Steam‐girdling of barley (Hordeum vulgare) leaves leads to carbohydrate accumulation and accelerated leaf senescence, facilitating transcriptomic analysis of senescence‐associated genes

Abstract

Summary• Leaf senescence can be described as the dismantling of cellular components during a specific time interval before cell death. This has the effect of remobilizing N in the form of amino acids that can be relocalized to developing seeds. High levels of carbohydrates have previously been shown to promote the onset of the senescence process.• Carbohydrate accumulation in barley ( Hordeum vulgare ) plants was induced experimentally by steam-girdling at the leaf base, occluding the phloem, and gene regulation under these conditions was investigated using the Affymetrix Barley GeneChip array and quantitative real-time reverse transcriptase polymerase chain reaction (qRT-PCR).• Transcript levels of plastidial (aminopeptidases, cnd41) and vacuolar (thiol and serine) proteases clearly increase in girdled leaves. Of special interest are cnd41, a plastidial aspartyl peptidase that has been implicated in Rubisco degradation in tobacco; and cp-mIII, a highly upregulated carboxypeptidase. SAG12 , hexokinases and other senescence-specific genes are also upregulated under these conditions.• Applying a genomic approach to the innovative experimental system described here significantly enhances our knowledge of leaf proteolysis and whole-plant N recycling.

Transcriptional and Metabolic Analysis of Senescence Induced by Preventing Pollination in Maize

“…In barley, hyperaccumulation of sugars due to stem girdling of leaves accelerated senescence, and transcriptional analysis revealed up-regulation of several senescence-related genes (Parrott et al, 2007). In Arabidopsis, transcriptome analysis of pho3, a mutant that lacks Suc transporter (SUC2) and hyperaccumulates soluble sugars and starch in the leaves (Gottwald et al, 2000), showed induction of several senescence-related and flavonoid biosynthesis genes (Lloyd and Zakhleniuk, 2004).…”

“…Sugar levels also increase during the period coinciding with the onset of natural senescence in maize (Crafts-Brandner et al, 1984b) and Arabidopsis (Quirino et al, 2001;Diaz et al, 2005;Pourtau et al, 2006). There is mounting evidence that sugar accumulation in aging leaves either triggers or enhances the rate of senescence (Pourtau et al, 2006;Parrott et al, 2007;Wingler and Roitsch, 2008). Conversely, the availability of an alternative sink after completion of grain filling could potentially slow down such sugar accumulation and delay the onset of senescence.…”

“…In Arabidopsis, for instance, direct application of sugars to the leaf cells resulted in premature senescence (Pourtau et al, 2006). In barley (Hordeum vulgare), hyperaccumulation of carbohydrates (CHOs) in the leaves due to stem girdling resulted in early senescence (Parrott et al, 2007). Manipulations of source-sink ratios by removing the developing seed/fruit sink often delay senescence as seen in many species including sunflower (Helianthus annuus; Ho et al, 1987), soybean (Glycine max; CraftsBrandner et al, 1984a), cowpea (Vigna unguiculata; Khanna-Chopra and Reddy, 1988), and wheat (Triticum aestivum; Biswas and Mandal, 1986).…”

New insights into plastid nucleoid structure and functionality

“…It could be shown that CND41 is involved in Rubisco degradation and in the translocation of nitrogen during senescence (Kato et al 2004). In senescent leaves of barley its expression was shown to be down-regulated (Parrott et al 2007). This suggests that the activity of CND41 is associated with the maintenance of chloroplast function and not to dismantling of chloroplasts.…”

Autophagic Recycling Plays a Central Role in Maize Nitrogen Remobilization

“…Although the routes for protein turnover in aging leaves are not fully understood, plastid resident proteases, senescenceassociated vacuoles (SAVs), and macroautophagy (hereafter referred to as autophagy) are considered to be three important routes (Otegui et al, 2005;Ishida et al, 2008;Wada et al, 2009;Roberts et al, 2012). As examples, the transcript levels of multiple chloroplast proteases and various components of the autophagic machinery are known to rise early during leaf senescence in Arabidopsis thaliana and various cereals (Parrott et al, 2007;Liu et al, 2008;Phillips et al, 2008;Ruuska et al, 2008;Breeze et al, 2011;Avila-Ospina et al, 2014;Penfold and Buchanan-Wollaston, 2014) and is concomitant with the export of major protein stores like Rubisco and glutamine synthetase into SAVs with intense proteolytic activity (Otegui et al, 2005;Martínez et al, 2008). In addition, direct connections between autophagy and the turnover of chloroplast and mitochondrial constituents was provided by the study of autophagy mutants during leaf senescence (Ishida et al, 2008;Wada et al, 2009;Izumi et al, 2010;Li et al, 2014).…”