Molecular cloning, functional analysis and localization of a novel gene encoding polygalacturonase-inhibiting protein in Chorispora bungeana

Phytohormones regulate the non-redundant response of ω-3 fatty acid desaturases to low temperatures in Chorispora bungeana

To explore the contributions of ω-3 fatty acid desaturases (FADs) to cold stress response in a special cryophyte, Chorispora bungeana, two plastidial ω-3 desaturase genes (CbFAD7, CbFAD8) were cloned and verified in an Arabidopsis fad7fad8 mutant, before being compared with the microsomal ω-3 desaturase gene (CbFAD3). Though these genes were expressed in all tested tissues of C. bungeana, CbFAD7 and CbFAD8 have the highest expression in leaves, while CbFAD3 was mostly expressed in suspension-cultured cells. Low temperatures resulted in significant increases in trienoic fatty acids (TAs), corresponding to the cooperation of CbFAD3 and CbFAD8 in cultured cells, and the coordination of CbFAD7 and CbFAD8 in leaves. Furthermore, the cold induction of CbFAD8 in the two systems were increased with decreasing temperature and independently contributed to TAs accumulation at subfreezing temperature. A series of experiments revealed that jasmonie acid and brassinosteroids participated in the cold-responsive expression of ω-3 CbFAD genes in both C. bungeana cells and leaves, while the phytohormone regulation in leaves was complex with the participation of abscisic acid and gibberellin. These results point to the hormone-regulated non-redundant contributions of ω-3 CbFADs to maintain appropriate level of TAs under low temperatures, which help C. bungeana survive in cold environments.

Integrated regulation triggered by a cryophyte ω-3 desaturase gene confers multiple-stress tolerance in tobacco

ω-3 fatty acid desaturases (FADs) are thought to contribute to plant stress tolerance mainly through linolenic acid (C18:3)-induced membrane stabilization, but a comprehensive analysis of their roles in stress adaptation is lacking. Here, we isolated a microsomal ω-3 FAD gene (CbFAD3) from a cryophyte (Chorispora bungeana) and elucidated its functions in stress tolerance. CbFAD3, exhibiting a high identity to Arabidopsis AtFAD3, was up-regulated by abiotic stresses. Its functionality was verified by heterogonous expression in yeast. Overexpression of CbFAD3 in tobacco constitutively increased C18:3 in both leaves and roots, which maintained the membrane fluidity, and enhanced plant tolerance to cold, drought, and salt stresses. Notably, the constitutively increased C18:3 induced a sustained activation of plasma membrane Ca2+-ATPase, thereby, changing the stress-induced Ca2+ signaling. The reactive oxygen species (ROS) scavenging system, which was positively correlated with the level of C18:3, was also activated in the transgenic lines. Microarray analysis showed that CbFAD3-overexpressing plants increased the expression of stress-responsive genes, most of which are affected by C18:3, Ca2+, or ROS. Together, CbFAD3 confers tolerance to multiple stresses in tobacco through the C18:3-induced integrated regulation of membrane, Ca2+, ROS, and stress-responsive genes. This is in contrast with previous observations that simply attribute stress tolerance to membrane stabilization.

Chilling- and Freezing-Induced Alterations in Cytosine Methylation and Its Association with the Cold Tolerance of an Alpine Subnival Plant, Chorispora bungeana

