Academic literature on the topic 'Xylem loading'

Abstract Although control of xylem ion loading is essential to confer salinity stress tolerance, specific details behind this process remain elusive. In this work, we compared the kinetics of xylem Na+ and K+ loading between two halophytes (Atriplex lentiformis and quinoa) and two glycophyte (pea and beans) species, to understand the mechanistic basis of the above process. Halophyte plants had high initial amounts of Na+ in the leaf, even when grown in the absence of the salt stress. This was matched by 7-fold higher xylem sap Na+ concentration compared with glycophyte plants. Upon salinity exposure, the xylem sap Na+ concentration increased rapidly but transiently in halophytes, while in glycophytes this increase was much delayed. Electrophysiological experiments using the microelectrode ion flux measuring technique showed that glycophyte plants tend to re-absorb Na+ back into the stele, thus reducing xylem Na+ load at the early stages of salinity exposure. The halophyte plants, however, were capable to release Na+ even in the presence of high Na+ concentrations in the xylem. The presence of hydrogen peroxide (H2O2) [mimicking NaCl stress-induced reactive oxygen species (ROS) accumulation in the root] caused a massive Na+ and Ca2+ uptake into the root stele, while triggering a substantial K+ efflux from the cytosol into apoplast in glycophyte but not halophytes species. The peak in H2O2 production was achieved faster in halophytes (30 min vs 4 h) and was attributed to the increased transcript levels of RbohE. Pharmacological data suggested that non-selective cation channels are unlikely to play a major role in ROS-mediated xylem Na+ loading.

Wild rice species provide a rich source of genetic diversity for possible introgression of salinity stress tolerance in cultivated rice. We investigated the physiological basis of salinity stress tolerance in Oryza species by using six rice genotypes (Oryza sativa L.) and four wild rice species. Three weeks of salinity treatment significantly (P < 0.05) reduced physiological and growth indices of all cultivated and wild rice lines. However, the impact of salinity-induced growth reduction differed substantially among accessions. Salt tolerant accessions showed better control over gas exchange properties, exhibited higher tissue tolerance, and retained higher potassium ion content despite higher sodium ion accumulation in leaves. Wild rice species showed relatively lower and steadier xylem sap sodium ion content over the period of 3 weeks analysed, suggesting better control over ionic sodium xylem loading and its delivery to shoots with efficient vacuolar sodium ion sequestration. Contrary to this, saline sensitive genotypes managed to avoid initial Na+ loading but failed to accomplish this in the long term and showed higher sap sodium ion content. Conclusively, our results suggest that wild rice genotypes have more efficient control over xylem sodium ion loading, rely on tissue tolerance mechanisms and allow for a rapid osmotic adjustment by using sodium ions as cheap osmoticum for osmoregulation.

Abstract Root-synthesized cytokinins are transported to the shoot and regulate the growth, development, and stress responses of aerial tissues. Previous studies have demonstrated that Arabidopsis (Arabidopsis thaliana) ATP binding cassette (ABC) transporter G family member 14 (AtABCG14) participates in xylem loading of root-synthesized cytokinins. However, the mechanism by which these root-derived cytokinins are distributed in the shoot remains unclear. Here, we revealed that AtABCG14-mediated phloem unloading through the apoplastic pathway is required for the appropriate shoot distribution of root-synthesized cytokinins in Arabidopsis. Wild-type rootstocks grafted to atabcg14 scions successfully restored trans-zeatin xylem loading. However, only low levels of root-synthesized cytokinins and induced shoot signaling were rescued. Reciprocal grafting and tissue-specific genetic complementation demonstrated that AtABCG14 disruption in the shoot considerably increased the retention of root-synthesized cytokinins in the phloem and substantially impaired their distribution in the leaf apoplast. The translocation of root-synthesized cytokinins from the xylem to the phloem and the subsequent unloading from the phloem is required for the shoot distribution and long-distance shootward transport of root-synthesized cytokinins. This study revealed a mechanism by which the phloem regulates systemic signaling of xylem-mediated transport of root-synthesized cytokinins from the root to the shoot.

