Dakora Felix D, Matiru Viviene N, Kanu Alfred S
Department of Chemistry, Tshwane University of Technology , Pretoria, South Africa.
Department of Botany, Jomo Kenyatta University of Agriculture and Technology , Nairobi, Kenya.
Front Plant Sci. 2015 Sep 14;6:700. doi: 10.3389/fpls.2015.00700. eCollection 2015.
Lumichrome and riboflavin are novel molecules from rhizobial exudates that stimulate plant growth. Reported studies have revealed major developmental changes elicited by lumichrome at very low nanomolar concentrations (5 nM) in plants, which include early initiation of trifoliate leaves, expansion of unifoliate and trifoliate leaves, increased stem elongation and leaf area, and consequently greater biomass accumulation in monocots and dicots. But higher lumichrome concentration (50 nM) depressed root development and reduced growth of unifoliate and second trifoliate leaves. While the mechanisms remain unknown, it is possible that lumichrome released by rhizobia induced the biosynthesis of classical phytohormones that caused the observed developmental changes in plants. We also showed in earlier studies that applying either 10 nM lumichrome, 10 nM ABA, or 10 ml of infective rhizobial cells (0.2 OD600) to roots of monocots and dicots for 44 h produced identical effects, which included decreased stomatal conductance and leaf transpiration in Bambara groundnut, soybean, and maize, increased stomatal conductance and transpiration in cowpea and lupin, and elevated root respiration in maize (19% by rhizobia and 20% by lumichrome). Greater extracellular exudation of lumichrome, riboflavin and indole acetic acid by N2-fixing rhizobia over non-fixing bacteria is perceived to be an indication of their role as symbiotic signals. This is evidenced by the increased concentration of lumichrome and riboflavin in the xylem sap of cowpea and soybean plants inoculated with infective rhizobia. In fact, greater xylem concentration of lumichrome in soybean and its correspondingly increased accumulation in leaves was found to result in dramatic developmental changes than in cowpea. Furthermore, lumichrome and riboflavin secreted by soil rhizobia are also known to function as (i) ecological cues for sensing environmental stress, (ii) growth factors for microbes, plants, and humans, (iii) signals for stomatal functioning in land plants, and (iv) protectants/elicitors of plant defense. The fact that exogenous application of ABA to plant roots caused the same effect as lumichrome on leaf stomatal functioning suggests molecular cross-talk in plant response to environmental stimuli.
核黄素和核黄素是来自根瘤菌分泌物的新型分子，可刺激植物生长。已报道的研究表明，核黄素在极低的纳摩尔浓度（5 nM）下就能引起植物的主要发育变化，包括三出复叶的早期形成、单叶和三出复叶的扩展、茎伸长和叶面积增加，从而导致单子叶植物和双子叶植物生物量积累增加。但较高浓度的核黄素（50 nM）会抑制根系发育，并减少单叶和第二片三出复叶的生长。虽然其机制尚不清楚，但根瘤菌释放的核黄素可能诱导了经典植物激素的生物合成，从而导致植物出现观察到的发育变化。我们在早期研究中还表明，将10 nM核黄素、10 nM脱落酸或10 ml感染性根瘤菌细胞（0.2 OD600）施用于单子叶植物和双子叶植物的根部44小时，会产生相同的效果，包括降低 Bambara 花生、大豆和玉米的气孔导度和叶片蒸腾作用，增加豇豆和羽扇豆的气孔导度和蒸腾作用，以及提高玉米的根系呼吸作用（根瘤菌提高19%，核黄素提高20%）。固氮根瘤菌比非固氮细菌更多地在细胞外分泌核黄素、核黄素和吲哚乙酸，这被认为表明它们作为共生信号的作用。接种感染性根瘤菌的豇豆和大豆植株木质部汁液中核黄素和核黄素浓度增加就证明了这一点。事实上，大豆中核黄素在木质部中的浓度更高，且其在叶片中的积累相应增加，这比豇豆导致了更显著的发育变化。此外，土壤根瘤菌分泌的核黄素和核黄素还具有以下功能：（i）感知环境胁迫的生态线索；（ii）微生物、植物和人类的生长因子；（iii）陆地植物气孔功能的信号；（iv）植物防御的保护剂/诱导剂。外源脱落酸施用于植物根部对叶片气孔功能产生与核黄素相同的影响，这一事实表明植物对环境刺激的反应中存在分子相互作用。
