Powdery mildews are phytopathogens whose growth and reproduction are entirely dependent on living plant cells. The molecular basis of this life-style, obligate biotrophy, remains unknown. We present the genome analysis of barley powdery mildew, Blumeria graminis f.sp. hordei (Blumeria), as well as a comparison with the analysis of two powdery mildews pathogenic on dicotyledonous plants. These genomes display massive retrotransposon proliferation, genome-size expansion, and gene losses. The missing genes encode enzymes of primary and secondary metabolism, carbohydrate-active enzymes, and transporters, probably reflecting their redundancy in an exclusively biotrophic life-style. Among the 248 candidate effectors of pathogenesis identified in the Blumeria genome, very few (less than 10) define a core set conserved in all three mildews, suggesting that most effectors represent species-specific adaptations.
Deficiens (defA+) is a homeotic gene involved in the genetic control of Antirrhinum majus flower development. Mutation of this gene (defA‐1) causes homeotic transformation of petals into sepals and of stamina into carpels in flowers displaying the ‘globifera’ phenotype, as shown by cross sections and scanning electronmicroscopy of developing flowers. A cDNA derived from the wild type defA+ gene has been cloned by differential screening of a subtracted ‘flower specific’ cDNA library. The identity of this cDNA with the defA+ gene product has been confirmed by utilizing the somatic and germinal instability of defA‐1 mutants. According to Northern blot analyses the defA+ gene is expressed in flowers but not in leaves, and its expression is nearly constant during all stages of flower development. The 1.1 kb long mRNA has a 681 bp long open reading frame that can code for a putative protein of 227 amino acids (mol. wt 26.2 kd). At its N‐terminus the DEF A protein reveals homology to a conserved domain of the regulatory proteins SRF (activating c‐fos) in mammals and GRM/PRTF (regulating mating type) in yeast. We discuss the structure and the possible function of the DEF A protein in the control of floral organogenesis.
Homeotic mutants have been useful for the study of animal development. Such mutants are also known in plants. The isolation and molecular analysis of several homeotic genes in Antirrhinum majus provide insights into the underlying molecular regulatory mechanisms of flower development. A model is presented of how the characteristic sequential pattern of developing organs, comprising the flower, is established in the process of morphogenesis.
GLOBOSA (GLO) is a homeotic gene whose mutants show sepaloid petals and carpelloid stamens. The similarity of Glo mutants to those of the DEFICIENS (DEFA) gene suggests that the two genes have comparable functions in floral morphogenesis. The GLO cDNA has been cloned by virtue of its homology to the MADS‐box, a conserved DNA‐binding domain also contained in the DEFA gene. We have determined the structure of the wild type GLO gene as well as of several glo mutant alleles which contain transposable element insertions responsible for somatic and germinal instability of Glo mutants. Analyses of the temporal and spatial expression patterns of the DEFA and GLO genes during development of wild type flowers and in flowers of various stable and unstable defA and glo alleles indicate independent induction of DEFA and GLO transcription. In contrast, organ‐specific up‐regulation of the two genes in petals and stamens depends on expression of both DEFA and GLO. In vitro DNA‐binding studies were used to demonstrate that the DEFA and GLO proteins specifically bind, as a heterodimer, to motifs in the promoters of both genes. A model is presented which proposes both combinatorial and cross‐regulatory interactions between the DEFA and GLO genes during petal and stamen organogenesis in the second and third whorls of the flower. The function of the two genes controlling determinate growth of the floral meristem is also discussed.
