RESUMO
Gametophytic self-incompatibility in the Solanaceae is controlled by a multiallelic locus called the S locus. Growth of pollen tubes in the pistil is inhibited when the pollen has one of the two S alleles carried by the pistil. The products of a number of pistil S alleles[mdash]S proteins or S RNases[mdash]have been identified, and their role in controlling the pistil's ability to reject self-pollen has been positively established. In contrast, the existence of pollen S allele products has so far been inferred entirely from genetic evidence. Here, we introduced a modified S3 gene of Petunia inflata encoding an S3 RNase lacking RNase activity into P. inflata plants of the S2S3 genotype to determine whether the production of the mutant protein, designated S3(H93R), would have any effect on the ability of the transgenic plants to reject S2 and S3 pollen. Analysis of the self-incompatibility behavior of 49 primary transgenic plants and the progeny of three plants (H30, H37, and H40) that produced S3(H93R) in addition to producing wild-type levels of endogenous S2 and S3 RNases revealed that S3(H93R) had a dominant negative effect on the function of the S3 RNase in rejecting self-pollen; however, it had no effect on the function of the S2 RNase. One likely explanation of the results is that S3(H93R) competes with the S3 RNase for binding to a common molecule, which is presumably the product of the pollen S3 allele.
RESUMO
Microtubules influence morphogenesis by forming distinct geometrical arrays in the cell cortex, which in turn affect the deposition of cellulose microfibrils. Although many chemical and physical factors affect microtubule orientation, it is unclear how cortical microtubules in elongating cells maintain their ordered transverse arrays and how they reorganize into new geometries. To visualize these reorientations in living cells, we constructed a microtubule reporter gene by fusing the microtubule binding domain of the mammalian microtubule-associated protein 4 (MAP4) gene with the green fluorescent protein (GFP) gene, and transient expression of the recombinant protein in epidermal cells of fava bean was induced. The reporter protein decorates microtubules in vivo and binds to microtubules in vitro. Confocal microscopy and time-course analysis of labeled cortical arrays along the outer epidermal wall revealed the lengthening, shortening, and movement of microtubules; localized microtubule reorientations; and global microtubule reorganizations. The global microtubule orientation in some cells fluctuates about the transverse axis and may be a result of a cyclic self-correcting mechanism to maintain a net transverse orientation during cellular elongation.
RESUMO
In cereals, gibberellin (GA) enhances the synthesis and secretion of hydrolytic enzymes from aleurone cells. These enzymes then mobilize the endosperm storage reserves that fuel germination. The dose-response curve of aleurone protoplasts to GA extends over a range of concentrations from 10(-11) to more than 10(-6) M. One hypothesis is that subpopulations of cells have different sensitivities to GA, with each cell having a threshold concentration of GA above which it is switched on. The dose-response curve therefore reflects a gradual recruitment of cells to the pool exhibiting a full GA response. Alternatively, all cells may gradually increase their responses as the GA level is increased. In the present study we found that at increasing GA concentrations, increasing numbers of barley (Hordeum vulgare) cells showed the enhanced amylase secretion and vacuolation characteristic of the GA response. We also observed that the region of aleurone tissue closest to the embryo contains the highest proportion of cells activated at the GA concentrations thought to occur naturally in germinating grain. These data indicate that an aleurone layer contains cells of varying sensitivities to GA and that recruitment of these differentially responding pools of cells may explain the broad dose response to GA.