Prohibitins, stomatins, and plant disease response genes compose a protein superfamily that controls cell proliferation, ion channel regulation, and death.

Prohibitins, stomatins, and a group of plant defense response genes are demonstrated to belong to a novel protein superfamily. This superfamily is bound by similar primary and secondary predicted protein structures and hydropathy profiles. A PROSITE-formatted regular expression was generated that is highly predictive for identifying members of this superfamily using PHI-BLAST. The superfamily is named PID (proliferation, ion, and death) because prohibitins are involved in proliferation and cell cycle control, stomatins are involved in ion channel regulation, and the plant defense-related genes are involved in cell death. The plant defense gene family is named HIR (hypersensitive induced reaction) because its members are associated with hypersensitive reactions involving cell death and pathogen resistance. For this study, eight novel maize genes were introduced: four closely related to prohibitins (Zm-phb1, Zm-phb2, Zm-phb3, and Zm-phb4), one to stomatins (Zm-stm1), and three to a gene implicated in plant disease responses (Zm-hir1, Zm-hir2, and Zm-hir3). The maize Zm-hir3 gene transcript is up-regulated in a disease lesion mimic mutation (Les9), supporting a role in maize defense responses. Members of this gene superfamily are involved in diverse functions, but their structural similarity suggests a conserved molecular mechanism, which we postulate to be ion channel regulation.

A porphyrin pathway impairment is responsible for the phenotype of a dominant disease lesion mimic mutant of maize.

The maize lesion mimic gene Les22 is defined by dominant mutations and characterized by the production of minute necrotic spots on leaves in a developmentally specified and light-dependent manner. Phenotypically, Les22 lesions resemble those that are triggered during a hypersensitive disease resistance response of plants to pathogens. We have cloned Les22 by using a Mutator-tagging technique. It encodes uroporphyrinogen decarboxylase (UROD), a key enzyme in the biosynthetic pathway of chlorophyll and heme in plants. Urod mutations in humans are also dominant and cause the metabolic disorder porphyria, which manifests itself as light-induced skin morbidity resulting from an excessive accumulation of photoexcitable uroporphyrin. The phenotypic and genetic similarities between porphyria and Les22 along with our observation that Les22 is also associated with an accumulation of uroporphyrin revealed what appears to be a case of natural porphyria in plants.

Plant-pathogen microevolution: molecular basis for the origin of a fungal disease in maize.

A new and severe disease of maize caused by a previously unknown fungal pathogen, Cochliobolus carbonum race 1, was first described in 1938. The molecular events that led to the sudden appearance of this disease are described in this paper. Resistance to C. carbonum race 1 was found to be widespread in maize and is conferred by a pair of unlinked duplicate genes, Hm1 and Hm2. Here, we demonstrate that resistance is the wild-type condition in maize. Two events, a transposon insertion in Hm1 and a deletion in Hm2, led to the loss of resistance, resulting in the origin of a new disease. None of the other plant species tested is susceptible to C. carbonum race 1, and they all possess candidate genes with high homology to Hm1 and Hm2. In sorghum and rice, these homologs map to two chromosomal regions that are syntenic with the maize Hm1 and Hm2 loci, indicating that they are related to the maize genes by vertical descent. These results suggest that the Hm-encoded resistance is of ancient origin and probably is conserved in all grasses.

A novel suppressor of cell death in plants encoded by the Lls1 gene of maize.

The Lls1 (lethal leaf spot1) locus of maize is defined by a recessive mutation characterized by the initiation, in a developmentally programmed manner, of necrotic lesions that expand to kill leaves cell autonomously. The loss-of-function nature of all Lls1 mutants implies that the Lls1 gene is required to limit the spread of cell death in mature leaves. We have cloned the Lls1 gene by tagging with Mutator, a transposable element system in maize, and we show that it encodes a novel protein highly conserved in plants. Two consensus binding motifs of aromatic ring-hydroxylating dioxygenases are present in the predicted LLS1 protein, suggesting that it may function to degrade a phenolic mediator of cell death.

Cloning and characterization of the maize An1 gene.

The Anther ear1 (An1) gene product is involved in the synthesis of ent-kaurene, the first tetracyclic intermediate in the gibberellin (GA) biosynthetic pathway. Mutations causing the loss of An1 function result in a GA-responsive phenotype that includes reduced plant height, delayed maturity, and development of perfect flowers on normally pistillate ears. The an1::Mu2-891339 allele was recovered from a Mutator (Mu) F2 family. Using Mu elements as molecular probes, an An1-containing restriction fragment was identified and cloned. The identity of the cloned gene as An1 was confirmed by using a reverse genetics screen for maize families that contain a Mu element inserted into the cloned gene and then by demonstrating that the insertion causes an an1 phenotype. The predicted amino acid sequence of the An1 cDNA shares homology with plant cyclases and contains a basic N-terminal sequence that may target the An1 gene product to the chloroplast. The sequence is consistent with the predicted subcellular localization of AN1 in the chloroplast and with its biochemical role in the cyclization of geranylgeranyl pyrophosphate, a 20-carbon isoprenoid, to ent-kaurene. The semidwarfed stature of an1 mutants is in contrast with the more severely dwarfed stature of GA-responsive mutants at other loci in maize and may be caused by redundancy in this step of the GA biosynthetic pathway. DNA gel blot analysis indicated that An1 is a single-copy gene that lies entirely within the deletion of the an1-bz2-6923 mutant. However, homozygous deletion mutants accumulated ent-kaurene to 20% of the wild-type level, suggesting that the function of An1 is supplemented by an additional activity.

Genetic patterns of plant host-parasite interactions.

A parasite's ability to infect and a host's ability to resist infection can be heritable traits. Patterns of inheritance suggest how host genes interact with parasite genes to determine whether or not infection occurs. Recent progress in the isolation and characterization of these genes in plants sheds new light on parasitism.

Reductase activity encoded by the HM1 disease resistance gene in maize.

The HM1 gene in maize controls both race-specific resistance to the fungus Cochliobolus carbonum race 1 and expression of the NADPH (reduced form of nicotinamide adenine dinucleotide phosphate)-dependent HC toxin reductase (HCTR), which inactivates HC toxin, a cyclic tetrapeptide produced by the fungus to permit infection. Several HM1 alleles were generated and cloned by transposon-induced mutagenesis. The sequence of wild-type HM1 shares homology with dihydroflavonol-4-reductase genes from maize, petunia, and snap-dragon. Sequence homology is greatest in the beta alpha beta-dinucleotide binding fold that is conserved among NADPH- and NADH (reduced form of nicotinamide adenine dinucleotide)-dependent reductases and dehydrogenases. This indicates that HM1 encodes HCTR.

A Biochemical Phenotype for a Disease Resistance Gene of Maize.

In maize, major resistance to the pathogenic fungus Cochliobolus (Helminthosporium) carbonum race 1 is determined by the dominant allele of the nuclear locus hm. The interaction between C. carbonum race 1 and maize is mediated by a pathogen-produced, low molecular weight compound called HC-toxin. We recently described an enzyme from maize, called HC-toxin reductase, that inactivates HC-toxin by pyridine nucleotide-dependent reduction of an essential carbonyl group. We now report that this enzyme activity is detectable only in extracts of maize that are resistant to C. carbonum race 1 (genotype Hm/Hm or Hm/hm). In several genetic analyses, in vitro HC-toxin reductase activity was without exception associated with resistance to C. carbonum race 1. The results indicate that detoxification of HC-toxin is the biochemical basis of Hm-specific resistance of maize to infection by C. carbonum race 1.