Chilling (0–18°C) and freezing (<0°C) are two distinct types of cold stresses. Epigenetic regulation can play an important role in plant adaptation to abiotic stresses. However, it is not yet clear whether and how epigenetic modification (i.e., DNA methylation) mediates the adaptation to cold stresses in nature (e.g., in alpine regions). Especially, whether the adaptation to chilling and freezing is involved in differential epigenetic regulations in plants is largely unknown. Chorispora bungeana is an alpine subnival plant that is distributed in the freeze-thaw tundra in Asia, where chilling and freezing frequently fluctuate daily (24 h). To disentangle how C. bungeana copes with these intricate cold stresses through epigenetic modifications, plants of C. bungeana were treated at 4°C (chilling) and -4°C (freezing) over five periods of time (0–24 h). Methylation-sensitive amplified fragment-length polymorphism markers were used to investigate the variation in DNA methylation of C. bungeana in response to chilling and freezing. It was found that the alterations in DNA methylation of C. bungeana largely occurred over the period of chilling and freezing. Moreover, chilling and freezing appeared to gradually induce distinct DNA methylation variations, as the treatment went on (e.g., after 12 h). Forty-three cold-induced polymorphic fragments were randomly selected and further analyzed, and three of the cloned fragments were homologous to genes encoding alcohol dehydrogenase, UDP-glucosyltransferase and polygalacturonase-inhibiting protein. These candidate genes verified the existence of different expressive patterns between chilling and freezing. Our results showed that C. bungeana responded to cold stresses rapidly through the alterations of DNA methylation, and that chilling and freezing induced different DNA methylation changes. Therefore, we conclude that epigenetic modifications can potentially serve as a rapid and flexible mechanism for C. bungeana to adapt to the intricate cold stresses in the alpine areas.

Immuno-affinity purification of PglPGIP1, a polygalacturonase-inhibitor protein from pearl millet: studies on its inhibition of fungal polygalacturonases and role in resistance against the downy mildew pathogen

Polygalacturonase-inhibitor proteins (PGIPs) are important plant defense proteins which modulate the activity of microbial polygalacturonases (PGs) leading to elicitor accumulation. Very few studies have been carried out towards understanding the role of PGIPs in monocot host defense. Hence, present study was taken up to characterize a native PGIP from pearl millet and understand its role in resistance against downy mildew. A native glycosylated PGIP (PglPGIP1) of ~43 kDa and pI 5.9 was immunopurified from pearl millet. Comparative inhibition studies involving PglPGIP1 and its non-glycosylated form (rPglPGIP1; recombinant pearl millet PGIP produced in Escherichia coli) against two PGs, PG-II isoform from Aspergillus niger (AnPGII) and PG-III isoform from Fusarium moniliforme, showed both PGIPs to inhibit only AnPGII. The protein glycosylation was found to impact only the pH and temperature stability of PGIP, with the native form showing relatively higher stability to pH and temperature changes. Temporal accumulation of both PglPGIP1 protein (western blot and ELISA) and transcripts (real time PCR) in resistant and susceptible pearl millet cultivars showed significant Sclerospora graminicola-induced accumulation only in the incompatible interaction. Further, confocal PGIP immunolocalization results showed a very intense immuno-decoration with highest fluorescent intensities observed at the outer epidermal layer and vascular bundles in resistant cultivar only. This is the first native PGIP isolated from millets and the results indicate a role for PglPGIP1 in host defense. This could further be exploited in devising pearl millet cultivars with better pathogen resistance.

An update on polygalacturonase-inhibiting protein (PGIP), a leucine-rich repeat protein that protects crop plants against pathogens

Polygalacturonase inhibiting proteins (PGIPs) are cell wall proteins that inhibit the pectin-depolymerizing activity of polygalacturonases secreted by microbial pathogens and insects. These ubiquitous inhibitors have a leucine-rich repeat structure that is strongly conserved in monocot and dicot plants. Previous reviews have summarized the importance of PGIP in plant defense and the structural basis of PG-PGIP interaction; here we update the current knowledge about PGIPs with the recent findings on the composition and evolution of pgip gene families, with a special emphasis on legume and cereal crops. We also update the information about the inhibition properties of single pgip gene products against microbial PGs and the results, including field tests, showing the capacity of PGIP to protect crop plants against fungal, oomycetes and bacterial pathogens.

