Plant growth enhancement and associated physiological responses are coregulated by ethylene and gibberellin in response to harpin protein Hpa1

The harpin protein Hpa1 produced by the bacterial blight pathogen of rice induces several growth-promoting responses in plants, activating the ethylene signaling pathway, increasing photosynthesis rates and EXPANSIN (EXP) gene expression levels, and thereby enhancing the vegetative growth. This study was attempted to analyze any mechanistic connections among the above and the role of gibberellin in these responses. Hpa1-induced growth enhancement was evaluated in Arabidopsis, tomato, and rice. And growth-promoting responses were determined mainly as an increase of chlorophyll a/b ratio, which indicates a potential elevation of photosynthesis rates, and enhancements of photosynthesis and EXP expression in the three plant species. In Arabidopsis, Hpa1-induced growth-promoting responses were partially compromised by a defect in ethylene perception or gibberellin biosynthesis. In tomato and rice, compromises of Hpa1-induced growth-promoting responses were caused by a pharmacological treatment with an ethylene perception inhibitor or a gibberellin biosynthesis inhibitor. In the three plant species, moreover, Hpa1-induced growth-promoting responses were significantly impaired, but not totally eliminated, by abolishing ethylene perception or gibberellin synthesis. However, simultaneous nullifications in both ethylene perception and gibberellin biosynthesis almost canceled the full effects of Hpa1 on plant growth, photosynthesis, and EXP2 expression. Theses results suggest that ethylene and gibberellin coregulate Hpa1-induced plant growth enhancement and associated physiological and molecular responses.     

A mutation of casein kinase 2 α4 subunit affects multiple developmental processes in <Emphasis Type="Italic">Arabidopsis</Emphasis>

Key message

Arabidopsis CK2 α4 subunit regulates the primary root and hypocotyl elongation, lateral root formation, cotyledon expansion, rosette leaf initiation and growth, flowering, and anthocyanin biosynthesis.

Abstract

Casein kinase 2 (CK2) is a conserved tetrameric kinase composed of two α and two β subunits. The inhibition of CK2 activity usually results in severe developmental deficiency. Four genes (CKA1–CKA4) encode CK2 α subunit in Arabidopsis. Single mutations of CKA1, CKA2, and CKA3 do not affect the normal growth of Arabidopsis, while the cka1 cka2 cka3 triple mutants are defective in cotyledon and hypocotyl growth, lateral root development, and flowering. The inhibition of CKA4 expression in cka1 cka2 cka3 background further reduces the number of lateral roots and delays the flowering time. Here, we report the characterization of a novel knockout mutant of CKA4, which exhibits various developmental defects including reduced primary root and hypocotyl elongation, increased lateral root density, delayed cotyledon expansion, retarded rosette leaf initiation and growth, and late flowering. The examination of the cellular basis for abnormal root development of this mutant revealed reduced root meristem cells with enhanced RETINOBLASTOMA-RELATED (RBR) expression that promotes cell differentiation in root meristem. Moreover, this cka4-2 mutant accumulates higher anthocyanin in the aerial part and shows an increased expression of anthocyanin biosynthetic genes, suggesting a novel role of CK2 in modulating anthocyanin biosynthesis. In addition, the complementation test using primary root elongation assay as a sample confirms that the changed phenotypes of this cka4-2 mutant are due to the lack of CKA4. Taken together, this study reveals an essential role of CK2 α4 subunit in multiple developmental processes in Arabidopsis.

     

Heterologous expression of a novel <Emphasis Type="Italic">Poa pratensis</Emphasis> gibberellin 2-oxidase gene, <Emphasis Type="Italic">PpGA2ox</Emphasis>, caused dwarfism,late flowering,and increased chlorophyll accumulation in <Emphasis Type="Italic">Arabidopsis</Emphasis>

