SHATTERPROOF MADS-box genes control seed dispersal in Arabidopsis

The fruit, which mediates the maturation and dispersal of seeds, is a complex structure unique to flowering plants. Seed dispersal in plants such as Arabidopsis occurs by a process called fruit dehiscence, or pod shatter. Few studies have focused on identifying genes that regulate this process, in spite of the agronomic value of controlling seed dispersal in crop plants such as canola. Here we show that the closely related SHATTERPROOF (SHP1) and SHATTERPROOF2 (SHP2) MADS-box genes are required for fruit dehiscence in Arabidopsis. Moreover, SHP1 and SHP2 are functionally redundant, as neither single mutant displays a novel phenotype. Our studies of shp1 shp2 fruit, and of plants constitutively expressing SHP1 and SHP2, show that these two genes control dehiscence zone differentiation and promote the lignification of adjacent cells. Our results indicate that further analysis of the molecular events underlying fruit dehiscence may allow genetic manipulation of pod shatter in crop plants.     

Assessing the redundancy of MADS-box genes during carpel and ovule development

Carpels are essential for sexual plant reproduction because they house the ovules and subsequently develop into fruits that protect, nourish and ultimately disperse the seeds. The AGAMOUS (AG) gene is necessary for plant sexual reproduction because stamens and carpels are absent from ag mutant flowers. However, the fact that sepals are converted into carpelloid organs in certain mutant backgrounds even in the absence of AG activity indicates that an AG-independent carpel-development pathway exists. AG is a member of a monophyletic clade of MADS-box genes that includes SHATTERPROOF1 (SHP1), SHP2 and SEEDSTICK (STK), indicating that these four genes might share partly redundant activities. Here we show that the SHP genes are responsible for AG-independent carpel development. We also show that the STK gene is required for normal development of the funiculus, an umbilical-cord-like structure that connects the developing seed to the fruit, and for dispersal of the seeds when the fruit matures. We further show that all four members of the AG clade are required for specifying the identity of ovules, the landmark invention during the course of vascular plant evolution that enabled seed plants to become the most successful group of land plants.     

A haemoprotein with kinase activity encoded by the oxygen sensor of Rhizobium meliloti.

The expression of the nitrogen-fixation genes of Rhizobium meliloti is controlled by oxygen. These genes are induced when the free oxygen concentration is reduced to microaerobic levels. Two regulator proteins, FixL and FixJ, initiate the oxygen-response cascade, and the genes that encode them have been cloned. The fixL product seems to be a transmembrane sensor that modulates the activity of the fixJ product, a cytoplasmic regulator. FixL and FixJ are homologous to a family of bacterial two-component regulators, for which the mode of signal transduction is phosphorylation. We report here the purification of both FixJ and a soluble truncated FixL (FixL*), overproduced from a single plasmid construct. FixL* catalyses its own phosphorylation and the transfer of the gamma-phosphate of ATP to Fix J. The resulting FixJ-phosphate linkage is sensitive to base, as are the aspartyl phosphates of homologous systems. Visible spectra of purified FixL* show that it is an oxygen-binding haemoprotein. We propose that FixL senses oxygen through its haem moiety and transduces this signal by controlling the phosphorylation of FixJ.     

B and C floral organ identity functions require SEPALLATA MADS-box genes

Abnormal flowers have been recognized for thousands of years, but only in the past decade have the mysteries of flower development begun to unfold. Among these mysteries is the differentiation of four distinct organ types (sepals, petals, stamens and carpels), each of which may be a modified leaf. A landmark accomplishment in plant developmental biology is the ABC model of flower organ identity. This simple model provides a conceptual framework for explaining how the individual and combined activities of the ABC genes produce the four organ types of the typical eudicot flower. Here we show that the activities of the B and C organ-identity genes require the activities of three closely related and functionally redundant MADS-box genes, SEPALLATA1/2/3 (SEP1/2/3). Triple mutant Arabidopsis plants lacking the activity of all three SEP genes produce flowers in which all organs develop as sepals. Thus SEP1/2/3 are a class of organ-identity genes that is required for development of petals, stamens and carpels.