Jinrong Peng

‘Green revolution’ genes encode mutant gibberellin response modulators

World wheat grain yields increased substantially in the 1960s and 1970s because farmers rapidly adopted the new varieties and cultivation methods of the so-called ‘green revolution’. The new varieties are shorter, increase grain yield at the expense of straw biomass, and are more resistant to damage by wind and rain,. These wheats are short because they respond abnormally to the plant growth hormone gibberellin. This reduced response to gibberellin is conferred by mutant dwarfing alleles at one of two Reduced height-1 (Rht-B1 and Rht-D1) loci,. Here we show that Rht-B1/Rht-D1 and maize dwarf-8 (d8), are orthologues of the Arabidopsis Gibberellin Insensitive (GAI) gene,. These genes encode proteins that resemble nuclear transcription factors and contain an SH2-like domain, indicating that phosphotyrosine may participate in gibberellin signalling. Six different orthologous dwarfing mutant alleles encode proteins that are altered in a conserved amino-terminal gibberellin signalling domain. Transgenic rice plants containing a mutant GAI allele give reduced responses to gibberellin and are dwarfed, indicating that mutant GAI orthologues could be used to increase yield in a wide range of crop species.

Phytochrome A null mutants of Arabidopsis display a wild-type phenotype in white light.

Phytochrome is a family of photoreceptors that regulates plant photomorphogenesis; the best-characterized member of this family is phytochrome A. Here, we report the identification of novel mutations at three Arabidopsis loci (fhy1, fhy2, and fhy3) that confer an elongated hypocotyl in far-red but not in white light. fhy2 mutants are phytochrome A deficient, have reduced or undetectable levels of PHYA transcripts, and contain structural alterations within the PHYA gene. When grown in white light, fhy2 mutants are morphologically indistinguishable from wild-type plants. Thus, phytochrome A appears to be dispensable in white light-grown Arabidopsis plants. fhy2 alleles confer partially dominant phenotypes in far-red light, suggesting that the relative abundance of phytochrome A can affect the extent of the far-red-mediated hypocotyl growth inhibition response. Plants homozygous for the recessive fhy1 and fhy3 mutations have normal levels of functional phytochrome A. The FHY1 and FHY3 gene products may be responsible for the transduction of the far-red light signal from phytochrome A to downstream processes involved in hypocotyl growth regulation.

Gibberellin regulates Arabidopsis floral development via suppression of DELLA protein function.

The phytohormone gibberellin (GA) regulates the development and fertility of Arabidopsis flowers. The mature flowers of GA-deficient mutant plants typically exhibit reduced elongation growth of petals and stamens. In addition, GA-deficiency blocks anther development, resulting in male sterility. Previous analyses have shown that GA promotes the elongation of plant organs by opposing the function of the DELLA proteins, a family of nuclear growth repressors. However, it was not clear that the DELLA proteins are involved in the GA-regulation of stamen and anther development. We show that GA regulates cell elongation rather than cell division during Arabidopsis stamen filament elongation. In addition, GA regulates the cellular developmental pathway of anthers leading from microspore to mature pollen grain. Genetic analysis shows that the Arabidopsis DELLA proteins RGA and RGL2 jointly repress petal, stamen and anther development in GA-deficient plants, and that this function is enhanced by RGL1 activity. GA thus promotes Arabidopsis petal, stamen and anther development by opposing the function of the DELLA proteins RGA, RGL1 and RGL2.

The Jasmonate-ZIM Domain Proteins Interact with the R2R3-MYB Transcription Factors MYB21 and MYB24 to Affect Jasmonate-Regulated Stamen Development in Arabidopsis

