| In order to study Brassinosteroids (BRs) promoting plant salt resistance, we chose24-epibrassinolide (EBL) as the delegate, and studied the effects of BRs on salt resistant regulation of the three dicotyledons of canola (Brassica napus L. cv. Nanyanyou1) which belongs to Cruciferae, tomato (Solarium lycomersicum L. cv. Guifeihong) which belongs to Solanaceae, and oil sunflower (Helianthus annuus L. cv. Gaoyou86F1) which belongs to Asteraceae. Further, we explored the mechanisms of BRs enhancing plant salt resistance from various aspects. We embarked on our studies mainly from following aspects:plant photosynthesis and antioxidantive plant pigments, osmotic adjustment ability, ion homeostasis, metabolism of polyamines (PAs) and so on. In which, we took the investigation on whether PAs and signal molecular—H2O2generated from PAs catabolism involving in BRs-induced plant salt resistance enhancement or not as our most important works. The main results showed as follows:(1) In the periods of seed germination and seedling establishment, higher concentration of EBL (10-6M) could induce a strange opposite effects on low-salt and high-salt stressed canola:EBL’s inhibition effect on growth of low-salt stressed canola, and EBL’s promotion effect on growth of high-salt stressed canola. Besides, EBL easily caused intumescence of lower site of seedling hypocotyl under high-salt stress. Further study found that the both strange phenomenons were closely related to the levels of PAs and H2O2generated from their oxidative metabolism;(2) In the seedling stage of canola, foliar application of10-6～10-12M EBL could promote the growth of plants under salt stress, and10"10M EBL achieved the peak values of plant growth under salt stress, and EBL mainly relieve the salt-induced inhibitory effects on shoots. However, EBL showed an inhibitory effect on the plant growth under normal conditions, especially on roots. Further study found that EBL treatment could significantly enhance the efficiency of photosynthesis and membrane stability of different organs under salt stress, and correspondingly, levels of various photosynthetic and non-photosynthetic plant pigments were affected by EBL application and elevated in different degrees. They could contribute to the enhancement of plant antioxidant capacity, the efficiency of photosynthesis and so on;(3) EBL also could improve the osmotic adjustment and ion homeostasis of salt-stressed canola, especially in the more tender leaves and roots. Correspondingly, EBL could promote the accumulation of soluble sugar, free amino acid, proline, organic acids and a variety of essential inorganic ions, and reduce Na+and Cl-levels in different organs of salt-stressed plants. EBL could improve K+/Na+homeostasis in leaves due to Na+decrease in leaves;(4) EBL could regulate the distribution and balance of various kinds or forms of PAs in the tissue and subcellular levels to improve the salt tolerance of canola. The main manifestations were:promoting conversion of Put to Spm, promoting transformation of free PAs to conjugated or bound state, enlarging accumulation of vacuolar Put pool, promoting Spm accumulation in the cell wall and so on. The most obvious places regulated by EBL were also more tender leaves and near the apical region of roots. EBL and salt stress had many positive synergistic effects in regulating PAs, indicating that PAs metabolism is an important strategy of canola exposed to salt stress, and EBL further enhanced it;(5)10-8M EBL could well relieve salt-induced inhibitory effects on growth and development in the whole-life-cycle of tomato. EBL mainly regulated ion balance through reducing Na+content of salt-stressed tomato. EBL inhibited Na+transportation to shoots under salinity, and Na+contents in flowers and apiculus were declined most obviously. EBL could elevate the level of PAs in nutritoriums of salt-stressed tomato, espacially Spm level. Also, EBL-induced leaf-PAs increase mainly appeared in younger leaves. EBL increased fruit-PAs levels and (Spd+Spm)/Put ratio in developmental midanaphase and premetaphase respectively, improving their salt resistance. However, flowers of salt-stressed tomato were animated by EBL, and PAs-levels were reduced. It was likely to be a comprehensive result of promoting flowering and decreasing Na+content partially. However, these positive effects of EBL weren’t shown in non-stressed tomato. Therefore, EBL alleviated salt-induced toxicities on vegetative and reproductive growth through regulations of Na+-shoot-translocation and PAs-levels in the whole-life-cycle, where young vegetative or generative organs were the protective cores. It led to improvement of tomato salt resistance. EBL also could stimulate flowring and fruit ripening to promotion of the salt adaptation of tomato;(6)10-8M EBL also could enhance the salt resistance of sunflower. However, in the periods of vegetative growth, it was difficult to determine whether EBL had a promoting effect on the growth of salt-stressed sunflower from the appearance or not. EBL increased leaf area and stem thickness of plants, rather than increased plant height. When entering the reproductive growth process, the promoting resistance effects of EBL on sunflower salt rssistance gradually appeared from its appearance. Once in the reproductive growth stage, a rapid wilting emergence of older leaves was appeared in salt-stressed sunflower, while EBL slowed this phenomenon obviously. The effects of EBL on enhancing salt resistance of sunflower were well reflected from biomass, and it is even more pronounced before blossom, and correspondingly, EBL could significantly reduce the accumulation of Na+in the shoots of salt-stressed sunflower before blossom, thereby increasing the ratio of K+/Na+. However, when entering the blossom period, in order to reduce the level of Na+in flowers as much as possible, the reduction effects of EBL on the Na+of leaf and stem almost completely disappeared, however, it provided a powerful safeguard for EBL to increase the following sunflower yield of sunflower under salt stress.In generally, we fully confirmed that EBL could improve salt resistance of three dicotyledons, and it had obvious concentration effect, also, it was depending on the developmental stages and the degree of salt stress. According to the special effects of EBL on salt-stressed canola in embryonic stage, we first proposed the hormonal stress-level-dependent biphasic effects (SLDB), and it was closely related to the regulation of PAs and H2O2generated from their catabolism. Besides, accumulation and balance of PAs also played an important role in the EBL-caused enlargement effects of early seedling hypocotyl, the growth-promoting effects of the vegetative organs and meliorating effects of reproductive organs development under high salinity. Thus, PAs played a very key role in the regulation of EBL on dicotyledonous plant growth and development process under salt stress. In view of regulations of osmotic adjustment ability and ion balance, EBL was priority to protect young tissues and reproductive organs of the salt-stressed plants, which could greatly increase the adaptation of plants to salinity. Where, in the ion balance regulation, EBL mainly reduced Na+accumulation to improve the K+/Na+ratio, but it much smaller impacted on K+. Lower Na+translocation from the roots to the shoots was an important strategy of EBL to control Na+accumulation of plants. In addition, plant pigments also played an important role in the process of EBL enhancing plant salt resistance. |
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