Journal of Plant Growth Regulation

Results from our previous study suggested that haem oxygenase-1/carbon monoxide (HO-1/CO) acts as a downstream signal system in the auxin-induced pathway leading to cucumber (Cucumis sativus) adventitious root formation. The objective of this study was to test whether HO-1 is also involved in hydrogen sulfide (H2S)-induced adventitious root formation. Cucumber explants were treated with HO-1 inducer haemin and H2S donor sodium hydrosulfide (NaHS) in combination with the specific inhibitor of HO-1 zinc protoporphyrin IX (ZnPPIX), and their effects on cucumber adventitious root development in IAA-depleted explants were compared. The results showed that similar to inducible responses of haemin, NaHS brought about the induction of cucumber HO-1 transcripts (CsHO-1) and its protein levels, and thereafter adventitious root formation. A further experiment verified that H2S or HS- rather than other sulfur-containing components derived from NaHS was ascribed to the stimulation response. The inducible effect is specific for CsHO-1 because ZnPPIX significantly suppressed the above responses, and the inhibitory effects were reversed partially when 30% CO-saturated aqueous solution was added. Molecular evidence further suggested that the NaHS-triggered upregulation of target genes responsible for HO-1/CO-induced adventitious root formation, including CsDNAJ-1 and CsCDPK1/5, was inhibited significantly by ZnPPIX. These decreases were reversed obviously by the addition of CO aqueous solution. However, hypotaurine (HT), the H2S scavenger, could not influence the haemin- and CO-induced adventitious rooting in IAA-depleted cucumber explants. Together, the above results suggested that HO-1 was involved in H2S-induced cucumber adventitious root formation.

Short brassinosteroid (BR) mutants lk, lka and lkb of pea (Pisum sativum L.) were investigated by immunofluorescence microscopy to elucidate the role of brassinosteroids in cell elongation via an effect on the microtubules (MTs). This study adds to our knowledge the fact that brassinolide (BL) can cause MT realignment in azuki bean and rescue the MT organization of BR mutants in Arabidopsis. It provides novel information on both cortical and epidermal cells and presents detailed information about the ratios of all MT orientations present, ranging from transverse (perpendicular to the elongating axis) to longitudinal (parallel to the elongating axis). Experiments were conducted in vivo using intact plants with direct application of a small amount of brassinolide (BL) to the internode. Employing a BR-receptor mutant, lka, and the BR-synthesis mutants, lk and lkb, allowed the identification and isolation of any BR-induced responses in the MT cytoskeleton following BL application. Increases in growth rate were noted in all pea lines including WT following BL application. These increases were strong in the BR-synthesis mutants, but weak in the BR-receptor mutant. Immunofluorescence revealed significant differences in the average MT orientation of cortical cells of mutants versus WTs. Importantly, these mutants possessed abundant MTs, unlike the BR-deficient bul1-1 mutant in Arabidopsis. Following BL application, the epidermal and cortical cells of lk and lkb plants showed a large and significant shift in MT orientation towards more transverse, whereas lka plants showed a small and nonsignificant response in these cells. These results suggest that the BR response pathway is linked to the regulation of MT orientation.

Salt stress is the main factor limiting plant growth. Researches on physiological and biochemical mechanisms of plant responses to salt stress are of great significance in exploring and improving plant salinity tolerance. Gladiolus gandavensis is an ornamental plant with beautiful flowers and good adversity adaptability that is native to the southeastern coast of China. The purpose of this study was to identify the salt tolerance level of G. gandavensis and explore the mechanism by which exogenous spermidine (Spd) adjusts the growth of G. gandavensis under different NaCl conditions. A pot experiment was conducted to examine the chlorophyll content, photosynthetic parameters, proline, reactive oxygen species (ROS) concentrations, antioxidant activities, and relative expression of the chlorophyll a/b response protein, CAT1, POD, MnSODM, P5CS, BADH genes and bZIP, DREB transcription factors in G. gandavensis seedlings under 0.3%, 0.6%, and 0.9% NaCl conditions, with and without 0.1 mmol·L−1 Spd. G. gandavensis seedlings exhibited reduced chlorophyll content; a decreased net photosynthetic rate (Pn); and increased relative electric conductivity (REC), malondialdehyde (MDA), proline and soluble protein contents, antioxidant activities, and relative expression of the CAT1, POD, MnSODM, P5CS, and BADH genes under 0.3%, 0.6%, and 0.9% NaCl conditions. And Spd spraying under 0.3% and 0.6% salt treatment reduced the decrease in chla content by approximately 55.2% and 23.4%, while increased Pn, proline content and the expression of CAT1, POD, MnSODM, P5CS, and BADH genes. Exogenously applied Spd effectively alleviated the damage caused by salt stress (0.3% and 0.6%) by upregulating the REC, proline content, gas exchange, antioxidant enzyme activity, and expression of CAT1, POD, MnSODM, P5CS, and BADH genes in G. gandavensis seedlings. However, when the seedlings were grown under 0.9% NaCl, no significant differences were found in the physiological and molecular responses between Spd-treated and non-Spd-treated plants. Therefore, the present study suggests that exogenous Spd can efficiently counteract the adverse effect of low (0.3%) and moderate (0.6%) salt stress on G. gandavensis seedlings.

