This title appears in the Scientific Report : 2015 

Root Cortical Senecence Influences Root Radial Water Uptake in Barley Plants
Schneider, Hannah (Corresponding author)
Wojciechowski, Tobias / Postma, Johannes Auke / van Dusschoten, Dagmar / Lynch, Jonathan
Pflanzenwissenschaften; IBG-2
2015
International Society of Root Research Symposium, Canberra (Australia), 2015-10-06 - 2015-10-09
Conference Presentation
Plant Science
Root cortical senescence (RCS) is a common phenomenon in many poaceae species involving the senescence and eventual loss of the root cortex. RCS is a type of programmed cell death with a predictable pattern (Henry & Deacon 1981) and apparent mechanistic similarities to apoptosis (Liljeroth & Bryngelsson 2001). RCS is distinctly different than the loss of the root cortex due to secondary root growth in many dicotyledonous plants and studies have suggested that RCS is a distinct process from aerenchyma formation (Deacon et al. 1986). A handful of studies have demonstrated how RCS is influenced by disease, plant species, or nutrient availability. However, RCS may be an adaptive response, by reducing root maintenance requirements and thereby permitting greater soil exploration. The objectives of this research were to test the hypothesis that root cortical senescence reduces radial water transport and root respiration in excised barley root segments. To test the hypothesis we created a spatial map of radial water uptake and respiration in barley roots as affected by anatomical traits and RCS. The objectives in this proposal are important to study as previous literature on root cortical senescence does not address these topics. Using a modified Pitman chamber (Lynch & Lauchlii 1984; Hu et al. 2014), time-course dynamics of radial water uptake into the xylem were measured from excised nodal and seminal roots of barley grown in solution culture with various stages of RCS. Specific three mm intervals of root zones were exposed to agar gel containing 18O-enriched water. Water was collected at specific time intervals from the cut end of the root and analyzed for 18O content. Root segments subjected to 18O enriched water treatment were collected, their respiration was measured, then they were fixed in 70% methanol, stained with acridine orange for cell viability and sudan red for endodermal development, cryo-sectioned, and imaged. Images were evaluated for RCS, endodermal development, and additional anatomical traits including stele diameter, number of xylem vessels, and cortical cell file number. Results were synthesized into a spatial map of root radial water uptake and anatomy based on genotype, root class, and root age. RCS develops in solution culture grown barley roots at four weeks of growth. The pattern of cell death begins in the outermost cortical cell file next to the epidermis, directly behind the zone of anucleate root hairs. RCS progresses inwards resulting in anucleate cortical cells with the exception of the endodermis which continues to be viable for the life of the plant. Root cortical cell files undergo senescence increasing in frequency basipetally. Cell layers around the base of the laterals typically do not undergo RCS or senescence is delayed even though the lateral branches themselves may have developed RCS. Clear genotype differences in the rate of root cortical senescence were observed and landraces had a faster progression of RCS compared to modern cultivars. Cryo-sectioning, acridine orange staining, and microscopy revealed that RCS could be classified into five stages ranging from absent to complete RCS (0% (absent), 25% (intermediate 1), 50% (intermediate 2), 75% (intermediate 3), and 100% (complete)). Pitman chamber preliminary results show that seminal roots with complete RCS have 75% less radial water uptake and 86% less root respiration than roots with no RCS. Nodal roots with complete RCS have 50% less radial water uptake and 45% less root respiration than roots with intermediate RCS. These results support the hypothesis that RCS reduces radial water uptake in barley roots. Understanding traits that enhance plant performance in drought conditions is important in increasing agricultural productivity, particularly in low-input systems. The utility of RCS under edaphic stresses by reducing carbon metabolic costs and conserving soil water can be tested in the future. RCS may be an adaptive response, by reducing root maintenance requirements and thereby permitting greater soil exploration.