One of the most significant manifestations of environmental stress in plants is the increased production of Reactive Oxygen Species (ROS). These ROS, if allowed to accumulate unchecked, can lead to cellular toxicity. A battery of antioxidant molecules is present in plants for keeping ROS levels under check and to maintain the cellular homeostasis under stress. Ascorbate peroxidase (APX) is a key antioxidant enzyme of such scavenging systems. It catalyses the conversion of H2O2 into H2O, employing ascorbate as an electron donor. The expression of APX is differentially regulated in response to environmental stresses and during normal plant growth and development as well. Different isoforms of APX show differential response to environmental stresses, depending upon their sub-cellular localization, and the presence of specific regulatory elements in the upstream regions of the respective genes. The present review delineates role of APX isoforms with respect to different types of abiotic stresses and its importance as a key antioxidant enzyme in maintaining cellular homeostasis.

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Chickpea (C. arietinum L.) is an important pulse crop in Asian and African countries that suffers significant yield losses due to attacks by insects like H. armigera. To obtain insights into early responses of chickpea to insect attack, a transcriptomic analysis of chickpea leaves just 20 minutes after simulated herbivory was performed, using oral secretions of H. armigera coupled with mechanical wounding. Expression profiles revealed differential regulation of 8.4% of the total leaf transcriptome with 1334 genes up-regulated and 501 down-regulated upon wounding at log2-fold change (|FC| ≤ −1 and ≥1) and FDR value ≤ 0.05. In silico analysis showed the activation of defenses through up-regulation of genes of the phenylpropanoid pathway, pathogenesis, oxidases and CYTP450 besides differential regulation of kinases, phosphatases and transcription factors of the WRKY, MYB, ERFs, bZIP families. A substantial change in the regulation of hormonal networks was observed with up-regulation of JA and ethylene pathways and suppression of growth associated hormone pathways like GA and auxin within 20 minutes of wounding. Secondary qPCR comparison of selected genes showed that oral secretions often increased differential expression relative to mechanical damage alone. The studies provide new insights into early wound responses in chickpea.

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The movement of petals during flower opening (anthesis) and their separation from the parent plant during abscission requires cell wall modification at the junction (abscission zone) of the petal and thalamus. The present study shows differential ethylene mediated temporal regulation of various members of the rose XTH gene family during flower opening and abscission in the ethylene sensitive, early abscising fragrant rose and the less sensitive late abscising hybrid rose. These studies indicate that large scale changes in xyloglucan crosslinking in cell wall mediated by XTHs may facilitate movement and separation during flower opening and abscission respectively.

Flowers of fragrant roses such as Rosa bourboniana are ethylene-sensitive and undergo rapid petal abscission while hybrid roses show reduced ethylene sensitivity and delayed abscission. To understand the molecular mechanism underlying these differences, a comparative transcriptome of petal abscission zones (AZ) of 0 h and 8 h ethylene-treated flowers from R. bourboniana was performed. Differential regulation of 3700 genes (1518 up, 2182 down) representing 8.5% of the AZ transcriptome was observed between 0 and 8 h ethylene-treated R. bourboniana petal AZ. Abscission was associated with large scale up-regulation of the ethylene pathway but prominent suppression of the JA, auxin and light-regulated pathways. Regulatory genes encoding kinases/phosphatases/F-box proteins and transcription factors formed the major group undergoing differential regulation besides genes for transporters, wall modification, defense and phenylpropanoid pathways. Further comparisons with ethylene-treated petals of R. bourboniana and 8 h ethylene-treated AZ (R. hybrida) identified a core set of 255 genes uniquely regulated by ethylene in R. bourboniana AZ. Almost 23% of these encoded regulatory proteins largely conserved with Arabidopsis AZ components. Most of these were up-regulated while an entire set of photosystem genes was prominently down-regulated. The studies provide important information on regulation of petal abscission in roses.

Summary The expression of insecticidal proteins under constitutive promoters in transgenic plants is fraught with problems like developmental abnormalities, yield drag, expression in unwanted tissues, and seasonal changes in expression. Rb PCD 1pro, a rapid, early acting wound‐inducible promoter from rose that is activated within 5 min of wounding, was isolated and characterized. Wounding increased transcript levels up to 150 and 500 folds within 5 and 20 min coupled with high translation as seen by histochemical GUS enzyme activity within 5–20 min. Rb PCD 1pro was activated by both sucking and chewing insects and showed wound‐inducible expression in various aerial tissues of plants representing commercially important dicot and monocot families. The promoter showed no expression in any vegetative tissue except upon wounding. Functionality of Rb PCD 1pro was tested by its ability to drive expression of the insecticidal protein gene cry IA c in transgenic Arabidopsis and tomato. Strong wound‐inducible Cry IA c expression was observed in both plants that increased 100–350 fold ( Arabidopsis ) and 280–600 fold (tomato) over the unwounded background within 5 min and over 1000–1600 fold within 20 min. The unwounded background level was just 3–6% of the Ca MV 35S promoter while wound‐induced expression was 5–27 folds higher than the best Ca MV 35S line in just 5 min and 80‐fold higher in 20 min. Transgenic plants showed strong resistance even to larger fourth instar larvae of H. armigera and no abnormalities in development and general plant growth. This is one of the earliest acting promoters with wide biotechnological application across monocot and dicot plants.

The existence of isoform diversity among antioxidant enzymes contributes to the spatial and temporal fine tuning of cellular responses. In plants heme binding ascorbate peroxidase (APX) (EC, 1.11.1.11) presents a crucial line of defense against reactive oxygen species. The present study aims to provide a comparative view of the functional attributes of major isoforms of APX in plants species. A total of 64 protein sequences of APX were subjected to homology search, multiple sequence alignment, phylogenetic tree construction, and motif analysis. The phylogenetic tree constructed revealed different clusters based on heme binding APX in respect of dicot and monocot plants such as different source of plant species represented by Oryza sativa, Arabidopsis thaliana, Sorghum bicolor, Zea mays, Ricinus communis, Populus trichocarpa, Vitis vinifera, and Selaginella moellendorffii. The multiple sequence alignment of these APX protein sequences from different plants showed conserved regions at different stretches with maximum homology in amino acid residues. The motif analysis revealed a conserved peroxidase domain uniformly observed in all APX irrespective of variable plant species suggesting its possible role in structural and enzymatic functions. The signature amino acids sequence of VFYQMGLSDKDIVALSGGHTLGRCH, NNGLHIAIRLCQPIKEQFPIITYADFYQLAGVVAVEVTGGPTIPMHPGRV and LFEDPSFRPYVEKYAKDQDAFFKDYAEAHMKLSELGF, related with the plant heme binding peroxidase as well as chloroplastic and cytosolic peroxidase signature was frequently observed and seemed to be related with the structure and enzymatic function in all APX protein sequences. The findings of the present study may be useful for designing degenerate primers or probes specific for APX and possibly presents the first line of defense amongst all the APX isoforms involved in the cellular antioxidant defense pathway, during exposure to abiotic stresses.