Root-synthesized cytokinins regulate the growth, development, and stress responses of aboveground tissues and follow the transport route via xylem tissue. Arabidopsis ATP-binding cassette (ABC) transporter G family member 14 (AtABCG14) is involved in the xylem loading of root-synthesized cytokinins. However, the phloem unloading of root-synthesized cytokinin and shoot distribution have remained elusive. The recent study by Zhao et al., (2021) proved that the AtABCG14 protein mediates the phloem unloading of cytokinins through the apoplastic pathway indicating the AtABCG14 is a master regulator of shoot cytokinin distribution.

Among trace elements not essential for plant growth and metabolism, Cd is of particular concern as it may exert phytotoxic effects and have direct consequences on human health by accumulating in staple food crops which make up a large proportion of dietary intake. Cd is generally present in the soil medium either naturally and/or from anthropogenic sources. Concerning agricultural activities, the application of sewage sludge and phosphate fertilizers containing Cd as an impurity, as well as the use of Cd containing irrigation water, are of particular relevance. Compared to other heavy metals, Cd constitutes a big issue in terms of food safety as it tends to be more mobile and thus more available to be translocated to the edible portion of the plant, causing acute or chronic toxicity to humans even at low soil concentrations. The well established tendency of rice (Oryza sativa L.) to accumulate Cd to levels often exceeding the international limits for the cereal grain trade highlights the need to apply sound strategies aimed at reducing the risk of grain Cd accumulation. Compared to the sole use of agronomic techniques, the selection of rice cultivars that accumulate low Cd in the grains by taking advantage from the broad variability in the Cd accumulation trait observed in Indica and Japonica cultivars is far more promising. Therefore, the general purpose of this study was to deepen the knowledge of the physiological basis governing Cd distribution in rice, with particular concern on Cd root retention and Cd translocation, as they have been seen to be crucial in determining Cd accumulation. Specifically, the role of phytochelatins (PCs) in chelation and subcellular compartimentalization of Cd in the roots was investigated, both by characterizing Cd-PCs complexes with respect of the external Cd concentration and examining the molecular basis of their synthesis. As Cd chelation by PCs has seen to be a crucial but not the only determinant in limiting the amount of Cd potentially available to be translocated to the shoots, the focus moved on the identification of the genes encoding transporters putatively involved in Cd xylem loading. Particularly we looked at two transporters, OsHMA2 and OsHMA4, belonging to the P1B-type ATPase subfamily, acknowledging the major role of such class of transporters in Cd translocation. While characterizing these transporters both by molecular and physiological analysis, the occurrence of clear competition effects of Cd over Zn at the translocation level emerged. Such an outcome highlighted that Cd movement determining its allocation through the plant is not strictly associated to Zn, which is likely to result from the existence of Cd transport pathways that are Zn-independent. These results, obtained by exposing rice plants to relatively low Cd concentrations aiming at simulating the real conditions in moderately contaminated soils, contributed to advance the understanding of the complex network of processes governing Cd accumulation in rice grains which, despite the economical and agricultural relevance of this crop, is still lacking. With regard to this, our study could be intended as a step further towards the development of molecular and/or physiological markers to early select rice genotypes able to exclude Cd from the grains with the intent of ensuring the food safety of the consumers.