Gibberellin 2-oxidases (GA2oxs) irreversibly convert bioactive gibberellins (GAs) and their immediate precursors into inactive GAs via 2-β hydroxylation and so regulate gibberellin content in plants. However, to the best of our knowledge, little has been known about the GA2oxs and its function in cool season turfgrass Poa pratensis. In this study, rapid amplification of cDNA end (RACE) was employed to isolate PpGA2ox from P. pratensis. The open reading frame of PpGA2ox was 1 047 bp in length, corresponding to 348 amino acids. PpGA2ox was localized in both nucleus and cytoplasm. The expression of PpGA2ox could be up-regulated by 10 μM gibberellic acid, 5 μM methyl jasmonate, or 10 μM indole-3-acetic acid. In addition, its native promoter could drive GUS expression in both leaf apex and shoot apical region. Moreover, overexpression of PpGA2ox in Arabidopsis led to GA-deficiency leading to dwarf phenotype, delayed flowering time, and increased chlorophyll content. Our study suggests that PpGA2ox could be a candidate gene for breeding new cultivars of P. pratensis.     

Enhancement of hypocotyl elongation by LOV KELCH PROTEIN2 production is mediated by auxin and phytochrome-interacting factors in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis>

Key message

Auxin and two phytochrome-interacting factors, PHYTOCHROME-INTERACTING FACTOR4 (PIF4) and PIF5, play crucial roles in the enhancement of hypocotyl elongation in transgenic Arabidopsis thaliana plants that overproduce LOV KELCH PROTEIN2 (LKP2).

Abstract

LOV KELCH PROTEIN2 (LKP2) is a positive regulator of hypocotyl elongation under white light in Arabidopsis thaliana. In this study, using microarray analysis, we compared the gene expression profiles of hypocotyls of wild-type Arabidopsis (Columbia accession), a transgenic line that produces green fluorescent protein (GFP), and two lines that produce GFP-tagged LKP2 (GFP-LKP2). We found that, in GFP-LKP2 hypocotyls, 775 genes were up-regulated, including 36 auxin-responsive genes, such as 27 SMALL AUXIN UP RNA (SAUR) and 6 AUXIN/INDOLE-3-ACETIC ACID (AUX/IAA) genes, and 21 genes involved in responses to red or far-red light, including PHYTOCHROME-INTERACTING FACTOR4 (PIF4) and PIF5; and 725 genes were down-regulated, including 15 flavonoid biosynthesis genes. Hypocotyls of GFP-LKP2 seedlings, but not cotyledons or roots, contained a higher level of indole-3-acetic acid (IAA) than those of control seedlings. Auxin inhibitors reduced the enhancement of hypocotyl elongation in GFP-LKP2 seedlings by inhibiting the increase in cortical cell number and elongation of the epidermal and cortical cells. The enhancement of hypocotyl elongation was completely suppressed in progeny of the crosses between GFP-LKP2 lines and dominant gain-of-function auxin-resistant mutants (axr2-1 and axr3-1) or loss-of-function mutants pif4, pif5, and pif4 pif5. Our results suggest that the enhancement of hypocotyl elongation in GFP-LKP2 seedlings is due to the elevated level of IAA and to the up-regulated expression of PIF4 and PIF5 in hypocotyls.

     

Cryptochrome 1b from Sweet Sorghum Regulates Photoperiodic Flowering,Photomorphogenesis, and ABA Response in Transgenic <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis>

Cryptochromes are blue/UV-A light receptors that mediate various aspects of plant growth and development. Here, we report the function and signal mechanism of cryptochrome 1b (SbCRY1b) from sweet sorghum [Sorghum bicolor (L.) Moench], a typical short-day cereal plant, to explore its potential for genetic improvement of sweet sorghum varieties. SbCRY1b mRNA enrichment showed almost 24-h diurnal rhythms in both short-day (SD) and long-day (LD) conditions. Overexpression of SbCRY1b rescued the late-flowering and the long hypocotyl phenotypes of cry1cry2 double mutant in the transgenic Arabidopsis. SbCRY1b mediated Arabidopsis FT mRNA expression in LD and HY5 protein accumulation in response to blue light. SbCRY1b protein was located in both the nucleus and cytoplasm and was degraded by 26S proteasomes in response to blue light. SbCRY1b interacted, respectively, with Arabidopsis suppressor of PHYA-1051 (AtSPA1), E3 ubiquitin ligase constitutive photomorphogenesis 1 (AtCOP1), and a putative COP1 from sweet sorghum (SbCOP1) instead of SbSPA1 in vitro in a blue light-dependent manner. The observations imply SbCRY1b functions as a major regulator of photoperiodic flowering and its function is more similar to that of Arabidopsis CRY2. Moreover, SbCRY1b-overexpressed transgenic Arabidopsis showed oversensitivity to abscisic acid (ABA) during seed germination and root development. The expression of abscisic acid-insensitive 4 (ABI4), ABI5, abscisic acid responsive element-binding 1 (ABF1), (sucrose non-fermenting 1)-related protein kinase (SnRK2.3), RD29A, and EM6 was upregulated in the transgenic Arabidopsis. The results demonstrated that SbCRY1b may integrate blue light and ABA signals to regulate plant development.     

