Effects of Helminthosporium carbonum Toxin on Absorption of Solutes by Corn Roots

Abstract

Susceptible corn roots exposed to the host-selective toxin of Helminthosporium carbonum took up and retained more NO3-, Na+, Cl-, 3-o-methylglucose, and leucine than did control roots. Stimulatory effects on uptake were more pronounced with freshly cut roots than with roots that were washed and aged.Solutes were accumulated against a concentration gradient, and toxin-treated tissues developed a steeper gradient than did control tissues. Toxin affected both the low and high affinity uptake systems for Na+ and C1. Toxin did not affect uptake of Na2_, K+, Ca'+, phosphate ion (H2PO4-and HP04-), S04-, and glutamic acid. No toxin-induced leakage of any solute tested was detected within 5 to 6 hr after initial exposure to toxin. The data suggest that toxin from H. carbonum does not cause the general plasma membrane derangement caused by other host-selective toxins. Instead, H. carbonum toxin may cause specific changes in characteristics of the plasmalemma, which result in increased uptake of certain solutes.Helminthosporium carbonum Ullstrup race 1 produces a toxic compound which selectively affects the corn genotypes that are hosts of the fungus. Susceptible genotypes are those with a homozygous recessive factor at the locus conditioning disease reaction. Several lines of evidence show that the toxin is required for pathogenicity. All visible and known biochemical symptoms of infection are reproduced by the toxin (26). Thus, an understanding of disease and disease resistance should be possible from studies of toxic action. Metabolites with selective toxicity are now known from nine plant-infecting fungi, including Helminthosporium victoriae, Helminthosporium maydis race T, Periconia circinata, Alternaria kikuchiana, and Phyllosticta maydis (26,29 (28). Likewise, the apparent effect on nitrate reductase is secondary, because the increase in activity was traced to toxin-stimulated uptake of substrate, not to increased synthesis of the enzyme (31). Some of the other stimulatory effects of HC toxin may be secondary to initial toxic lesions. Metabolic activity induced by toxin at damaging concentrations is followed some hours later by decreased activity, cell leakiness, and death.The purpose of this work was to determine effects of toxin on uptake and retention of representative solutes in addition to nitrate. Nitrite, C1-, SO,-, Na+, K+, Ca2+, phosphate ion (H2PO-and HP042-), 3-o-methylglucose, leucine, and glutamic acid were selected for further study. HC toxin is selective for susceptible corn in its effects on NO3-uptake, and the increase in uptake is the earliest effect of toxin reported to date (31). An abstract describing some of the work has been published (30).Plant cells are thought to have at least two uptake systems for each of several ions (2, 12). Mechanism 1 of ion transport has a high affinity for ions and functions when ion concentrations are below 0.5 mM; this system is generally thought to be located in the plasmalemma, and it accumulates ions against a concentration gradient. Ion transport mech...

Effects of Helminthosporium carbonum Toxin on Nitrate Uptake and Reduction by Corn Tissues

“…The pH change was not caused by aeration, presence of cheesecloth, string, solutes under test, or root excision. Similar pH changes were obtained when roots were rinsed in CaSO4 solutions (22). Uptake rates of toxin-treated and control roots were similar in unbuffered solutions and in solutions buffered at pH 5.8 with 2 mm 2-(N-morpholino)ethanesulfonic acid.…”

Effects of Host-Specific Toxins on Electropotentials of Plant Cells

“…Control cells without CCCP maintained a constant potential difference. ous work to differ from the other toxins in several respects; e.g., it increases the uptake of several ions and organic compounds by maize roots within 2 hr of exposure (8).…”

“…Toxins were prepared as described in previous papers (2,3,8). The HV toxin preparation completeiy inhibited root growth of susceptible oat seedlings at 0.001 p,g/ml; PC toxin completely inhibited susceptible sorghum growth at 0.1 I g/ml.…”

“…The earliest known effect of HC toxin from H. carbonumn Ullstrup race 1 (a maize pathogen) occurs within 2 hr as an increase in uptake of NO. and other solutes (8); the increase in uptake is followed 12 to 24 hr later by losses of materials from tissues.…”

“…Maize (Zea mnays L.) hybrids were Pr X K61 and Prl X K61, which are susceptible and resistant to H. carbonum race 1 :nd to HC toxin. Seeds were surface sterilized and planted in moist Vermiculite.Toxins were prepared as described in previous papers (2,3,8). The HV toxin preparation completeiy inhibited root growth of susceptible oat seedlings at 0.001 p,g/ml; PC toxin completely inhibited susceptible sorghum growth at 0.1 I g/ml.…”