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Alkaline phosphatases (APases) are important enzymes in organophosphate utilization. Three prokaryotic APase gene families, PhoA, PhoX, and PhoD, are known; however, their functional characterization in cyanobacteria largely remains to be clarified. In this study, we cloned the phoD gene from a halotolerant cyanobacterium, Aphanothece halophytica (phoD Ap ). The deduced protein, PhoD Ap , contains Tat consensus motifs and a peptidase cleavage site at the N terminus. The PhoD Ap enzyme was activated by Ca 2؉ and exhibited APase and phosphodiesterase (APDase) activities. Subcellular localization experiments revealed the secretion and processing of PhoD Ap in a transformed cyanobacterium. Expression of the phoD Ap gene in A. halophytica cells was upregulated not only by phosphorus (P) starvation but also under salt stress conditions. Our results suggest that A. halophytica cells possess a PhoD that participates in the assimilation of P under salinity stress.Phosphorus (P) is an essential nonmetal nutrient for all living cells. Despite its relative abundance in the ecosystem, P is sometimes a limiting factor for organisms in terrestrial, oceanic, and freshwater environments (1, 11). This is because while all organisms can utilize soluble inorganic phosphate (P i ), the P-containing organic compounds generally found in nature are complex insoluble forms that are not readily available to cells (1, 11). Alkaline phosphatases (APases) release free P i from organic compounds. To date, three prokaryotic APase gene families-PhoA, PhoX, and PhoD-have been documented (7). In addition to different levels of homology among these APases, dissimilar metal requirements for their activities have been reported (7). A recent metagenomics analysis revealed that PhoX, a recently characterized phosphatase, is more abundant in marine bacteria than the previously considered classical PhoA (12). However, it has also been shown that PhoD is more abundant in marine bacteria than PhoA or PhoX (7), suggesting an important role for PhoD in marine bacteria. PhoD encompasses a family of phosphatase/phosphodiesterases (APase/APDase). Except for Bacillus subtilis PhoD (PhoD Bs ) (3), little is known on other PhoD proteins. PhoD Bs was shown to be secreted into extracellular medium by the Tat pathway, which recognizes targeted proteins by their N-terminal twin-arginine signal peptides containing the Tat consensus (SRRXFLK) motif (3, 9).Cyanobacteria inhabit a broad range of ecosystems and play a vital role in the global cycling of nutrients, including P.Hitherto, extensive studies on the regulation of gene expression during phosphate stress have been carried out (14). In contrast, functional properties of cyanobacterial APases have been reported only on classical PhoA (8), atypical APase (11), PhoV (21), and PhoX (5), and there is no report on the cyanobacterial PhoD. Moreover, little is known on the role of APase under high-salinity conditions, and this encouraged us to examine the role of cyanobacterial PhoD under high-salinity conditions.Aphanot...

Water deficit limits plant growth and yield. Arbuscular mycorrhizal (AM) symbiosis is viewed as one of the several methods to improve growth under water deficit. The present study investigated the growth performance in relation to water deficit in two cultivars (''H2'' and ''660'') of AM treated macadamia (Macadamia tetraphylla L.) plants. AM treatment significantly improved the growth in macadamia plants that have been subjected to water deficit (7 % soil water content) for 14 days. Leaf water content (LWC) and maximum quantum yield of PSII (F v /F m ) in AM-associated plants were maintained better than those in the control (well-watered) plants. A positive correlation was observed between LWC and F v /F m in ''H2'' cultivar. AM treatment enhanced proline and soluble sugar content in ''H2'' cultivar under water deficit stress. In contrast, only soluble sugars were accumulated in the AMassociated plants of ''660'' cultivar under water deficit stress. The study concludes that soluble sugars and proline are involved as key signals of osmoregulation defense response, improve water relation in plant tissues, and thereby resulting in improved growth in AM-associated macadamia plants.

The aim of this research was to investigate betaine aldehyde dehydrogenase (BADH) and glycinebetaine (Glybet) biosynthesis in photoautotrophic rice seedlings. The role of Glybet on physiological and growth responses to salt stress in both salt-tolerant and salt-sensitive lines is to be investigated. The BADH activity in salt-tolerant seedlings cultured under extreme salt stress (342 mm NaCl) progressively increased during the first few hours until it peaked after 72 h. This was about 2.5 times greater than in salt-sensitive plants. Similarly, the amount of Glybet detected in salt-tolerant lines was 1.3 times more than in salt-sensitive lines at 96 h salt exposure. The BADH activities were positively related to Glybet accumulation in both salt-tolerant and salt-sensitive lines. The accumulation of Glybet in salt-tolerant lines was directly correlated with pigment stabilization. Relative water content in the salt-tolerant lines was closely related to water oxidation in photosystem II (PSII), defined by maximum quantum yield of PSII (F v /F m ). In addition, a high concentration of total chlorophyll is more efficient in capturing light energy, defined by photochemical quenching. The concentrations of chlorophyll a and total carotenoid were positively related to the quantum efficiency of PSII (U PSII ) and nonphotochemical quenching, respectively, resulting in a high net-photosynthetic rate (NPR) and the promotion of growth. The high level of Glybet in salt-tolerant lines plays a role as a salt defensive response mechanism in terms of pigment stabilization and water oxidation in PSII, resulting in high NPR and growth efficiency.

Soluble carbohydrates play a key role as osmolytes and significantly contribute in salt defence mechanism, especially in halophyte species. The objective of this study is to investigate the transcriptional expression of starch-related genes, sugar profile and physiological performances of two contrasting rice genotypes, Pokkali (salt tolerant) and IR29 (salt sensitive), in response to salt stress. Total soluble sugars, glucose and fructose levels in the flag leaf of salt-stressed Pokkali rice were enhanced relative to soluble starch accumulation in plants exposed to EC = 13.25 dS m(-1) (salt stress) for 3 days. In Pokkali, the net photosynthetic rate and starch metabolism may play a key role as energy resources under salt stress. In contrast, photosynthetic performance, indicated by photosynthetic pigment levels and chlorophyll fluorescence parameters, in salt-stressed IR29 was significantly reduced, leading to delayed starch biosynthesis. The reduction in photosynthetic ability and lack of defence mechanisms in IR29 caused growth inhibition and yield loss. Soluble starch and soluble sugar enrichment in Pokkali rice may function alternatively as osmotic adjustment in salt defence mechanism and strengthen carbon energy reserves, greater survival prospects under salt stress and enhanced productivity.

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Soil salinity may limit plant growth and development, and cause yield loss in crop species. This study aimed at remediating saline soil using organic matter (OM) treatment, before the cultivation of RD6 rice (Oryza sativa L. spp. indica). Physiological and morphological characters of rice plants, as well as crop yield, were evaluated from salt-affected soil with varying levels of salinity. The chlorophyll a and total chlorophyll pigments of rice plants grown in salt-affected soil (2% salt level) with the application of OM were maintained better than in plants grown without OM treatment. The degree of reduced photosynthetic pigments in rice plants was dependent on the level of salt contamination. Pigment content was positively related to maximum quantum yield of PSII (F v /F m ) and quantum efficiency of PSII (Φ PSII ), leading to reduced net photosynthetic rate (P n ) and reduced total grain weight (TGW). Photosynthetic abilities, including chlorophyll a and total chlorophyll pigments and Φ PSII , in rice plants grown with OM treatment were greater than in those cultivated in soil without the OM treatment, especially in high salt levels (1-2% salt). The remediation of salt-affected soil in paddy fields using OM should be applied further, as an effective way of enhancing food crop productivity.