Jasmonate is essential for diverse biological processes, including male fertility and plant defense in Arabidopsis. This work shows that the R2R3-MYB transcription factors MYB21 and MYB24 function as direct targets of JAZ proteins to mediate jasmonate-regulated stamen development. The Arabidopsis thaliana F-box protein CORONATINE INSENSITIVE1 (COI1) perceives jasmonate (JA) signals and subsequently targets the Jasmonate-ZIM domain proteins (JAZs) for degradation by the SCFCOI1-26S proteasome pathway to mediate various jasmonate-regulated processes, including fertility, root growth, anthocyanin accumulation, senescence, and defense. In this study, we screened JAZ-interacting proteins from an Arabidopsis cDNA library in the yeast two-hybrid system. MYB21 and MYB24, two R2R3-MYB transcription factors, were found to interact with JAZ1, JAZ8, and JAZ11 in yeast and in planta. Genetic and physiological experiments showed that the myb21 myb24 double mutant exhibited defects specifically in pollen maturation, anther dehiscence, and filament elongation leading to male sterility. Transgenic expression of MYB21 in the coi1-1 mutant was able to rescue male fertility partially but unable to recover JA-regulated root growth inhibition, anthocyanin accumulation, and plant defense. These results demonstrate that the R2R3-MYB transcription factors MYB21 and MYB24 function as direct targets of JAZs to regulate male fertility specifically. We speculate that JAZs interact with MYB21 and MYB24 to attenuate their transcriptional function; upon perception of JA signal, COI1 recruits JAZs to the SCFCOI1 complex for ubiquitination and degradation through the 26S proteasome; MYB21 and MYB24 are then released to activate expression of various genes essential for JA-regulated anther development and filament elongation.

Gibberellin-mediated proteasome-dependent degradation of the barley DELLA protein SLN1 repressor

DELLA proteins are nuclear repressors of plant gibberellin (GA) responses. Here, we investigate the properties of SLN1, a DELLA protein from barley that is destabilized by GA treatment. Using specific inhibitors of proteasome function, we show that proteasome-mediated protein degradation is necessary for GA-mediated destabilization of SLN1. We also show that GA responses, such as the aleurone α-amylase response and seedling leaf extension growth, require proteasome-dependent GA-mediated SLN1 destabilization. In further experiments with protein kinase and protein phosphatase inhibitors, we identify two additional signaling steps that are necessary for GA response and for GA-mediated destabilization of SLN1. Thus, GA signaling involves protein phosphorylation and dephosphorylation steps and promotes the derepression of GA responses via proteasome-dependent destabilization of DELLA repressors.

Gibberellin Acts through Jasmonate to Control the Expression of MYB21, MYB24, and MYB57 to Promote Stamen Filament Growth in Arabidopsis

Precise coordination between stamen and pistil development is essential to make a fertile flower. Mutations impairing stamen filament elongation, pollen maturation, or anther dehiscence will cause male sterility. Deficiency in plant hormone gibberellin (GA) causes male sterility due to accumulation of DELLA proteins, and GA triggers DELLA degradation to promote stamen development. Deficiency in plant hormone jasmonate (JA) also causes male sterility. However, little is known about the relationship between GA and JA in controlling stamen development. Here, we show that MYB21, MYB24, and MYB57 are GA-dependent stamen-enriched genes. Loss-of-function of two DELLAs RGA and RGL2 restores the expression of these three MYB genes together with restoration of stamen filament growth in GA-deficient plants. Genetic analysis showed that the myb21-t1 myb24-t1 myb57-t1 triple mutant confers a short stamen phenotype leading to male sterility. Further genetic and molecular studies demonstrate that GA suppresses DELLAs to mobilize the expression of the key JA biosynthesis gene DAD1, and this is consistent with the observation that the JA content in the young flower buds of the GA-deficient quadruple mutant ga1-3 gai-t6 rga-t2 rgl1-1 is much lower than that in the WT. We conclude that GA promotes JA biosynthesis to control the expression of MYB21, MYB24, and MYB57. Therefore, we have established a hierarchical relationship between GA and JA in that modulation of JA pathway by GA is one of the prerequisites for GA to regulate the normal stamen development in Arabidopsis.

Gibberellin Mobilizes Distinct DELLA-Dependent Transcriptomes to Regulate Seed Germination and Floral Development in Arabidopsis

