Nodulin 26-like intrinsic protein Cs NIP2;2 is a silicon influx transporter in Cucumis sativus L.

Nodulin 26-like intrinsic proteins(NIPs) are a family of channel-forming transmembrane proteins that function in the transport of water and other small molecules.Some NIPs can mediate silicon transport across plasma membranes and lead to silicon accumulation in plants,which is beneficial for the growth and development of plants.Cucumber is one of the most widely consumed vegetables;however,the functions of NIPs in this crop are still largely unknown.Here,we report the functional characteristics of Cs NIP2;2.It was found that Cs NIP2;2 is a tandem repeat of Cs NIP2;1,which had been demonstrated to be a silicon influx transporter gene.Cs NIP2;2 has a selectivity filter composed of cysteine,serine,glycine and arginine(CSGR),which is different from all previously characterized silicon influx transporters in higher plants at the second helix position.Xenopus laevis oocytes injected with Cs NIP2;2 c RNA demonstrated a higher uptake of silicon than the control,and the uptake remained unchanged under low temperature.Cs NIP2;2 was found to be expressed in the root,stem,lamina and petiole,and exogenous silicon treatment decreased its expression in the stem but not in other tissues.Transient expression of Cs NIP2;2-e GFP fusion sequence in onion epidermal cells showed that Cs NIP2;2 was localized to the cell nucleus,plasma membrane and an unknown structure inside the cell.The results suggest that Cs NIP2;2 is a silicon influx transporter in cucumber,and its subcellular localization and the selectivity filter are different from those of the previously characterized silicon influx transporters in other plants.These findings may be helpful for understanding the functions of NIPs in cucumber plants.

Carbon Dioxide Concentrations and Light Levels on Growth and Mineral Nutrition of Juvenile Cacao Genotypes

In many countries cacao (Theobroma cacao L.) is invariably grown as an understory crop in agroforestry types of cropping systems and subjected to low levels photosynthetic photon flux density (PPFD) due to presence of large number of upper story shade trees with poorly managed canopy structure. In recent years carbon dioxide concentration in the atmosphere is steadily increasing and it is unclear what impact this will have on performance of cacao grown under shade of upper story shade trees. A climatically controlled greenhouse experiment was undertaken to evaluate the effects of ambient and elevated carbon dioxide (400 and 700 μmol·mol-1) and three levels of PPFD (100, 200, and 400 μmol·m-2·s-1) on growth, and macro- and micronutrient use efficiency of three genetically contrasting cacao genotypes (CCN 51, VB 1117 and NO 81). Intraspecific variations were observed in cacao genotypes for growth parameters at ambient to elevated carbon dioxide and low to adequate levels of PPFD. With the exceptions of total root length and leaf area, irrespective of carbon dioxide and PPFD levels, all three genotypes showed significant differences in all the growth parameters. For all the cacao genotypes, increasing PPFD from 100 to 400 μmol·m-2·s-1 and carbon dioxide from 400 to 700 μmol·mol-1 increased overall growth parameters such as leaf, shoot and root biomass accumulation, stem height, leaf area, relative growth rate and net assimilation rate. Irrespective of carbon dioxide and PPFD, invariably genotypes differed significantly in macro-micronutrient uptake parameters such as concentration, uptake, influx, transport and use efficiency. With few exceptions, raising PPFD from 100 to 400 μmol·m-2·s-1 and carbon dioxide from 400 to 700 μmol·mol-1 increased nutrient use efficiency for all the cacao genotypes. Elevated carbon dioxide and adequate PPFD are beneficial in improving cacao growth and mineral nutrient uptake and use efficiency.

Negative Feedback Regulation of Auxin Signaling by ATHB8/ACL5-BUD2 Transcription Module

The role of auxin as main regulator of vascular differentiation is well established, and a direct correlation between the rate of xylem differentiation and the amount of auxin reaching the （pro）cambial cells has been proposed. It has been suggested that thermospermine produced by ACAULIS5 （ACL5） and BUSHY AND DWARF2 （BUD2） is one of the factors downstream to auxin contributing to the regulation of this process in Arabidopsis. Here, we provide an in-depth characterization of the mechanism through which ACL5 modulates xylem differentiation. We show that an increased level of ACL5 slows down xylem differentiation by negatively affecting the expression of homeodomain-leucine zipper （HD- ZIP） III and key auxin signaling genes. This mechanism involves the positive regulation of thermospermine biosynthesis by the HD-ZIP III protein ARABIDOPSIS THALIANA HOMEOBOX8 tightly controlling the expression of ACL5 and BUD2. In addition, we show that the HD-ZIP III protein REVOLUTA contributes to the increased leaf vascularization and long hypocotyl phenotype of acl5 likely by a direct regulation of auxin signaling genes such as LIKE AUXIN RESISTANT2 （LAX2） and LAX3. We propose that proper formation and differentiation of xylem depend on a balance between positive and negative feedback loops operating through HD-ZIP III genes.

A Critical Role of AMT2;1 in Root-To-Shoot Translocation of Ammonium in Arabidopsis

Ammonium uptake in plant roots is mediated by AMT/MEP/Rh-type ammonium transporters. Out of five AMTs being expressed in Arabidopsis roots, four AMT1-type transporters contribute to ammonium uptake, whereas no physiological function has so far been assigned to the only homolog belonging to the MEP subfamily, AMT2;1. Based on the observation that under ammonium supply, the transcript levels of AMT2;1 increased and its promoter activity shifted preferentially to the pericycle, we assessed the contribution of AMT2;1 to xylem loading. When exposed to ^15N-labeled ammonium, amt2;1 mutant lines translocated less tracer to the shoots and contained less ammonium in the xylem sap. Moreover, in an amtl;1 amtl;2 amtl ;3 amt2;1 quadruple mutant （qko）, co-expression of AMT2;1 with either AMT1;2 or AMT1;3 significantly enhanced ^15N translocation to shoots, indicating a cooperative action between AMT2;1 and AMT1 transporters. Under N deficiency, proAMT2;1-GFP lines showed enhanced promoter activity predominantly in cortical root cells, which coincided with elevated ammonium influx conferred by AMT2;1 at millimolar sub- strate concentrations. Our results indicate that in addition to contributing moderately to root uptake in the low-affinity range, AMT2;1 functions mainly in root-to-shoot translocation of ammonium, depending on its Cell-type-specific expression in response to the plant nutritional status and to local ammonium gradients.