Dying in the Underground: Characterization of Root Senescence in Barley

Plant root senescence is still an uncharacterized biological process. At the global scale, root senescence or turnover rate is crucial for global carbon allocation from the atmospheric pool to the soil pool. At the field scale, a better understanding of root senescence may help to reduce fertilizer inputs, because delayed root senescence will allow maintaining a high level of nutrient uptake also after flowering when root activity usually drops.

In order to characterize root senescence, hydroponically cultivated barley plants were harvested weekly over a period of 54 days for structural, physiological and molecular analyses. Interestingly, we identified day 39 as an important transition point in the development of seminal roots, as several events fell together: i) seminal roots stopped elongating, ii) root epidermal and cortical tissues started decaying, iii) nitrate uptake capacity started decreasing. This coincided exactly with a sudden increase in abscisic acid (ABA) levels in seminal roots, which was confirmed by enhanced transcript levels of ABA biosynthesis genes.

To identify possible regulators of root senescence, 60K barley microarrays were used for transcriptome analysis of the apical and basal root zone. Several NAC-, WRKY- and AP2-type transcription factors (TFs) were identified, which showed strong up-regulation just before root senescence set in.

The present study indicates that root senescence is a highly coordinated developmental process, which involves an interplay of TFs and phytohormones.