Severe Arabidopsis (Arabidopsis thaliana) gibberellin (GA)-deficient mutant ga1-3 fails to germinate and is impaired in floral organ development. In contrast, the ga1-3 gai-t6 rga-t2 rgl1-1 rgl2-1 mutant confers GA-independent seed germination and floral development. This fact suggests that GA-regulated transcriptomes for seed germination and floral development are DELLA dependent. However, it is currently not known if all GA-regulated genes are GA regulated in a DELLA-dependent fashion and if a similar set of DELLA-regulated genes is mobilized to repress both seed germination and floral development. Here, we compared the global gene expression patterns in the imbibed seeds and unopened flower buds of the ga1-3 mutant with that of the wild type and of the ga1-3 gai-t6 rga-t2 rgl1-1 rgl2-1 mutant. We found that about one-half of total GA-regulated genes are apparently regulated in a DELLA-dependent fashion, suggesting that there might be a DELLA-independent or -partially-dependent component of GA-dependent gene regulation. A cross-comparison based on gene identity revealed that the GA-regulated DELLA-dependent transcriptomes in the imbibed seeds and flower buds are distinct from each other. Detailed ontology analysis showed that, on one hand, DELLAs differentially regulate the expression of different individual members of a gene family to run similar biochemical pathways in seeds and flower. Meanwhile, DELLAs control many functionally different genes to run specific pathways in seeds or flower buds to mark the two different developmental processes. Our data shown here not only confirm many previous reports but also single out some novel aspects of DELLA functions that are instructive to our future research.

The role of GA-mediated signalling in the control of seed germination.

Seed germination is promoted by gibberellin (GA) in many plant species. Several GA signalling factors are known to induce the expression of genes encoding enzymes that mobilise food reserves, including starches, proteins and lipids, stored in the endosperm during seed germination. However, these factors do not control seed germination. Two recent reports have indicated that RGL1 and RGL2, both homologous to the GA-response height-regulating factors GAI/RGA/RHT/d8/SLR1/SLN1, are repressors of seed germination in Arabidopsis. These reports provide new clues as to how GA controls seed germination. The induction of RGL2 expression by imbibition and its repression by GA are of particular interest because they imply that RGL2 acts as an integrator of environmental and endogenous cues for germination.

Loss of function of four DELLA genes leads to light- and gibberellin-independent seed germination in Arabidopsis

The Arabidopsis severe gibberellin-deficient mutant ga1-3 does not germinate even when the optimal light and temperature conditions are provided. This fact suggests that (1) gibberellin (GA) is absolutely necessary for the germination of an intact seed and (2) the ga1-3 mutant can be used as a good system to identify factors that repress seed germination. In this report, using ga1-3 mutation as the genetic background, we confirm that RGL2, one member of the DELLA family, encodes the predominant repressor of seed germination in Arabidopsis and show that the other DELLA genes GAI,RGA and RGL1 enhance the function of RGL2. More importantly, we show that ga1-3 seeds lacking RGA, RGL1 and RGL2 or GAI, RGL1 and RGL2, confer GA-independent germination in the light but not in the darkness whilst ga1-3 seeds lacking GAI, RGA and RGL2 germinate both in the light and darkness. This suggests that the destabilization or inactivation of RGA and GAI is not only triggered by GA but also possibly by light. In addition, ga1-3 seeds lacking in all the aforementioned four DELLA genes have elongated epidermal cells and confer light-, cold- and GA-independent seed germination. Therefore, DELLA proteins likely act as integrators of environmental and endogenous cues to regulate seed germination.

Derivative Alleles of the Arabidopsis Gibberellin-Insensitive (gai) Mutation Confer a Wild-Type Phenotype

The gai mutation of Arabidopsis confers a dwarf phenotype resembling that of mutants defective in gibberellin (GA) biosynthesis. However, gai mutant plants differ from GA biosynthesis mutants because they fail to respond to exogenous GAs and accumulate endogenous GA species to higher (rather than lower) levels than found in wild-type controls. The gai mutation, therefore, identifies a gene that modulates the response of plant cells to GA. We have mapped gai with respect to visible and restriction fragment length polymorphism (RFLP) markers from chromosome 1. To observe the phenotype exhibited by individuals potentially lacking wild-type (GAI) function, we have also isolated novel irradiation-induced derivative alleles of gai. When homozygous, these alleles confer a revertant phenotype that is indistinguishable from the wild type. gai is a semidominant mutation that exerts its effects either because it is a gain-of-function mutation or because it is a loss-of-function or reduced-function mutation. The genetic and physiological properties of the derivative alleles are considered with reference to these alternative modes of dominance of gai. Because these alleles are potential deletion or rearrangement mutations, together with the closely linked RFLP markers identified in the linkage mapping experiments, they provide useful resources for the isolation of the gai locus via a map-based cloning approach.