VOIGT, E. L. Transporte e homeostase Na+/K+ sob condições de sodicidade em feijão caupi. 2008. 143 f. Tese (Doutorado em Bioquímica) - Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2008.<br>Submitted by Francisco Lacerda (lacerda@ufc.br) on 2014-12-03T13:35:44Z No. of bitstreams: 1 2008_tese_elvoigt.pdf: 869527 bytes, checksum: 111481cec40b8990af0276048cc2efd9 (MD5)<br>Approved for entry into archive by José Jairo Viana de Sousa(jairo@ufc.br) on 2015-01-19T21:22:09Z (GMT) No. of bitstreams: 1 2008_tese_elvoigt.pdf: 869527 bytes, checksum: 111481cec40b8990af0276048cc2efd9 (MD5)<br>Made available in DSpace on 2015-01-19T21:22:09Z (GMT). No. of bitstreams: 1 2008_tese_elvoigt.pdf: 869527 bytes, checksum: 111481cec40b8990af0276048cc2efd9 (MD5) Previous issue date: 2008<br>High soil salinity can be associated with K+ starvation enabling interactive stresses on plant nutrition that impair crop production. The transport mechanisms on the soil-root symplast-xylem boundaries are crucial to the establishment of Na+ toxicity under salt stress, especially under low K+ availability. Then, the aim of this work was the physiological characterization of Na+ and K+ uptake and xylem loading in the roots of cowpea [Vigna unguiculata (L.) Walp.]. Low-affinity Na+ uptake, investigated by influx experiments using detached roots, was mediated by Ca2+-sensitive and Ca2+-insensitive pathways. The Ca2+-sensitive pathway may involve non-selective cation channels (NSCCs), while the Ca2+-insensitive pathway may depend on K+ channels and NH4+-sensitive K+ transporters. High-affinity K+ uptake, examined by the excided root technique, involved NH4+-sensitive and NH4+-insensitive pathways. The NH4+-sensitive pathway may be mediated by K+ transporters from the KT/HAK/KUP e HKT families and the NH4+-insensitive pathway may depend on K+ channels. Xylem loading, assessed by root exudation experiments, indicates that the high K+/Na+ selectivity showed by cowpea was collapsed under external Na+ concentrations above 20 mM in the presence of 1 mM K+. The Na+ access to the root xylem was almost unrestricted and it was compensated by enhanced K+ release to the sap. The Na+ compartmentation into the root cells may act as a physiological barrier to Na+ exclusion from the leaves, avoiding Na+ toxicity in cowpea due to the maintenance of the high leaf K+/Na+ ratio. K+ starvation associated with salt stress intensified the Na+ flux and restricted the K+ flux into the root xylem, as it enhanced Na+ accumulation in the young leaves, allowing unfavourable conditions to ionic homeostasis in cowpea in comparison with salt stress applied individually.<br>A salinidade elevada do solo pode estar associada à escassez de K+, propiciando estresses interativos sobre a nutrição das plantas, os quais prejudicam a produção agrícola. Os mecanismos de transporte de Na+ e K+ nas interfaces solo-simplasto radicular-xilema são decisivos para o estabelecimento da toxicidade de Na+ nas plantas submetidas ao estresse salino, especialmente sob baixa disponibilidade de K+. Assim sendo, o objetivo desse trabalho foi caracterizar fisiologicamente os mecanismos de transporte de Na+ e K+ envolvidos com a absorção e o carregamento do xilema nas raízes de feijão caupi [Vigna unguiculata (L.) Walp.]. A absorção de Na+ por mecanismos de baixa afinidade, investigada por experimentos de influxo em raízes destacadas, foi mediada por uma via sensível e uma via insensível ao Ca2+. A via sensível ao Ca2+ deve envolver os canais de cátions não-seletivos (NSCCs), enquanto a via insensível deve depender dos canais de K+ e dos transportadores de K+ sensíveis ao NH4+. A absorção de K+ por mecanismos de alta afinidade, examinada pela técnica de influxo em raízes destacadas, envolveu uma via sensível e uma via insensível ao NH4+. A via sensível ao NH4+ deve ser mediada pelos transportadores das famílias KT/HAK/KUP e HKT e a via insensível, pelos canais de K+. O carregamento de Na+ e K+ no xilema, estudado por experimentos de exudação radicular, indicou que a elevada seletividade K+/Na+ apresentada por feijão caupi foi colapsada sob concentrações externas de Na+ superiores a 20 mM, na presença de K+ 1 mM. O acesso quase irrestrito de Na+ ao xilema radicular foi compensado pela liberação aumentada de K+ na seiva e pela manutenção do fluxo de K+ para a parte aérea. A compartimentalização de Na+ nas raízes deve atuar como barreira fisiológica para excluir Na+ das folhas e, conseqüentemente, deve evitar a toxicidade desse íon em feijão caupi pela manutenção da alta relação K+/Na+ foliar. A privação de K+ aliada ao tratamento salino intensificou o fluxo de Na+ e restringiu o fluxo de K+ no xilema radicular, além de aumentar a acumulação de Na+ nas folhas novas, propiciando condições menos favoráveis à homeostase iônica em feijão caupi, em comparação com o estresse salino aplicado isoladamente.

