Avocado (Persea americana) fruit experience a rapid and extensive loss of firmness during ripening. In this study, we examined whether the chelator solubility and molecular weight of avocado polyuronides paralleled the accumulation of polygalacturonase (PC) activity and loss in fruit firmness. Polyuronides were derived from ethanolic precipitates of avocado mesocarp prepared using a procedure to rapidly inactivate endogenous enzymes. During ripening, chelator (cyclohexane-trans-l,2-diamine tetraacetic acid [CDlA])-soluble polyuronides increased from approximately 30 to 40 pg of galacturonic acid equivalents (mg alcohol-insoluble solids)-' in preripe fruit to 150 to 170 pg mg-' in postclimacteric fruit. In preripe fruit, chelator-extractable polyuronides were of high molecular weight and were partially excluded from Sepharose CL-26-300 gel filtration media. Avocado polyuronides exhibited marked downshifts in molecular weight during ripening. At the postclimacteric stage, nearly all chelator-extractable polyuronides, which constituted from 75 to 90% of total cell wall uronic acid content, eluted near the total volume of the filtration media. Rechromatography of low molecular weight polyuronides on BioCel P-4 disclosed that oligomeric uronic acids are produced in vivo during avocado ripening. l h e gel filtration behavior and pattern of depolymerization of avocado polyuronides were not influenced by the polyuronide extraction protocol (imidazole versus CDTA) or by chromatographic conditions designed to minimize interpolymeric aggregation. Polyuronides from ripening tomato (fycopersicon esculentum) fruit extracted and chromatographed under conditions identical with those used for avocado polyuronides exhibited markedly less rapid and less extensive downshifts in molecular weight during the transition from mature-green to fully ripe. Even during a 9-d period beyond the fully ripe stage, tomato fruit polyuronides exhibited limited additional depolymerization and did not include oligomeric species. A comparison of the data for the avocado and tomato fruit indicates that downshifts in polyuronide molecular weight are a prominent feature of avocado ripening and may also explain why molecular down-regulation of PC (EC 3.2.1.15) in tomato fruit has resulted in minimal effects on fruit performance until the terminal stages of ripening.Avocado fruit soften extensively during ripening, with mesocarp firmness, measured in terms of resistance to penetration, exceeding 450 N at the preripe stage and decreasing severa1 orders of magnitude during ripening (Pesis et al., 1978; Awad and Young, 1979; O'Donoghue and Huber,

No abstract

Cysteine protease inhibitors delayed the senescence of Sandersonia aurantiaca Hook. flowers. Tepal fading and wilting occurred later in the 2,2� -dipyridyl-treated flowers, and these flowers had a greater soluble protein content and less active endoproteases compared with control flowers that were held in water. Biochemical analysis revealed the presence of several protease-active bands in the soluble protein fraction of Sandersonia tepals. Activity of the polypeptides increased as flower senescence progressed. Western analysis with an antibody raised against the castor bean cysteine proteinase identified homologous proteins in Sandersonia flowers (ca 46, 41 and 31�kDa). Three cDNAs encoding cysteine proteinases were isolated from Sandersonia tepals (PRT5, PRT15 and PRT22). Expression of all three increased in tepals as senescence progressed. mRNAs for PRT5 were detected only in senescing flower tissue, whereas PRT15 and PRT22 were expressed in leaf, stem and root tissue. PRT5 has significant homology to C-terminus KDEL proteins, which have a role in the degradation of plant cell contents during programmed cell death. PRT15 is most similar to cysteine proteinases with a long C-terminal extension, whereas PRT22 is homologous to stress-induced cysteine proteinases.

No abstract

Visual symptoms of the onset of senescence in Sandersonia aurantiaca flowers begin with fading of flower colour and wilting of the tissue. When fully senescent, the flowers form a papery shell that remains attached to the plant. The cell walls of these flowers have been examined to determine whether there are wall modifications associated with the late stages of expansion and subsequent senescence-related wilting. Changes in the average molecular size of pectin were limited through flower opening and senescence, although there was a loss of neutral sugar-containing side-branches from pectins in opening flowers, and the total amounts of pectin and cellulose continued to rise in cell walls of fully senescent sandersonia flowers. Xyloglucan endotransglycosylase activity increased in opening and mature flowers, but declined sharply as flowers wilted. Concomitantly, the proportion of hemicellulose polymers of increasing molecular weight increased from flower expansion up to the point at which wilting occurred. Approximately 50% of the non-cellulosic neutral sugar in mature flower cell walls was galactose, primarily located in an insoluble cell wall fraction. Total galactose in this fraction increased per flower with maturity, then declined at the onset of wilting. Beta-galactosidase activity was low in expanding tepals, but increased as flowers matured and wilted.

Senescence is genetically controlled and activated in mature tissues during aging. However, immature plant tissues also display senescence-like symptoms when continuously exposed to adverse energy-depleting conditions. We used detached dark-held immature inflorescences of Arabidopsis (Arabidopsis thaliana) to understand the metabolic reprogramming occurring in immature tissues transitioning from rapid growth to precocious senescence. Macroscopic growth of the detached inflorescences rapidly ceased upon placement in water in the dark at 21°C. Inflorescences were completely degreened by 120 h of dark incubation and by 24 h had already lost 24% of their chlorophyll and 34% of their protein content. Comparative transcriptome profiling at 24 h revealed that inflorescence response at 24 h had a large carbon-deprivation component. Genes that positively regulate developmental senescence (ARABIDOPSIS NAC DOMAIN CONTAINING PROTEIN92) and shadeavoidance syndrome (PHYTOCHROME INTERACTING FACTOR4 [PIF4] and PIF5) were up-regulated within 24 h. Mutations in these genes delayed degreening of the inflorescences. Their up-regulation was suppressed in dark-held inflorescences by glucose treatment, which promoted macroscopic growth and development and inhibited degreening of the inflorescences. Detached inflorescences held in the dark for 4 d were still able to reinitiate development to produce siliques upon being brought out to the light, indicating that the transcriptional reprogramming at 24 h was adaptive and reversible. Our results suggest that the response of detached immature tissues to dark storage involves interactions between carbohydrate status sensing and light deprivation signaling and that the dark-adaptive response of the tissues appears to utilize some of the same key regulators as developmental senescence.

No abstract

The storage, soluble, and structural carbohydrates of two onion cultivars, the hard, pungent Pukekohe Longkeeper (PLK) and the softer, milder Houston Grano, were analyzed to determine differences that might be related to their response to sulfur nutrition received during growth as well as their postharvest attributes and end-use suitability. PLK tissue contained 1.37 times more dry matter than Grano and was composed of more fructan and sucrose and less glucose and fructose than Grano [corrected] There were also differences in neutral sugar content, especially galactose, and the amount, size, and content of pectin fractions soluble in chelator and weak alkali. These two onion cultivars differed in their capacity to take up sulfur, but there was no statistical association between sulfur supply and any measured dry matter component.