Tolerance to deacclimation is an important physiological feature in plants in the face of global warming, which is resulting in incidents of increases in winter temperatures. The aim of the work was to explore how disturbances in the signalling and synthesis of brassinosteroids (BR) influence the deacclimation tolerance of barley. One group of mutants and their reference cultivars (Bowman and Delisa) was cold-acclimated, deacclimated and then tested for frost tolerance at − 12 °C. After cold acclimation, the second group of plants was additionally exposed to frost (− 6 °C) and then, deacclimated and tested for frost tolerance at − 12 °C. The deacclimated brassinosteroid mutants were characterised by an increased tolerance to frost, and consequently, had a higher tolerance to deacclimation than their wild-type cultivars. The mechanism of this phenomenon may be partly explained by analysing the hormonal homeostasis in the crowns. For all of the tested plants, a characteristic feature of the response to the deacclimation phase was an increase in the growth-promoting hormones and abscisic acid compared to the cold acclimation phase. The increase was greater in the BR-deficient (BW084) and BR-insensitive (BW312) mutants compared to the Bowman reference cultivar. Mutant 522DK was characterised by a lower accumulation of total cytokinins and gibberellins as well as an enhanced auxin deactivation compared to the Delisa. In the second group, when the plants were exposed to a temperature of − 6 °C before deacclimation, the hormonal homeostasis was further altered in both the mutants and reference cultivars, but all of the mutants had a higher frost tolerance than the wild types.

Strigolactones are a group of carotenoid-derived phytohormones which are synthesized in plastids and cytosol. Strigolactones also act as molecular cue that assist plants to liaise with surroundings. These active biomolecules are also called as non-traditional phytohormones or plant growth regulators. Strigolactones are generated in roots mainly and released into rhizosphere, although their synthesis has been reported in other plant parts in trace amount. More than one thousand types of strigolactones are reported in plants but only 30 strigolactones have been identified yet. Strigolactones play a significant role in regulation of biochemical processes such as seed germination, plant growth and development (root system architecture, shoot branching and tillering), and delaying leaf senescence during plant survival under environmental constraints. Strigolactones usually involve oxidative responses in plant cells and promote osmolytes production in stressed cells for maintenance of homeostasis under environmental stress, including salt, light, temperature, drought, nutrient deficiency, and heavy metals. The present review highlights biosynthesis and functions of strigolactones, their crosstalk with other phytohormones and functions in abiotic stress resistance mechanism. Application of strigolactones for plant growth under harsh conditions provides channel for development of novel technologies which are environmentally benign and sustainable. Further in-depth investigations are required about the exogenous treatment or transgenic approaches for the accumulation of strigolactones inside the plants which can be a potential strategy for protection of plants against multiple environmental stresses.

Our understanding of the developmental changes that occur during top leader elongation in gymnosperms lags behind that in angiosperms. We developed a semiquantitative method for determining epidermal cell size, by measuring the Feret diameter after cell wall staining of stem epidermal peels. This method allowed a large number of cells to be measured at various locations in the top leader of the Christmas tree Abies nordmanniana. Further, we have identified the growth rate of individual sections of the top leader, and the relationship between cell length and needle arrangement throughout the top leader. At bud break, all stem units begin to elongate simultaneously, but growth ceases from the base upwards during top leader elongation. Long top leaders were characterized by having up to three times as long cells at the base compared to short top leaders, whereas the cell lengths were similar in the apical region independent of the given plant growth capacity. In the basal sector, the level of auxin was much higher, whereas the levels of cytokinins were lower than in the apical sector, causing the auxin/cytokinin ratio to change from about 3 in the apical sector to more than 20 in the basal part. The Fibonacci number changed in the apical sector due to an increased cell number in the stem units and therefore longer distance between the needles. We conclude that the general growth pattern during top leader elongation in A. nordmanniana is similar to angiosperms but differs at the cellular level.

