John E. Mullet

Jasmonic Acid Signaling Modulates Ozone-Induced Hypersensitive Cell Death

Recent studies suggest that cross-talk between salicylic acid (SA)–, jasmonic acid (JA)–, and ethylene-dependent signaling pathways regulates plant responses to both abiotic and biotic stress factors. Earlier studies demonstrated that ozone (O3) exposure activates a hypersensitive response (HR)–like cell death pathway in the Arabidopsis ecotype Cvi-0. We now have confirmed the role of SA and JA signaling in influencing O3-induced cell death. Expression of salicylate hydroxylase (NahG) in Cvi-0 reduced O3-induced cell death. Methyl jasmonate (Me-JA) pretreatment of Cvi-0 decreased O3-induced H2O2 content and SA concentrations and completely abolished O3-induced cell death. Cvi-0 synthesized as much JA as did Col-0 in response to O3 exposure but exhibited much less sensitivity to exogenous Me-JA. Analyses of the responses to O3 of the JA-signaling mutants jar1 and fad3/7/8 also demonstrated an antagonistic relationship between JA- and SA-signaling pathways in controlling the magnitude of O3-induced HR-like cell death.

Turgor-responsive gene transcription and RNA levels increase rapidly when pea shoots are wilted. Sequence and expression of three inducible genes

Reduction of turgor in pea shoots caused the accumulation of several poly(A) RNAs. cDNA clones derived from three different poly(A) RNAs which accumulate in wilted pea shoots were isolated, sequenced and expression of the corresponding genes examined. Clone 7a encoded a 289 amino acid protein. The C-terminal 180 amino acids of this protein were homologous to soybean nodulin-26. RNA hybridizing to cDNA 7a was abundant in roots, and induced in shoots by dehydration, heat shock and to a small extent by ABA. Hydropathic plots indicate that the protein encoded by cDNA 7a contains six potential membrane spanning domains similar to proteins which form ion channels. Clone 15a encoded a 363 amino acid protein with high homology to cysteine proteases. RNA hybridizing to cDNA 15a was more abundant in roots than shoots of control plants. Dehydration of pea shoots induced cDNA 15a mRNA levels whereas heat shock or ABA treatment did not. Clone 26g encoded a 508 amino acid protein with 30% residue identity to several aldehyde dehydrogenases. RNA hybridizing to cDNA 26g was induced by dehydration of shoots but not roots and heat shock and ABA did not modulate RNA levels. Levels of the three poly(A) RNAs increased 4–6-fold by 4 h after wilting and this increase was not altered by pretreatment of shoots with cycloheximide. When wilted shoots were rehydrated, RNA hybridizing to cDNA 26g declined to pre-stress levels within 2 h. Run-on transcription experiments using nuclei from pea shoots showed that transcription of the genes which encode the three poly(A) RNAs was induced within 30 min following reduction of shoot turgor. One of the genes showed a further increase in transcription by 4 h after dehydration whereas transcription of the other 2 genes declined. These results indicate that plant cells respond to changes in cell turgor by rapidly increasing transcription of several genes. Furthermore, the expression of the turgor-responsive genes varies with respect to the time course of induction and reversibility of the wilting-induced changes.

Oligosaccharins, brassinolides, and jasmonates: nontraditional regulators of plant growth, development, and gene expression.

Each of the nontraditional plant hormones reviewed in this article, oligosaccharins, brassinolides, and JA, can exert major effects on plant growth and development. However, in many cases, the mechanisms by which these compounds are involved in the endogenous regulation of morphogenesis remain to be established. Nevertheless, the use of mutant or transgenic plants with altered levels or perception of these hormones is leading to phenomenal increases in our understanding of the roles they play in the life cycle of plants. It is likely that in the future, novel modulators of plant growth and development will be identified; some will perhaps be related to the peptide encoded by ENOD40 (Van de Sande et al., 1996), which modifies the action of auxin.

