Expression of an Endopolygalacturonase Gene During Growth and Abscission of Peach Fruits

Polygalacturonase (PG, EC 3.2.1.15) is one of the cell wall hydrolases involved in the cell separation processes which occur during ripening of some fleshy fruits (FISHER and BENNETT 1991), as well as during abscission of leaves and fruits (HUBERMANN and GOREN 1979; RASC~O et al. 1985; TAYLOR et al. 1990; BONGHI et al. 1992; TAYLOR et al. 1993). In particular, endopolygalacturonase of tomato fruit is the most widely known form of this enzyme, having been characterized at molecular and biochemical level (see literature in ZHENG et al. 1992). Endopolygalacturonase activity has also been found in peach during both abscission and development of fruits, while no such activity could be detected during leaf abscission (BONGHI et al. 1992; ZANCHIN et al. 1993). A few years ago it was observed that polyclonal antibodies raised against a tomato fruit PG (i.e., PGZA) recognized, in soft ripe fruits of peach, a polypeptide with molecular mass similar to that of PG2A. Furthermore, using as a probe a cDNA coding for tomato fruit endopolygalacturonase, the same researchers were able to clone and characterize a 3.5 kb fragment of peach genomic DNA (LEE et al. 1990). On the basis of sequence analysis it was concluded that, besides an unidentified sequence, it contained about the 3' half of a gene which showed, in the coding regions, extensive homology with the tomato PG gene. According to the same researchers, such homology could explain the observed cross-reaction between the antibody to tomato fruit PG and a peach polypeptide, which was therefore suggested to be a peach endopolygalacturonase and the product of the partly characterized gene (LEE et al. 1990). In tomato the gene encoding the fruit endopolygalacturonase seems to be expressed during the fruit ripening, but not during the leaf abscission. In fact, despite a significative rise in PG activity, an antibody to fruit PG did not recognize any leaf abscission protein. Moreover, a cDNA encoding a tomato fruit PG gave no hybridization to mRNA obtained from activated abscission zones of tomato leaves (TAYLOR et al. 1990). In peach it has recently been shown that a cDNA coding for tomato fruit PG hybridized to mRNA obtained from fruit abscission zones but not from leaf ones where, in any case, no PG activity had been detected (BONGHI et al. 1992). In peach, cell separation events which show an involvement of endopolygalacturonase, are not restricted to fruit softening and abscission. Recently, it has been found that PG activity can also be detected throughout the fruit growth ( ZANCHIN et al. in press). On the basis of the above findings we considered it of some interest to see whether the endopolygalacturonase activity, observed in the course of different cell separation events in peach, is due to expression of the partly known PG gene (LEE et al. 1990) or, as already observed in tomato (TAYLOR et al. 1990), only some of that activity can be ascribed to expression of that gene

The maize fused leaves1 (fdl1) gene controls organ separation in the embryo and seedling shoot and promotes coleoptile opening

The fdl1-1 mutation, caused by an Enhancer/Suppressor mutator (En/Spm) element insertion located in the third exon of the gene, identifies a novel gene encoding ZmMYB94, a transcription factor of the R2R3-MYB subfamily. The fdl1 gene was isolated through co-segregation analysis, whereas proof of gene identity was obtained using an RNAi strategy that conferred less severe, but clearly recognizable specific mutant traits on seedlings. Fdl1 is involved in the regulation of cuticle deposition in young seedlings as well as in the establishment of a regular pattern of epicuticular wax deposition on the epidermis of young leaves. Lack of Fdl1 action also correlates with developmental defects, such as delayed germination and seedling growth, abnormal coleoptile opening and presence of curly leaves showing areas of fusion between the coleoptile and the first leaf or between the first and the second leaf. The expression profile of ZmMYB94 mRNA\u2014determined by quantitative RT-PCR\u2014 overlaps the pattern of mutant phenotypic expression and is confined to a narrow developmental window. High expression was observed in the embryo, in the seedling coleoptile and in the first two leaves, whereas RNA level, as well as phenotypic defects, decreases at the third leaf stage. Interestingly several of the Arabidopsis MYB genes most closely related to ZmMYB94 are also involved in the activation of cuticular wax biosynthesis, suggesting deep conservation of regulatory processes related to cuticular wax deposition between monocots and dicots

The life underwater of secondarily aquatic plants: some problems and solutions

The freshwater secondarily aquatic plants, most of which are higher plants, are those returned to the water environment after spending a period of time living on land. The readaptation to living underwater has made it necessary for these plants to put in place morphological and functional strategies to cope with some major problems due to features of the aquatic environment, but also deriving from the specialized organization of their \u201cterrestrial\u201d bodies. The poor O2 availability underwater accounted for the evolution of wide aerenchyma tissues throughout the plant organs to improve the photosynthetic O2 flux from the shoot to the roots buried in anoxic sediments and to the neighboring rhizosphere. This favors sediment oxygenation, sustains the aerobic metabolism of roots, and improves the availability and uptake of mineral nutrients, whose delivery to the entire plants, without a transpirational flux, is ensured by an acropetal mass transport depending on root pressure, guttation from hydathodes and channeling by apoplast closure around the vascular tissues. A great expansion of leaf surfaces and an enhanced surface:volume ratio of chloroplast-rich photosynthetic cells help to contact the water medium and to increase the cell/environment exchanges to gain inorganic carbon. Furthermore, different physiological mechanisms operate to cope with the scarce availability of CO2 and the prevalence of HCO3 12 as inorganic carbon form in water. Some of them, like cell wall acidification through H+ extrusion by a light-dependent APTase or activation of an apoplastic carbonic anhydrase, operate outside the cells, leading to a conversion of HCO3 12 to CO2, which then diffuses into the cells. Others, on the contrary, act inside the cells to load the active site of Rubisco with CO2, thus favoring photosynthesis and lowering photorespiration. Aquatic macrophytes with isoetid life form, moreover, can obtain most ot the fixed CO2 from sediments. In submerged species, in additin to the C3 cycle, the C4 and CAM-like photosynthetic metabolisms can also operate, and are modulated by the environmental inorganic carbon availability and the plant photosynthetic demand. Interestingly, in the aquatic plants the C4 pathway, which can be concomitant with the C3 one, does not depend on the Kranz anatomy of leaves, but relies on the intracellular compartmentation of carboxylative and decarboxylative enzymes. The CAM-like pathway, defined AAM, which also coexists with the C3, allows the submerged plants to fix CO2 in the dark, thus exploiting the higher CO2 availability in the water medium during the night, and extending to 24\u2009h the period of inorganic carbon assimilation. In almost all the aquatic macrophytes the AAM is only expressed in the submersion state, whereas it is quickly inactivated in emerging leaves in a cell by cell way