VFRC Publications - Page 23

Devouring Nutrients (organic acids or metal chelators (phytosiderophors) from their roots that dissolve and/or chelate soil nutrients for uptake. The VFRC report provides an overview of important classes of exudates, their mechanisms and effectiveness, which are found to be species or even cultivar dependent and strongly related to the nutrient deficiency status.22 Overcoming the lack of synchronization between plant demand and supply of nutrients will be a powerful strategy to increase uptake and utilization efficiency by plants. Whereas nutrients are taken up in ionic forms (Table 1), plant cells also can engulf larger particles such as bacteria in their entirety, possibly through endocytosis (Figure 10). Hence, tiny particles of nutrients administered through roots or leaves have been found to be devoured by plants, and some entering the plant through stomata and other channels, in the case of leaves. Favorable responses in growth of roots and shoot have been found for several nutrients delivered to plants as particles of 10-200 nanometers, as compared to bacteria of 1,000 nanometers.26 These particles tend to dissolve in a rather slow-release pattern that may enable synchronization of nutrient release from fertilizers according to crop demand. Support to plants for nutrient acquisition by beneficial soil organisms appears to be the result of amazingly complex interactions among plants, soils, micro-organisms and nutrients.23 Micro-organisms may help plants to better scavenge soil nutrients, reduce hazards such as drought, toxicity from heavy metals, or resistance against harmful organisms, and may also improve plant fitness. Some microbes are even considered to be bio-fertilizers due to their production of plant hormones and nutrient-mobilizing compounds. Therefore, maintaining a diverse population of micro-organisms, e.g., through prevention of overuse of mineral fertilizers, may be beneficial in the long run. Yet, human interventions targeting the exploitation of particular beneficial microbial interactions would be highly specific regarding plant species, soil type, micro-organism and nutrients, which may cause the beneficial impact of currently available inoculants to vary greatly, while many may lack rigorous scientific evidence of their impact. Figure 10. Different forms of endocytosis where plants encapsulate entire bacteria (Phagocytosis), small undefined particles (pintocytosis) and specific particles (receptormediate).27 In-situ interventions with micro-organisms will then be confronted with the edaphic complexities, whereas ex-situ utilization of microorganisms, such as in the industrial fermentation processes, could inherit opportunities for fertilizer production. Virtually all P fertilizers are still manufactured via sulfuric acid-based wet processes. Phosphate-solubilizing (PS) Gram-negative bacteria such as Pseudomonads could be an interesting candidate for the industrial development of phosphate fertilizers from phosphate rock24 (Figure 9). In view of its biological basis, the use of microbes to dissolve rock phosphate may be both an economically and environmentally sustainable technology with increased crop performance benefit