Soil salinization is the accumulation of water-soluble salts in the soil to a level that impacts on agricultural production. Approximately 20% of the world’s cultivated land, or 6% of the world total area, is threatened by salinity. Hence, soil salinization is becoming the main challenge of the modern agriculture. Of all the cereals, wheat (Triticum aestivum) is a moderately salt-tolerant crop while barley (Hordeum vulgare) is classified as relatively salt tolerant. Within Triticum genera, durum wheat (Triticum turgidum ssp. durum) is less salt-tolerant than bread wheat (Triticum aestivum). Salinity tolerance is a complex trait, both physiologically and genetically. Both rapid and efficient osmotic adjustment and effective control of Na+ delivery to the shoot are absolutely critical for salinity stress tolerance. Reducing Na+ delivery from root to shoot can be achieved by either minimizing entry of Na+ to the xylem from the root symplast or maximizing Na+ retrieval from the xylem (e.g. by active means) or by reduced rate of transpiration resulted from stomata closure (e.g. by passive means). Specific details of their coordination and the relative contribution of these components towards salinity stress tolerance in wheat and barley have been not fully revealed until now. Hence, the major aim of this PhD project was to investigate the physiological and molecular mechanisms of regulating Na+ transport from root to shoot and its link with plant osmotic adjustment and the overall plant performance under saline conditions. The following specific objectives were addressed: * To quantify the relative contribution of organic and inorganic osmolytes towards osmotic adjustment in barley * To link osmotic adjustment and stomatal characteristics with salinity stress tolerances in contrasting barley accessions * To evaluate predictive values of various physiological indices for salinity stress tolerance in wheat and barley * To investigate the physiological and molecular mechanisms mediating xylem Na+ loading in wheat and barley Abstract ii Working along these lines, a broad range of barley (Hordeum vulgare and Hordeum spontaneum) genotypes contrasting in salinity stress tolerance were used to investigate the causal link between plant stomatal characteristics, tissue ionic relations, and salinity tolerance. In total, 46 genotypes (including two wild barleys) were grown under glasshouse conditions and exposed to a moderate salinity stress (200 mM NaCl) for five weeks. The overall salinity tolerance correlated positively with stomata density, leaf K+ concentration and the relative contribution of inorganic ions towards osmotic adjustment in the shoot. At the same time, no correlation between salinity tolerance and stomatal conductance or leaf Na+ content in shoot was found. Taken together, these results indicate the importance of increasing stomata density as an adaptive tool to optimise efficiency of CO2 assimilation under moderate saline conditions, as well as benefits of the predominant use of inorganic osmolytes for osmotic adjustment, in barley. Another finding of note was that wild barleys showed rather different strategies dealing with salinity, as compared with cultivated varieties. Then, a large number of wheat (Triticum aestivum and Triticum turgidum) cultivars were screened by using a broad range of physiological indices, to evaluate predictive values of various physiological indices for salinity stress tolerance in wheat cultivars. In general, most of the bread wheats showed better Na+ exclusion that was associated with higher relative yield. Leaf K+/Na+ ratio and leaf and xylem K+ contents were the major factors determining salinity stress tolerance in wheat. Other important traits included high xylem K+ content, high stomatal conductance, and low osmolality. Bread wheat and durum wheat showed different tolerance mechanisms, with leaf K+/Na+ content in durum wheat making no significant contributions to salt tolerance, while the important traits were leaf and xylem K+ contents. These results indicate that Na+ sequestration ability is much stronger in durum compared with bread wheat, most likely as a compensation for its lesser efficiency to exclude Na+ from transport to the shoot. Based on the large screening of barley genotypes for their ability to exclude Na+ for its loading into the xylem and delivery to the shoot, four genotypes contrasting in salinity stress tolerance were selected for further studies of molecular and physiological mechanisms mediating xylem Na+ and K+ loading and linking it with overall