SA Grain July 2017

Plant growth promoting rhizobacteria

– key to making soil more accessible to crops

oil is a dynamic resource that sustains life processes and

is an important key in food security. Pathogens affect plant

health, which threaten sustainable agriculture. Chemical fer-

tilisers are known to kill pathogens and weeds and increase

yields, but have detrimental side effects on plant and soil health.

Alternatively, biological agents such as plant growth promot-

ing rhizobacteria show potential to lower the risk of such fertilis-

ers. Rhizobacteria can play a determining role in soil fertility, the

suppression of pathogens and improvement of plant growth.

What are plant growth

promoting rhizobacteria?

The rhizosphere (the 1 mm - 2 mm thick layer of soil surrounding

plant roots) is the zone of maximum microbial activity where the

majority of soil micro-organisms (bacteria and fungi) reside to utilise

compounds released from crop roots for energy.

Some micro-organisms present in the rhizosphere appear to have

no direct consequence on plant growth while others are deleteri-

ous and some beneficial to plant growth. Plant growth promoting

rhizobacteria are beneficial soil bacteria naturally present in the

rhizosphere that enhance plant growth and reduce abiotic stresses

through a variety of mechanisms.

They live on or inside plant roots and their modes of action to en-

hance plant growth and reduce abiotic stresses may be direct or in-

direct. Bacteria such as

Agrobacterium, Arthrobacter, Azotobacter,

Azospirillum, Bacillus, Bradyrhizobium, Burkholderia, Caulobacter,

Chromobacterium, Erwinia, Flavobacterium, Micrococcus, Pseu-

domonas, Rhizobium

and

Serratia

resort under plant growth pro-

moting rhizobacteria.

Direct and indirect modes of action

The direct mode of action by plant growth promoting rhizobacteria

includes the production of bacterial volatiles (stimulatory effects),

auxins, cytokinins (phytohormones that lower ethylene levels in

the plant), improvement of the plant nutrient status (liberation of

phosphates and micronutrients from insoluble sources and non-

symbiotic nitrogen fixation) and stimulation of disease-resistance

mechanisms (induced systemic resistance).

Indirect mode of action includes plant growth promoting rhizobac-

teria acting like biocontrol agents by reducing diseases through

stimulation of other beneficial symbioses and protection of the plant

through the degradation of xenobiotics such as pesticides in soils.

For plant growth promoting rhizobacteria to be effective, the follow-

ing conditions should be taken into consideration:

Overall soil condition, including soil pH, water, minerals, and

soil porosity.

Extent of bacterial colonisation of the root zone.

Amount and types of plant exudates released.

Relevance of plant growth promoting

rhizobacteria to agriculture

Plant growth promoting rhizobacteria promote the recycling of

plant nutrients and maintenance of soil structure while increasing

root size and root strength in plants. Healthier roots help to pro-

duce stronger, more productive plants, which lead to increased ag-

ronomic activity and better yields.

Furthermore, fertiliser costs and runoff are reduced because plant

growth promoting rhizobacteria permit the efficient use of ferti-

lisers while reducing the need for high quantities of traditional ferti-

lisers such as N, P and K.

Numerous associations between plants and beneficial bacteria

show a protective response under restrictive environmental condi-

tions such as drought. Metabolites like polysaccharides produced

by plant growth promoting rhizobacteria change the soil structure,

and have a positive effect on plants grown during water stress.

Prospects of beneficial micro-organisms

The advantages of plant growth promoting rhizobacteria are in-

creasingly acknowledged by scientists and producers alike. An in-

creased interest in plant growth promoting rhizobacteria indicate

that there is a need for additional research and the development of

research technologies in this field.

For more information, contact Owen Rhode at

RhodeO@arc.agric.za.

OWEN RHODE,

ARC-Grain Crops, Potchefstroom

Dit is versoenbaar met alle plantvoedingstowwe, behalwe water-

oplosbare fosfate. Dit bevat geen NO

, CL

of SO

nie.

Aangesien Ca so ‘n belangrike rol in plantgesondheid speel, sal

voldoende Ca-vlakke die volgende funksies ondersteun:

Speel ‘n belangrike rol in fotosintese – ATP en ADP.

Belangrike element vir endofitiese mikrobes in die grond.

Versterk selwande.

Fisiese beskerming teen meganiese skade.

Saam met calmodulin (Ca-afhanklike ensieme) het dit die

volgende noodsaaklike funksies in die sel:

Polariteit vir diffusie en die opname van voedingstowwe;

afskeiding van ensieme en hormone;

groeikragtigheid;

seldeling;

genetiese uitdrukking (RNA) vir voortplanting en vrugvor-

ming; en

Ca-ensieme vir beskerming teen swamme en ander patogeen-

aanvalle.

Aanwending

Complex SuperCa kan as blaarvoeding gebruik en teen ‘n standaard

toediening toegedien word. 1 liter/ha tot 2 liter/ha toegedien met

‘n maksimum van 100 liter water deur middel van blaarbespuiting.

FOCUS

Fertiliser

Special

July 2017