Cloning and functional analysis of three genes encoding polygalacturonase-inhibiting proteins from Capsicum annuum and transgenic CaPGIP1 in tobacco in relation to increased resistance to two fungal pathogens

Polygalacturonase-inhibiting proteins (PGIPs) are plant cell wall glycoproteins that can inhibit fungal endopolygalacturonases (PGs). The PGIPs directly reduce the aggressive potential of PGs. Here, we isolated and functionally characterized three members of the pepper (Capsicum annuum) PGIP gene family. Each was up-regulated at a different time following stimulation of the pepper leaves by Phytophthora capcisi and abiotic stresses including salicylic acid, methyl jasmonate, abscisic acid, wounding and cold treatment. Purified recombinant proteins individually inhibited activity of PGs produced by Alternaria alternata and Colletotrichum nicotianae, respectively, and virus-induced gene silencing in pepper conferred enhanced susceptibility to P. capsici. Because three PGIP genes acted similarily in conferring resistance to infection by P. capsici, and because individually purified proteins showed consistent inhibition against PG activity of both pathogens, CaPGIP1 was selected for manipulating transgenic tobacco. The crude proteins from transgenic tobacco exhibited distinct enhanced resistance to PG activity of both fungi. Moreover, the transgenic tobacco showed effective resistance to infection and a significant reduction in the number of infection sites, number of lesions and average size of lesions in the leaves. All results suggest that CaPGIPs may be involved in plant defense response and play an important role in a plant's resistance to disease.

Spatial distribution of polygalacturonase-inhibiting proteins in Arabidopsis and their expression induced by Stemphylium solani infection

Disease-induced polygalacturonase-inhibiting proteins (PGIPs) are the major defense proteins which play an important role in resistance to infection of pathogens. To date, the AtPGIP expression in Arabidopsis induced by Stemphylium solani (S. solani) was not described. Here the distribution of AtPGIPs and their expression induced by S. solani infection in Arabidopsis was reported. Notably, immunofluorescence localization showed that the AtPGIPs were distributed in leaves, petioles, stems and roots of 5 week old Arabidopsis, but they were mainly localized in epidermis, vascular bundles and vascular cylinder. Further studies indicated that the transcription level of AtPGIP1 and AtPGIP2 was both up-regulated in response to infection with S. solani which caused hypersensitive cell death, but the transcription level of AtPGIP2 was less induced than AtPGIP1. Consistently, the bulk AtPGIPs of Arabidopsis showed a higher activity in leaves infected by S. solani. Taken together, our preliminary results showed that AtPGIPs were spatially distributed and AtPGIP expression might take part in resistance to infection of S. solani. This study might highlight the potential importance of AtPGIPs and plant disease resistance.

Cloning and characterization of a Verticillium wilt resistance gene from Gossypium barbadense and functional analysis in Arabidopsis thaliana

Verticillium wilt causes enormous loss to yield or quality in many crops. In an effort to help controlling this disease through genetic engineering, we first cloned and characterized a Verticillium wilt resistance gene (GbVe) from cotton (Gossypium barbadense) and analyzed its function in Arabidopsis thaliana. Its nucleotide sequence is 3,819 bp long, with an open reading frame of 3,387 bp, and encoding an 1,128-aa protein precursor. Sequence analysis shows that GbVe produces a leucine-rich repeat receptor-like protein. It shares identities of 55.9% and 57.4% with tomato Ve1 and Ve2, respectively. Quantitative real-time PCR indicated that the Ve gene expression pattern was different between the resistant and susceptible cultivars. In the resistant Pima90-53, GbVe was quickly induced and reached to a peak at 2 h after inoculation, two-fold higher than that of control. We localized the GbVe-GFP fusion protein to the cytomembrane in onion epidermal cells. By inserting GbVe into Arabidopsis via Agrobacterium-mediated transformation, T(3) transgenic lines were obtained. Compared with the wild-type control, GbVe-overexpressing plants had greater levels of resistance to V. dahliae. This suggests that GbVe is a useful gene for improving the plant resistance against fungal diseases.