plant performance and sequestration of Na+ and K+ in leaf tissues. We report that both leaf and xylem K+/Na+ ratios correlated positively with overall plant salt tolerance after prolonged (3 weeks) exposures to salinity stress. Interestingly, it was Na+ but not K+ content that determined this correlation. At the same time, it was found that accumulation of Na+ in xylem sap in salt-tolerant genotypes (TX9425 and CM72) reached a peak 5 days after salt application and then declined. In contrast, salt-sensitive genotypes were less efficient in controlling xylem Na+ loading and showed a progressive increase in xylem Na+ concentrations. We then used the MIFE (microelectrode ion flux measurement) technique to study some aspects of salt stress signalling and Na loading into the xylem. This was achieved by measuring net fluxes of Ca2+, K+ and Na+ from xylem parenchyma tissue of control- and salt-grown plants in response to a range of known second messengers such as H2O2, ABA, or cGMP. Our results indicate that NADPH oxidase-mediated apoplastic H2O2 production acts upstream of xylem Na+ loading and is causally related to ROS-inducible Ca2+ uptake systems in the xylem parenchyma tissue. ABA was also able, directly or in-directly, to regulate the process of Na+ retrieval from xylem. The above findings were further supported by molecular experiments revealing that salt-tolerant barley genotypes (CPI and CM72) upregulate transcript levels of HvHKT1;5 and HvSOS1 to take up Na+ and transport them to shoot for osmotic adjustment shortly after salt addition. Salinity stress tolerance in durum wheat is strongly associated with plant’s ability to control Na+ delivery to the shoot. Two loci, termed Nax1 and Nax2, were recently identified as being critical for this process and were suggested to confer activity of HKT1;4 and HKT1;5 transporters from HKT gene family, respectively. At the functional level these transporters are assumed to actively retrieve Na+ from the xylem thus limiting the rates of Na+ transport from roots to shoots. In this work we show that Nax loci also affect activity and expression levels of SOS1-like Na+/H+ exchanger in both root cortical and stelar tissues. Net Na+ efflux measured from salt-treated stele by non-invasive ion flux measuring MIFE technique declined in the following sequence Tamaroi (parental line) > Nax1 = Nax2 > Nax1:Nax2 lines. This efflux was amiloride (a known inhibitor of Na+/H+ exchanger)-sensitive and was mirrored by net H+ flux changes. SOS1 relative transcript levels were 6 to 10 fold lower in Nax lines compared with Tamaroi. Thus, it appears that Nax loci confer two highly complementary mechanisms, both contributing to reducing xylem Na+ content. One of them is enhanced retrieval of Na+ back into the root stele via HKT, and another one reduced rate of Na+ loading into the xylem via SOS1. It is suggested that such duality may play important adaptive role by providing plant with a greater versatility to respond to changing environment and control Na+ delivery to the shoot. In conclusion, this project has found that different crop species and genotypes adopt different strategies to resist salinity stress. In barley, it was found that the predominant use of inorganic osmolytes contributes to osmotic adjustment in the shoot. While rapid Na+ delivery to the shoot seems to be an effective strategy to ensure normal shoot growth under saline condition, xylem Na+ loading should be tightly controlled and stopped once sufficient amount of Na+ was delivered to the shoot. Salt-sensitive barley cultivars failed to slow down the xylem Na+ loading once osmotic adjustment was achieved, while tolerant genotypes were efficient in controlling this process. Both HvHKT1;5 and HvSOS1 transporters were found to be involved in control of xylem Na+ loading and delivery to the shoot. The above process is also intrinsically linked with NADPH oxidase-mediated apoplastic H2O2 production that acts upstream of xylem Na+ loading and is causally related to ROS-inducible Ca2+ uptake systems in the xylem parenchyma tissue. In wheat, bread and durum wheat can be differentiated by their reliance on Na+ exclusion and Na+ sequestration, respectively. The discovered role of NAX loci as controller of expression level and activity of SOS1-like transporters and existence of two highly complementary mechanisms conferring xylem Na+ loading/retrieval and reported insights into regulation of activity of membrane transporters expressed at xylem parenchyma interface open new prospects of cereal breeding for salinity tolerance.