted in retarded germination and seedling growth under drought stress. In contrast, the transgenic plants overexpressing PIP1;4 or PIP2;5 showed enhanced water flow and facilitated germination under cold stress. The expression of several PIPs was noticeably affected by the overexpression of PIP1;4 or PIP2;5 in Arabidopsis under dehydration stress, suggesting that the expression of one aquaporin isoform influences the expression levels of other aquaporins under stress conditions. Taken together, our results demonstrate that overexpression of an aquaporin affects the expression of endogenous aquaporin genes and thereby impacts on seed germination, seedling growth, and stress responses of the plants under various stress conditions.

Hydrogen peroxide permeability of plasma membrane aquaporins of Arabidopsis thaliana

Although aquaporins have been known to transport hydrogen peroxide (H2O2) across cell membranes, the H2O2-regulated expression patterns and the permeability of every family member of the plasma membrane intrinsic protein (PIP) toward H2O2 have not been determined. This study investigates the H2O2-regulated expression levels of all plasma membrane aquaporins of Arabidopsis thaliana (AtPIPs), and determines the permeability of every AtPIP for H2O2 in yeast. Hydrogen peroxide treatment of Arabidopsis down-regulated the expression of AtPIP2 subfamily in roots but not in leaves, whereas the expression of AtPIP1 subfamily was not affected by H2O2 treatment. The growth and survival of yeast cells that expressed AtPIP2;2, AtPIP2;4, AtPIP2;5, or AtPIP2;7 was reduced in the presence of H2O2, while the growth of yeast cells expressing any other AtPIP family member was not affected by H2O2. These results show that only certain isoforms of AtPIPs whose expression is regulated by H2O2 treatment are permeable for H2O2 in yeast cells, and suggest that the integrated regulation of aquaporin expression by H2O2 and the capacity of individual aquaporin to transport H2O2 are important for plant response to H2O2.

Aquaporin as a membrane transporter of hydrogen peroxide in plant response to stresses.

Hydrogen peroxide (H2O2) is a reactive oxygen species that signals between cells, and H2O2 signaling is essential for diverse cellular processes, including stress response, defense against pathogens, and the regulation of programmed cell death in plants. Although plasma membrane intrinsic proteins (PIPs) have been known to transport H2O2 across cell membranes, the permeability of each family member of PIPs toward H2O2 has not yet been determined in most plant species. In a recent study, we showed that certain isoforms of Arabidopsis thaliana AtPIPs, including AtPIP2;2, AtPIP2;4, AtPIP2;5, and AtPIP2;7, are permeable for H2O2 in yeast cells. Since the expression of PIPs is differently modulated in Arabidopsis by abiotic stress or H2O2 treatment, it is important to investigate the integrated regulation of aquaporin expression and their physiological significance in H2O2 transport and plant response to diverse abiotic stresses.

Early response in water relations influenced by NaCl reflects tolerance or sensitivity of barley plants to salinity stress via aquaporins

Barley varieties, K305 and I743, which are sodium chloride (NaCl) tolerant and sensitive respectively, were hydroponically grown to determine the short-term effects of NaCl on the cell water relations in roots using a cell pressure probe, and on the transcript levels of 10 barley PIP aquaporin genes (HvPIPs) in roots. Stomatal conductance, as an indicator of sensitivity to NaCl, was decreased to less than half values of control upon exposure to 100 mmol L–1 NaCl for 24 h in I743 whereas tolerant variety, K305, was able to maintain original conductance. Osmotic half-times of water exchange in cortical cells allowed for a clear distinction between the two varieties up to 200 mmol L–1 NaCl. With treatment duration of up to 12 h with 100 mmol L–1 NaCl, the elastic modulus was reduced in I743 but increased in K305. Hydrostatic half-times of water exchange in K305 increased rapidly, whereas this value remained unchanged in I743. Application of abscisic acid (ABA) after 1 h NaCl treatment restored the hydraulic conductivity of cells (Lp) in K305 but not in I743 whereas the opposite results were obtained when mercury chloride (HgCl2) was applied, verifying the contrasting gating response of aquaporins in two varieties. Reduced expression of HvPIPs was consistent with the reduction of hydraulic conductivity of both varieties after 24 h NaCl, but without any significant differences between them, indicating the importance of the activities of existing aquaporins rather than de novo synthesis to cope with short-term effects of salt stress.

Effect of nutrient deficiencies on the water transport properties in figleaf gourd plants

Effects of nitrogen, phosphorus, and potassium deficiencies on water transport properties in figleaf gourd plants were studied. Plants were treated for different period of deficiency and physiological parameters such as stomatal conductance, photosynthesis and transpiration were measured. Cell and root pressure probes were utilized to measure turgor and root pressures, half-times of water exchange and hydraulic conductivities to analyze water transport properties. When plants were grown in nitrogen or phosphorus deficient nutrient solutions, they became insensitive to mercury, suggesting that aquaporin was closed resulting in reduced hydraulic conductivity. Inclusion of tungstate, however, restored the sensitivity of cells to mercury, indicating the importance of internal nutrient concentration, not the incoming nutrient supply. The hydrostatic hydraulic conductivity of roots grown in nitrogen deficient solution, representing apoplastic pathway of water transport, was reduced but this reduction was dramatically recovered by the application of tungstate, indicating the importance of nutrient availability from storage pools in relation to water status of plants.

Functionally redundant LNG3 and LNG4 genes regulate turgor-driven polar cell elongation through activation of XTH17 and XTH24

Key messageIn this work, we genetically characterized the function of Arabidopsis thaliana, LONGIFOLIA (LNG1), LNG2, LNG3, LNG4, their contribution to regulate vegetative architecture in plant. We used molecular and biophysical approaches to elucidate a gene function that regulates vegetative architecture, as revealed by the leaf phenotype and later effects on flowering patterns in Arabidopsis loss-of-function mutants. As a result, LNG genes play an important role in polar cell elongation by turgor pressure controlling the activation of XTH17 and XTH24.AbstractPlant vegetative architecture is related to important traits that later influence the floral architecture involved in seed production. Leaf morphology is the primary key trait to compose plant vegetative architecture. However, molecular mechanism on leaf shape determination is not fully understood even in the model plant A. thaliana. We previously showed that LONGIFOLIA (LNG1) and LONGIFOLIA2 (LNG2) genes regulate leaf morphology by promoting longitudinal cell elongation in Arabidopsis. In this study, we further characterized two homologs of LNG1, LNG3, and LNG4, using genetic, biophysical, and molecular approaches. Single loss-of-function mutants, lng3 and lng4, do not show any phenotypic difference, but mutants of lng quadruple (lngq), and lng1/2/3 and lng1/2/4 triples, display reduced leaf length, compared to wild type. Using the paradermal analysis, we conclude that the reduced leaf size of lngq is due to decreased cell elongation in the direction of longitudinal leaf growth, and not decreased cell proliferation. This data indicate that LNG1/2/3/4 are functionally redundant, and are involved in polar cell elongation in Arabidopsis leaf. Using a biophysical approach, we show that the LNGs contribute to maintain high turgor pressure, thus regulating turgor pressure-dependent polar cell elongation. In addition, gene expression analysis showed that LNGs positively regulate the expression of the cell wall modifying enzyme encoded by a multi-gene family, xyloglucan endotransglucosylase/hydrolase (XTH). Taking all of these together, we propose that LNG related genes play an important role in polar cell elongation by changing turgor pressure and controlling the activation of XTH17 and XTH24.