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