The importance of phytohormones and microbes in biostimulants: Mass spectrometric evidence and their positive effects on plant growth

Many studies have shown that organic fertilizers (with low NPK values) can sometimes produce similar growth promotion effect of comparable chemical fertilizer (high NPK values) application. Hence, some putative biostimulants must be present in the organic fertilizer which drives plant growth when macro- and micro-nutrients are applied in smaller quantities. We deduced that phytohormones (especially cytokinins (CKs) that regulate cell division, chloroplast greening, and other important processes) are the likely biostimulants responsible for growth enhancement. Using an earthworm-based vermicompost (VC) system, we showed unequivocally the presence of phytohormones (auxins and CKs like N6-Isopentenyladenine (iP), N6-Isopentenyladenosine, zeatin (Z)) using mass spectrometery. The high levels of the CK iP is a good reflection of the microbial origin for phytohormones present in VC (and their leachate, "vermi-tea") due to its importance in CK biosynthesis pathways and high abundance provided by microorganisms. To demonstrate directly the microbial-phytohormone linkage in regulating plant development, pigeon pea inoculated with Rhizobium strain IC3342, showed a 3-fold elevation in xylem cytokinin delivery (mainly Z and dihydrozeatin, with some iP) to the shoot and altered various aspects of leaf physiology and shoot development. It is noteworthy that culture filtrates of Rhizobium strain IC3342 contained mainly Z and iP type of CKs. Our combined studies revealed that certain types of phytohormones are important components of the biostimulants currently in use by the plant industry.