Do rhizobia produce cytokinins?

Area of Science:

• Plant-microbe interactions
• Plant hormone signaling
• Rhizobium research

Background:

• Rhizobium bacteria form symbiotic relationships with legumes, influencing plant growth.
• Plant hormones, such as cytokinins, play critical roles in regulating plant development.
• Abnormal plant growth can result from altered hormone levels.

Purpose of the Study:

• To investigate the cytokinin profiles of two related Rhizobium strains, one normal and one unusual.
• To determine if differences in cytokinin production correlate with observed abnormal shoot development in nodulated plants.
• To identify and quantify specific cytokinins produced by Rhizobium in culture and in planta.

Main Methods:

• Culturing of Rhizobium strains (ANU240 and IC3342) on a defined medium.
• Analysis of culture supernatants using mass spectrometry and radioimmunoassay to detect and quantify cytokinins.
• Measurement of cytokinin levels in the xylem sap of pigeonpea plants inoculated with different Rhizobium strains.

Main Results:

• Both Rhizobium strains produced a range of cytokinins, with N6-(2-isopentenyl)adenine (iP) and zeatin (Z) being dominant.
• Strain IC3342 produced 26 times more zeatin and 8 times more iP than strain ANU240.
• Plants inoculated with strain IC3342 showed markedly higher cytokinin levels in xylem sap and abnormal lateral bud proliferation.

Conclusions:

• This study provides the first unambiguous identification of cytokinins in Rhizobium culture media.
• The elevated cytokinin levels in plants nodulated by strain IC3342 are likely responsible for the observed abnormal shoot development.
• Rhizobium-derived cytokinins can significantly impact plant development through xylem transport.