Studies on senescing tobacco leaf disks with special reference to peroxidase : I. The effects of cutting, and of inhibition of nucleic-acid and protein synthesis

Role of chloroplastidial proteases in leaf senescence

In this report the effect of hydrogen peroxide (H2O2) on peroxidase (POD) activity during leaf senescence was studied with and without phenylmethylsulfonyl fluoride (PMSF) pre-treatment in detached neem (Azadirachta indica A. juss) leaf chloroplasts. Increased POD activity was detected in natural and H2O2-promoted senescent leaf chloroplasts compared to untreated control mature green leaf chloroplasts. However, under H2O2 POD activity markedly increased at 1 day, and then significantly decreased until 4 days. In the presence of H2O2, PMSF, the induction of POD activity was alleviated at 1 day, whereas reduced after 4 days. In contrast, in the presence of H2O2, cycloheximide (CX), the induction of POD activity was reduced at 1 day, whereas alleviated after 4 days. The was a partial reduction in H2O2-induced POD activity with PMSF and CX, indicating the presence of pre-existing inactive PODs in chloroplasts. We also propose a new role for chloroplastidial proteases as activators of pre-existing inactive PODs during leaf senescence.

The metabolism of indoleacetic acid by barley grains

It has been shown that indoleacetic acid (IAA) does not occur in developing grains of Hordeum vulgare L. (barley), but that an unidentified indolic compound does. This compound, designated 'A', was also found to be a product of the metabolism of exogenous IAA by barley. The expression of the gibberellic acid effect was delayed for at least 8 h if grains were imbibed in a solution of IAA, and during this time, the IAA was metabolised. The enzyme system involved could be peroxidase, which was active in the grains at all stages of their development and at maturity, and partially purified extracts of peroxidase were found to have considerable IAA oxidase activity.

The lysosome-concept in plants : I. Peroxidases associated with subcellular and wall fractions of maize root tips: Implications for vacuole development

Only 11% of the total peroxidase activity in a root tip homogenate of Zea mays (L.) was sedimentable. The majority of sedimentable activity was bound to membrane fractions with which acid hydrolases were also associated.Peroxidase activity was released from a cell wall fraction exhaustively washed using homogenization and detergent. The specific activities of the washings were 3-10 fold higher than in the sedimentable and soluble fractions. A quarter of the wall associated activity was only released after treatment of the washed wall preparation with cellulase.Histochemistry shows peroxidase associated with cell walls. The only activity within the cell was observed associated with the inner surface of the provacuole membranes. The provacuoles were observed fusing in maturing cells. No peroxidase was seen in the Golgi.One peroxidase isozyme was apparently specifically associated with the sedimentable membrane fractions and may represent the activity observed in the provacuole membranes. Three fast migrating isozymes were restricted to the soluble fractions while at least three slow migrating isozymes were wall associated. One of the latter was specifically released from the wall by calcium nitrate. The relationship between the isozymes is discussed.A scheme for the origin of vacuoles is proposed.

Non-uniformities in the metabolism of excised leaves and leaf discs

In intact tobacco and Chinese-cabbage (Brassica pekinensis) leaves an "ageing" process begins as soon as the leaves are excised. The terminal few millimetres of the petiole increasingly preempt materials such as phosphate and uracil taken up by the leaf. Actinomycin D treatment blocks this process and leads to increased uptake of such materials into the lamina.Immediately after excision there is a radial "geographical" gradient, in the ability of excised leaf discs to accumulate phosphate and uracil from solution. Tissue near the cut edge accumulates much more of these materials than that near the centre, and total nucleic acids isolated from the outer zone have a higher specific activity.Over the first day after excision there is a marked increase in this ability of the outer tissue of discs to accumulate labelled precursors but the changes taking place with time are complex and variable. Specific activity of total nucleic acids increases somewhat less than the increased uptake of labelled precursor. Actinomycin D becomes very unevenly distributed in leaf discs floated on solutions of the drug. These various effects are of sufficient magnitude to affect the interpretation of studies using excised leaf tissues.

Studies on senescing tobacco leaf disks with special reference to peroxidase : II. The effects and interactions of proline, hydroxyproline and kinetin

Senescing tobacco leaf disks were treated with hydroxyproline (hypro). Chlorophyll breakdown and α-amino-nitrogen increase were partially inhibited, but the decrease in soluble protein was stimulated. The normal increase in absolute acid-phosphatase activity was reversed, but the increase in peroxidase activity occurred although after a lag phase. Kinetin inhibition of senescence was reversed by hypro and kinetin actually reinforced the effects of hypro. Proline only partially overcame the effects of hypro plus kinetin. In the presence of kinetin, proline considerably stimulated the increase in peroxidase activity.Discussed are the way in which hypro may be acting, and the possibility of peroxidase being involved in proline hydroxylation.