Characterization of an Arabidopsis Lipoxygenase Gene Responsive to Methyl Jasmonate and Wounding

A cDNA corresponding to the gene AtLox2 was isolated from an Arabidopsis thaliana library using a lipoxygenase (LOX) probe from soybean. AtLox2 encodes a 102-kD protein, AtLOX2, which has 42 to 45% amino acid sequence identity with other plant LOX sequences. The AtLOX2 sequence is more than 30 amino acids longer at the amino terminus than other plant LOX sequences, and this extension has features reminiscent of chloroplast transit peptides, suggesting that AtLOX2 may be chloroplast localized. AtLox2 mRNA levels are high in leaves and inflorescences but very low in seeds, roots, and stems. AtLox2 mRNA accumulation is rapidly induced in leaves in response to methyl jasmonate. Leaves that have been wounded and adjacent leaves on the same plant also accumulate AtLox2 mRNA.

Dynamic Regulation of Chloroplast Transcription

This paper examines the coordinated expression of plastid and nuclear genes for chlorplast development and provides an opportunity to understand how plants sense and alter gene expression in response to light. Topic areas covered include the following: changing perspectives of plastid transcription; plastid genome organization; protein stoichiometry, mRNA abundance, and transcription rates; significance of plastid mRNA stability; overall dynamics of chloroplast transcription; differential transcription during chloroplast development;special role for a nuclear-encoded plastid-localized RNA polymerase. 27 refs., 1 fig., 1 tab.

Transcription and RNA stability are important determinants of higher plant chloroplast RNA levels.

Transcription in lysed barley plastids and Northern slot blot analyses were used to determine the relationship between changes in RNA levels and transcription during plastid development. Transcription in plastids of 4.5–9‐day‐old dark‐grown or illuminated barley seedlings declined up to 10‐fold as a function of plant age. Decreased transcription of some plastid genes (rbcL, psaA‐psaB) was paralleled by decreased levels of mRNA. In other cases (16SrDNA, psbA) the changes in transcription were not followed by proportional changes in RNA levels indicating that RNA stability is important in establishing the amount of plastid RNA for these genes. Further analysis showed that transcription of the plastid rRNA transcription unit is regulated differently than the transcription of protein coding genes such as psbA or rbcL.

Two Methyl Jasmonate-Insensitive Mutants Show Altered Expression of AtVsp in Response to Methyl Jasmonate and Wounding.

Jasmonates are plant signal molecules that are derived from lipids through the action of lipoxygenase. Jasmonates regulate gene expression during plant development and in response to water deficit, wounding, and pathogen elicitors. The signal transduction chain that mediates jasmonate action was investigated by isolating and studying two methyl jasmonate (MeJA)-insensitive mutants of Arabidopsis thaliana. The recessive mutants, jin1 and jin4, are nonallelic and neither corresponds to coi1, a previously identified MeJA-insensitive mutant. Both mutants showed reduced sensitivity to MeJA-mediated root growth inhibition as well as reduced MeJA induction of AtVsp in leaves. Expression of AtVsp in flowers was not altered in the mutants. Furthermore, MeJA modulation of the jasmonate-responsive lipoxygenase and phenylalanine ammonia lyase genes was not altered in the mutants. jin4 plants exhibited increased sensitivity to abscisic acid in seed germination assays, whereas jin1 plants showed wild-type sensitivity. Neither mutant showed altered sensitivity to ethylene in hypocotyl growth inhibition assays. jin1 and jin4 identify genes that modulate the response of AtVsp to MeJA in leaves of A. thaliana.

Arabidopsis thaliana Atvsp is homologous to soybean VspA and VspB, genes encoding vegetative storage protein acid phosphatases, and is regulated similarly by methyl jasmonate, wounding, sugars, light and phosphate

The soybean vegetative storage proteins, VSPα and VSPβ, are acid phosphatases that accumulate to very high levels in hypocotyls, young leaves and flowers and pods. The genes encoding the soybean VSP are activated by jasmonate, wounding, sugars and light and down regulated by phosphate and auxin. In this study, expression of an Arabidopsis thaliana gene (Atvsp) encoding a protein homologous to soybean Vspα and Vspβ, was examined and compared to expression of the soybean Vsp genes. Atvsp mRNA was present at high levels in flowers and buds and at low levels in roots, stems, leaves and siliques. Expression of Atvsp in leaves could be induced by wounding or by treatment of illuminated plants with methyl jasmonate and sucrose. Roots of plants with wounded leaves also accumulated Atvsp mRNA indicating that this gene can be regulated by a transmissible wound signal. Phosphate partially inhibited expression of Atvsp. Arabidopsis proteins of 29 and 30 kDa crossreacted with antibodies against soybean VSP. These proteins were very abundant in flowers and the proteins accumulated in leaves and roots of plants treated with methyl jasmonate. The level of these proteins in flowers was similar to the levels of soybean VSP in young soybean leaves. Overall, these data indicate that Arabidopsis Atvsp and soybean VspA/B genes are regulated similarly and that in both plants, the gene products can accumulate to high levels. This suggests that genes homologous to VspA/B may be of greater general significance than previously recognized.

