Molybdenum transport in plants.

Abstract

Molybdenum (Mo) is an essential micronutrient required by plants. It is biologically inactive until bound in a pterin compound named the molybdenum cofactor (Moco) that binds to apoproteins used in both reductive and oxidative reactions such as nitrate reductase (NR), xanthine dehydrogenase (XDH), aldehyde oxidase (AO) and sulfite oxidase (SOX). In Vitis vinifera cv. Merlot, molybdenum deficiency is common amongst vines grown on own roots in acidic soils often resulting in yield reductions. Foliar application of molybdenum sprays increases yield and remedies deficiency indicating that Merlot grown on own roots has a reduced capacity for molybdenum uptake from the soil. Molybdenum generally occurs as molybdate (MoO₄ ²⁻) within the soil solution. The mechanism(s) involved in molybdenum transport have recently been discovered in plants, although are well characterised within prokaryotic systems. Unfortunately, no homologues of prokaryotic genes involved in molybdate transport exist within eukaryotes. It has been suggested that molybdenum transport in plants may occur through other systems including sulfate transporters due to chemical similarities between sulfate and molybdate. A yeast functional complementation approach using a sulfate transport mutant was initially used to identify novel putative plant molybdenum transport proteins. A cDNA library derived from Pinot noir roots starved of molybdenum was screened for transporters. Unfortunately, no cDNAs were identified that met the requirements of a molybdenum transporter when screened on media containing low amounts of molybdenum. However, a number of putative cDNA’s partially complemented the yeast mutant YSD1, however none of these could be validated in second round screens. A candidate gene approach was then initiated to identify pre-characterised genes that may also have capacity to transport molybdenum. The plant sulfate transporter, SHST1, restored growth of YSD1 on media containing low amounts of molybdenum. Kinetic analysis using ⁹⁹MoO₄²⁻ to trace molybdenum transport in yeast cells demonstrated that SHST1 enhanced the uptake of molybdenum at nM concentrations. The uptake was not inhibited by sulfate, but the transport of sulfate was reduced with molybdenum. Further analysis demonstrated that SHST1 did prefer sulfate as the substrate but molybdenum could compete at higher concentrations. This result is the first measurement of molybdenum being transported through a pre characterised sulfate transport protein. Whole plant experiments using rooted grapevine cuttings and ⁹⁹MoO₄²⁻ to trace molybdenum movement into plants indicated that Merlot did not have reduced capacity to uptake molybdenum compared to other varieties that do not suffer from molybdenum deficiencies such as Chardonnay. When plants were grown with molybdenum, Merlot accumulated more molybdenum than Chardonnay, with the reverse being true when plants were grown without molybdenum. Similar experiments were performed on symbiosomes isolated from Glycine max grown with and without molybdenum. Symbiosomes absorbed more molybdenum when plants were grown without molybdenum. A field site was established to look at the molybdenum profiles within petioles against yield responses over a 3-year period. Molybdenum application did not increase the yield amongst vines despite all vines initially being deficient in molybdenum. There were no cumulative effects of molybdenum application over the trial, however, molybdenum did have limited translocation ability within the vine system.