The role of plasma membrane aquaporins (PIPs) in water relations of Arabidopsis was studied by examining plants with reduced expression of PIP1 and PIP2 aquaporins, produced by crossing two different antisense lines. Compared with controls, the double antisense (dAS) plants had reduced amounts of PIP1 and PIP2 aquaporins, and the osmotic hydraulic conductivity of isolated root and leaf protoplasts was reduced 5-to 30-fold. The dAS plants had a 3-fold decrease in the root hydraulic conductivity expressed on a root dry mass basis, but a compensating 2.5-fold increase in the root to leaf dry mass ratio. The leaf hydraulic conductance expressed on a leaf area basis was similar for the dAS compared with the control plants. As a result, the hydraulic conductance of the whole plant was unchanged. Under sufficient and under water-deficient conditions, stomatal conductance, transpiration rate, plant hydraulic conductance, leaf water potential, osmotic pressure, and turgor pressure were similar for the dAS compared with the control plants. However, after 4 d of rewatering following 8 d of drying, the control plants recovered their hydraulic conductance and their transpiration rates faster than the dAS plants. Moreover, after rewatering, the leaf water potential was significantly higher for the control than for the dAS plants.From these results, we conclude that the PIPs play an important role in the recovery of Arabidopsis from the water-deficient condition.Water transport through cellular membranes is facilitated by aquaporins, proteins that form waterselective channels. The presence of aquaporins in a membrane can increase the osmotic hydraulic conductivity of the membrane (L P , meters per second per megapascal) by 10-to 20-fold (Preston et al., 1992). In plants, the physiological importance of aquaporins is currently mainly inferred from their widespread occurrence (Johansson et al., 2000) and the use of HgCl 2 , a nonspecific inhibitor (Tyerman et al., 2002). Aquaporins, which are found in almost all types of tissues (Maurel, 1997), have changed the way we think about plant water relations (Maurel and Chrispeels, 2001).Water movement through a living organ such as a root or a leaf can take an apoplastic route, which has a low resistance to flow, or a transcellular route, which has a higher resistance because water has to move through lipid bilayer membranes . Bulk water flow associated with the transpiration stream is mostly apoplastic, except in the root exo-and endodermis (Zimmermann et al., 2000) and in the leaf bundle sheath (Koroleva et al., 2002), where apoplastic barriers (Casparian band, suberin lamellae, and secondary cell wall thickening) restrict the apoplastic path. Other important processes such as cell enlargement, refilling of embolized vessels, and movement of guard cells and pulvini may require rapid transport of water across membranes. Furthermore, the considerable growth-associated water potential difference (0.1-0.3 MPa) found in most growing organs of herbaceous plants (e.g. Nonami and Boyer, 1...
Citrus is the main fruit tree crop in the world and therefore has a tremendous economical, social and cultural impact in our society. In recent years, our knowledge on plant reproductive biology has increased considerably mostly because of the work developed in model plants. However, the information generated in these species cannot always be applied to citrus, predominantly because citrus is a perennial tree crop that exhibits a very peculiar and unusual reproductive biology. Regulation of fruit growth and development in citrus is an intricate phenomenon depending upon many internal and external factors that may operate both sequentially and simultaneously. The elements and mechanisms whereby endogenous and environmental stimuli affect fruit growth are being interpreted and this knowledge may help to provide tools that allow optimizing production and fruit with enhanced nutritional value, the ultimate goal of the Citrus Industry. This article will review the progress that has taken place in the physiology of citrus fruiting during recent years and present the current status of major research topics in this area. Key words: abiotic stresses, abscission, color break, flowering, fruit set, ripening Fisiologia da frutificação em citrus. Citrus é a principal fruteira no mundo, tendo, portanto, profundos impactos econômicos, sociais e culturais em nossa sociedade. Nos últimos anos, o conhecimento sobre a biologia reprodutiva de plantas tem aumentado consideravelmente, principalmente em função de trabalhos desenvolvidos com plantas-modelo. Todavia, a informação produzida nessas espécies nem sempre pode ser aplicada a citrus, fundamentalmente porque citrus é uma cultura arbórea perene com uma biologia reprodutiva muito peculiar e incomum. A regulação do crescimento e desenvolvimento do fruto em citrus é um fenômeno complexo e dependente de muitos fatores externos e internos que podem operar tanto seqüencialmente como simultaneamente. Os elementos e mecanismos pelos quais estímulos ambientes e endógenos afetam o crescimento do fruto vêm sendo interpretados, e esse conhecimento pode auxiliar a prover ferramentas que permitiriam otimizar a produção per se, além da obtenção de frutos com maior valor nutricional, o objetivo precípuo da Industria de Citrus. Neste artigo, revisam-se os avanços que vêm ocorrendo na fisiologia da frutificação de citrus durante os últimos anos; apresenta-se, também, o status atual de pesquisas mais relevantes nessa área. Palavras-chave: estresses abióticos, floração, maturação, vingamento de frutos
Aquaporins are channel proteins that facilitate the transport of water across plant cell membranes. In this work, we used a combination of pharmacological and reverse genetic approaches to investigate the overall significance of aquaporins for tissue water conductivity in Arabidopsis (Arabidopsis thaliana). We addressed the function in roots and leaves of AtPIP1;2, one of the most abundantly expressed isoforms of the plasma membrane intrinsic protein family. At variance with the water transport phenotype previously described in AtPIP2;2 knockout mutants, disruption of AtPIP1;2 reduced by 20% to 30% the root hydrostatic hydraulic conductivity but did not modify osmotic root water transport. These results document qualitatively distinct functions of different PIP isoforms in root water uptake. The hydraulic conductivity of excised rosettes (K ros ) was measured by a novel pressure chamber technique. Exposure of Arabidopsis plants to darkness increased K ros by up to 90%. Mercury and azide, two aquaporin inhibitors with distinct modes of action, were able to induce similar inhibition of K ros by approximately 13% and approximately 25% in rosettes from plants grown in the light or under prolonged (11-18 h) darkness, respectively. Prolonged darkness enhanced the transcript abundance of several PIP genes, including AtPIP1;2. Mutant analysis showed that, under prolonged darkness conditions, AtPIP1;2 can contribute to up to approximately 20% of K ros and to the osmotic water permeability of isolated mesophyll protoplasts. Therefore, AtPIP1;2 can account for a significant portion of aquaporin-mediated leaf water transport. The overall work shows that AtPIP1;2 represents a key component of whole-plant hydraulics.
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