To cope with growth in low-phosphate (Pi) soils, plants have evolved adaptive responses that involve both developmental and metabolic changes. PHOSPHATE STARVATION RESPONSE 1 (PHR1) and related transcription factors play a central role in the control of Pi starvation responses (PSRs). How Pi levels control PHR1 activity, and thus PSRs, remains to be elucidated. Here, we identify a direct Pi-dependent inhibitor of PHR1 in Arabidopsis, SPX1, a nuclear protein that shares the SPX domain with yeast Pi sensors and with several Pi starvation signaling proteins from plants. Double mutation of SPX1 and of a related gene, SPX2, resulted in molecular and physiological changes indicative of increased PHR1 activity in plants grown in Pi-sufficient conditions or after Pi refeeding of Pi-starved plants but had only a limited effect on PHR1 activity in Pi-starved plants. These data indicate that SPX1 and SPX2 have a cellular Pi-dependent inhibitory effect on PHR1. Coimmunoprecipitation assays showed that the SPX1/PHR1 interaction in planta is highly Pi-dependent. DNA-binding and pull-down assays with bacterially expressed, affinity-purified tagged SPX1 and ΔPHR1 proteins showed that SPX1 is a competitive inhibitor of PHR1 binding to its recognition sequence, and that its efficiency is highly dependent on the presence of Pi or phosphite, a nonmetabolizable Pi analog that can repress PSRs. The relative strength of the SPX1/PHR1 interaction is thus directly influenced by Pi, providing a link between Pi perception and signaling.phosphate sensor | phosphate starvation signaling S ince the beginning of molecular genetics, phosphate (Pi) starvation rescue systems, especially the Pi starvation rescues systems of bacteria and yeast, have served as emblematic models for studies of regulation of gene activity. In plants, these systems have gained additional interest because of the complexity and multicellular nature of plants (1, 2), and especially due to their potential for improving Pi acquisition and use in crops, a major goal toward sustainable agriculture. Considerable information has been gathered in the past decade on the components of the Pi starvation signaling pathway (reviewed in refs. 3-6). Major findings in plants include (i) identification of PHOSPHATE STARVATION RESPONSE 1 (PHR1) and related transcription factors as master regulators of Pi starvation responses (PSRs) (7-11); (ii) demonstration of the involvement of ubiquitin system components, including PHO2 and NLA, in Pi signaling (12-16); (iii) identification of miRNAs as mobile signals in Pi homeostasis (17, 18); and (iv) identification of Pi starvation-induced (PSI) riboregulators of miRNA activity, based on target mimicry (19) and natural antisense RNA that activates translation of PHO1 mRNA (20). In addition, a singular characteristic of nutrient starvation responses in plants is that several of these responses are at long distance, systemically controlled by plant shoot nutrient status, whereas others are controlled by local nutrient concentration. Transcriptomic a...
Anomalous flowering of the Antirrhinum majus mutant squamosa (squa) is characterized by excessive formation of bracts and the production of relatively few and often malformed or incomplete flowers. To study the function of squamosa in the commitment of an inflorescence lateral meristem to floral development, the gene was cloned and its genomic structure, a well as that of four mutant alleles, was determined. SQUA is a member of a family of transcription factors which contain the MADS‐box, a conserved DNA binding domain. In addition, we analysed the temporal and spatial expression pattern of the squa gene. Low transcriptional activity of squa is detectable in bracts and in the leaves immediately below the inflorescence. High squa transcript levels are seen in the inflorescence lateral meristems as soon as they are formed in the axils of bracts. Squa transcriptional activity persists through later stages of floral morphogenesis, with the exception of stamen differentiation. Although necessary for shaping a normal racemose inflorescence, the squa function is not absolutely essential for flower development. We discuss the function of the gene during flowering, its likely functional redundancy and its possible interaction with other genes participating in the genetic control of flower formation in Antirrhinum.
We have determined the structure of the floral homeotic deficiens (defA) gene whose mutants display sepaloid petals and carpelloid stamens, and have analysed its spatial and temporal expression pattern. In addition, several mutant alleles (morphoalleles) were studied. The results of these analyses define three functional domains of the DEF A protein and identify in the deficiens promoter a possible cis‐acting binding site for a transcription factor which specifically upregulates expression of deficiens in petals and stamens. In vitro DNA binding studies show that DEF A binds to specific DNA motifs as a heterodimer, together with the protein product of the floral homeotic globosa gene, thus demonstrating that the protein encoded by deficiens is a DNA binding protein. Furthermore, Northern analysis of a temperature sensitive allele at permissive and non‐permissive temperatures provides evidence for autoregulation of the persistent expression of deficiens throughout flower development. A possible mechanism of autoregulation is discussed.
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