Mapping of post-flowering drought resistance traits in grain sorghum: association between QTLs influencing premature senescence and maturity

Abstract The identification of genetic factors underlying the complex responses of plants to drought stress provides a solid basis for improving drought resistance. The stay-green character in sorghum (Sorghum bicolor L. Moench) is a post-flowering drought resistance trait, which makes plants resistant to premature senescence under drought stress during the grainfilling stage. The objective of this study was to identify quantitative trait loci (QTLs) that control premature senescence and maturity traits, and to investigate their association under post-flowering drought stress in grain sorghum. A genetic linkage map was developed using a set of recombinant inbred lines (RILs) obtained from the cross B35 × Tx430, which were scored for 142 restriction fragment length polymorphism (RFLP) markers. The RILs and their parental lines were evaluated for post-flowering drought resistance and maturity in four environments. Simple interval mapping identified seven stay-green QTLs and two maturity QTLs. Three major stay-green QTLs (SGA, SGD and SGG) contributed to 42% of the phenotypic variability (LOD 9.0) and four minor QTLs (SGB, SGI.1, SGI.2, and SGJ) significantly contributed to an additional 25% of the phenotypic variability in stay-green ratings. One maturity QTL (DFB) alone contributed to 40% of the phenotypic variability (LOD 10.0), while the second QTL (DFG) significantly contributed to an additional 17% of the phenotypic variability (LOD 4.9). Composite interval mapping confirmed the above results with an additional analysis of the QTL × Environment interaction. With heritability estimates of 0.72 for stay-green and 0.90 for maturity, the identified QTLs explained about 90% and 63% of genetic variability for stay-green and maturity traits, respectively. Although stay-green ratings were significantly correlated (r=0.22, P ≤ 0.05) with maturity, six of the seven stay-green QTLs were independent of the QTLs influencing maturity. Similarly, one maturity QTL (DFB) was independent of the stay-green QTLs. One stay-green QTL (SGG), however, mapped in the vicinity of a maturity QTL (DFG), and all markers in the vicinity of the independent maturity QTL (DFB) were significantly (P ≤ 0.1) correlated with stay-green ratings, confounding the phenotyping of stay-green. The molecular genetic analysis of the QTLs influencing stay-green and maturity, together with the association between these two inversely related traits, provides a basis for further study of the underlying physiological mechanisms and demonstrates the possibility of improving drought resistance in plants by pyramiding the favorable QTLs.

The Sorghum Photoperiod Sensitivity Gene, Ma3, Encodes a Phytochrome B'

The Ma3 gene is one of six genes that regulate the photoperiodic sensitivity of flowering in sorghum (Sorghum bicolor [L.] Moench). The ma3R mutation of this gene causes a phenotype that is similar to plants that are known to lack phytochrome B, and ma3R sorghum lacks a 123-kD phytochrome that predominates in light-grown plants and that is present in non-ma3R plants. A population segregating for Ma3 and ma3R was created and used to identify two randomly amplified polymorphic DNA markers linked to Ma3. These two markers were cloned and mapped in a recombinant inbred population as restriction fragment length polymorphisms. cDNA clones of PHYA and PHYC were cloned and sequenced from a cDNA library prepared from green sorghum leaves. Using a genome-walking technique, a 7941-bp partial sequence of PHYB was determined from genomic DNA from ma3R sorghum. PHYA, PHYB, and PHYC all mapped to the same linkage group. The Ma3- linked markers mapped with PHYB more than 121 centimorgans from PHYA and PHYC. A frameshift mutation resulting in a premature stop codon was found in the PHYB sequence from ma3R sorghum. Therefore, we conclude that the Ma3 locus in sorghum is a PHYB gene that encodes a 123-kD phytochrome.