In apple (Malus) cultivation, the adaptability of rootstocks has a fundamental influence on multiple abiotic-stress resistance properties of their scions. Here, we investigated the high-salt resistance of apple rootstocks, by treating 2-year-old apple trees consisting of the ‘Fuji’ cultivar grafted onto three different dwarfing rootstocks (SH6, M9-T337 and G935) with a high-salt solution for 30 days. We found that the high-salt stress caused leaf yellowing and growth inhibition to all apple plants, and that the most serious reduction in relative chlorophyll content, plant height and biomass accumulation was associated with the SH6 rootstock. In addition, based on the salt injury index, the high-salt resistance of the ‘Fuji’ scion on the three dwarfing rootstocks was ranked as G935 > M9-T337 > SH6. In addition, analysis of mineral elemental composition showed that the Na+ content in ‘Fuji’ leaves on the G935 rootstock increased less than twofold compared to the control, whereas those on the SH6 rootstock increased nearly 20-fold, accompanied with reduced K and Fe element absorption under high-salt treatments. Expression analysis of nine Na+ transport-related genes indicated that in the salt overly sensitive (SOS) pathway, the SOS1, SOS2 and SOS3 genes showed considerably greater induction by high-salt stress in G935 roots than in SH6 roots. Given the Na+ content detected in roots of different rootstocks, we propose that more efficient Na+ extracellular efflux in G935 roots, rather than intracellular Na+ compartmentalization, contributed to the improved characteristics of the G935 rootstock under high-salt conditions.

Nickel (Ni) element is strongly phytotoxic at high concentrations for several plants, but due to its dual behavior and complicated chemistry, it has received little attention in plant nutrition and relevant experimental data are limited. The current research was carried out to study the effect of Ni on maize (Zea mays L.) growth and phytoextraction potential with EDTA assistance, a process termed as chemical assisted phytoremediation. Treatments included two levels of EDTA (0 and 0.5 mM), two levels of Ni (0 and 40 µM) and their combination (EDTA+Ni) that were applied to maize plants grown in a pot experiment. Application of Ni alone or in combination with EDTA reduced maize root and shoot length by 7.8% to 13.3% and by 15.6% to 21.1%, respectively, compared with control, as well as root and shoot dry weight by 42.0% to 60.0% and by 29.8% to 46.6%, respectively. A similar declining trend was observed also for the content of photosynthetic pigments (chl-a, chl-b, total chlorophyll, and carotenoids) as well as total proteins. However, proline, total soluble sugars, and free amino acids showed an increasing trend with application of Ni and EDTA alone or in combination. These treatments significantly decreased P and Na content in maize roots, stems, leaves, and grains, while increased K content compared with control. Application of EDTA with Ni was the most effective treatment to enhance Ni accumulation in maize (50.23 mg per plant) compared with Ni alone (40.62 mg per plant), EDTA alone (27.75 mg per plant), and control (15.51 mg per plant). Application of EDTA in combination with Ni enhanced Ni accumulation by 4.9 folds in maize shoots and by 2.6 folds in roots over control. In conclusion, application of EDTA in suitable concentrations may enhance Ni uptake by maize providing an effective and economic phytoremediation method of Ni-contaminated soils.

This study evaluated the overall influence of ectomycorrhizal fungi (ECMF) and exogenous Ca2+ supplementation on the growth, physiological, and metabolic traits of Pinus sylvestris var. mongolica seedlings under water stress and well-watered conditions. Several analytical methodologies pertinent to mycology, biochemistry, and ecology were applied. Exogenous Ca2+ was applied to 1-year-old mycorrhizal P. sylvestris var. mongolica, and the growth and physiological traits of various parts of P. sylvestris var. mongolica were assessed under water stress and well-watered conditions. The results showed that Ca2+ has an additional effect on the growth of P. sylvestris var. mongolica ECMF-inoculated under water stress. Ca2+ and ECMF together can enhance the activity of antioxidant enzymes, such as CAT, POD, SOD, and APX in the seedlings, and resist the accumulation of reactive oxygen species (ROS) caused by water stress. An appropriate combination of Ca2+ and ECMF can reduce the MDA content more efficiently in P. sylvestris var. mongolica and alleviate the extent of cellular stress. However, whether the free proline content is related to ECMF/Ca2+ inoculation remains to be further studied. The current findings indicate that, in the early stage of seedling development, it is feasible to apply exogenous Ca2+ and ECMF to improve growth and drought resistance.