Identification and expression pattern analysis of the glucosinolate biosynthetic gene <Emphasis Type="Italic">BoCYP83B1</Emphasis> from broccoli

Glucosinolates are a branch of amino acid-derived metabolites, which are specifically found in Brassicales. In Arabidopsis, tryptophan derived indolic glucosinolates are required for plant defense against a wide range of pathogens and herbivores due to their strong antimicrobial activity and potential signaling function. An important enzyme in indolic glucosinolate biosynthesis pathway is CYP83B1, which oxidizes indole-3-acetaldoxime, a precursor of indole-3-acetic acid (IAA). In this study, we reported isolation and expression characterization of a CYP83B1 gene from Brassica oleracea L. var. italica Plenck, which we termed BoCYP83B1. Overexpression of BoCYP83B1 in Arabidopsis resulted in an altered glucosinolate profile and early flowering phenotype. By expressing the reporter gene β-glucuronidase under the control of the BoCYP83B1 promoter in Arabidopsis, we analyzed the spatial expression pattern of BoCYP83B1 under normal growth conditions as well as in response to several hormones and stresses. The BoCYP83B1 was primarily expressed in vascular tissue through the almost whole plant. It was strongly induced by methyl jasmonate, 1-amino-1-cyclopropanecarboxylic acid, salicylic acid (SA), gibberellin, and IAA, suggesting its involvement in complex signaling pathways. Mannitol, NaCl, UV, and Flagelin 22 significantly up-regulated BoCYP83B1 expression, indicating its possible role in stress response. Interestingly, the response of BoCYP83B1 to SA and NaCl showed tissue specificity. Thus, BoCYP83B1 might have different functions in different tissues.     

Endophytic microbes <Emphasis Type="Italic">Bacillus</Emphasis> sp. LZR216-regulated root development is dependent on polar auxin transport in <Emphasis Type="Italic">Arabidopsis</Emphasis> seedlings

Key message

Endophytic microbes Bacillus sp. LZR216 isolated from Arabidopsis root promoted Arabidopsis seedlings growth. It may be achieved by promoting the lateral root growth and inhibiting the primary root elongation.

Abstract

Plant roots are colonized by an immense number of microbes, including epiphytic and endophytic microbes. It was found that they have the ability to promote plant growth and protect roots from biotic and abiotic stresses. But little is known about the mechanism of the endophytic microbes-regulated root development. We isolated and identified a Bacillus sp., named as LZR216, of endophytic bacteria from Arabidopsis root. By employing a sterile experimental system, we found that LZR216 promoted the Arabidopsis seedlings growth, which may be achieved by promoting the lateral root growth and inhibiting the primary root elongation. By testing the cell type-specific developmental markers, we demonstrated that Bacillus sp. LZR216 increases the DR5::GUS and DR5::GFP expression but decreases the CYCB1;1::GUS expression in Arabidopsis root tips. Further studies indicated that LZR216 is able to inhibit the meristematic length and decrease the cell division capability but has little effect on the quiescent center function of the root meristem. Subsequently, it was also shown that LZR216 has no significant effects on the primary root length of the pin2 and aux1-7 mutants. Furthermore, LZR216 down-regulates the levels of PIN1-GFP, PIN2-GFP, PIN3-GFP, and AUX1-YFP. In addition, the wild-type Arabidopsis seedlings in the present of 1 or 5 µM NPA (an auxin transport inhibitor) were insensitive to LZR216-inhibited primary root elongation. Collectively, LZR216 regulates the development of root system architecture depending on polar auxin transport. This study shows a new insight on the ability of beneficial endophytic bacteria in regulating postembryonic root development.