Influence of Helminthosporium maydis, Race T, Toxin on Potassium Uptake in Maize Roots

Abstract

Basal K+ uptake in the root midzone region (cm 2 + 3 + 4) of N and T cytoplasmic versions of each of four maize inbreds was equaly sensitive to the toxin(s) ofHelminthosporium maydis, race T. Basal K+ uptake in the root apex (0-1 cm) and augmented K+ uptake in the root midzone were more toxin-sensitive in inbreds W64A(T) and Mol7(T) than in inbreds W64A(N) and Mol7(N). This differential response of N and T cytoplasms to toxins was not found for corresponding cytoplasms of inbreds WF9 and B37.Development of the augmented K+ uptake rate in midzone segments of W64A(T) was blocked by a toxin concentration which did not affect augmentation development in W64A(N). Augmentation development was more toxin-sensitive in T than in N cytoplasmic versions of ani inbreds tested. Fertility-restoring nudear loci decreased but did not eliminate the toxin sensitivity of augmentation development as observed in root midzones of inbred A619(T). Chloramphenkol-and/or cydoheximide-sensitive protein synthesis was required for augmentation development, but not for expression of either basal or augmented K+ uptake.rates increase with increasing distance from the apex through 6 cm from the apex (4). Basal K+ uptake in T versions is more toxin-sensitive than in N versions only in the apical root region of inbred W64A; in more mature regions (cm segments 2 + 3 + 4 from the apex), basal uptake is equally toxin-sensitive in N and T. Augmented K+ uptake in these more mature regions of N and T roots is more toxin-sensitive than basal K+ uptake, and the augmented increment in K+ uptake rate is inhibited 20 to 25% by toxin in N and 70 to 80% in T. Thus, despite the likelihood that the mechanism of K+ uptake is in all cases the same, differences exist in toxin sensitivity of K+ uptake between basal and augmented rates for the same root region, N and T versions for the same root region, and apex and midzone regions of the same root (4).The present report distinguishes the expression of basal and augmented K+ uptake from the phenomenon of augmentation development using several maize inbreds. Only augmentation development in excised root segments of T versions of several inbreds is consistently more toxin-sensitive than in the corresponding N versions. In addition, toxin sensitivity is only partially decreased in T by restoration of fertility. Augmentation development in W64A is also sensitive to CAP5 and CYC.Texas cytoplasmic male-sterile (T) maize (Zea mays L.) is more susceptible to Helminthosporium maydis (Nisikado and Miyake), race T, than corresponding normal (N) cytoplasm genotypes (3,8,10,14). With the exception of the leaf chlorosis bioassay (3, 9, 11, 15), the effects of the toxin(s) of H. maydis, race T, are not restricted to T cytoplasm. Thus, a qualitative distinction between plants with N and T cytoplasms is rarely observed since N plants are sensitive at high toxin concentrations (4,6, 11, 16).A complicated interaction exists between toxin and root in inbred W64A with respect to K+ uptake (4). K+ absorption rates vary along the...

Cytoplasm-specific Effects of Helminthosporium maydis Race T Toxin on Survival of Corn Mesophyll Protoplasts

“…Although mitochondria in situ (1) or isolated from (19) susceptible (but not resistant) corn are highly sensitive to toxin, results of certain experiments using tissues have led to the proposal that the plasma membrane is the more likely site of toxin action (2,3,8,18 (26). Liquid from cultures was filtered through cheesecloth, Whatman No.…”

“…Results of experiments using such materials can be variable, misleading, and/or difficult to interpret because of plant to plant variation, tissue heterogeneity, toxin penetration problems, or tendencies toward nonspecific responses. The problem is illustrated by the controversy over the site of toxin action in the susceptible cell.Although mitochondria in situ (1) or isolated from (19) susceptible (but not resistant) corn are highly sensitive to toxin, results of certain experiments using tissues have led to the proposal that the plasma membrane is the more likely site of toxin action (2,3,8,18 (26). Liquid from cultures was filtered through cheesecloth, Whatman No.…”