Despite being underground, rhizomes are stems, not roots. Like other stems, they have nodes that can push up other stems. As they’re protected underground they’re also useful places for storing carbohydrates, water and nutrients. So for plants like grasses, a strong rhizome network can be a major aid in survival and propagation. But what controls their development? Xiqing Ma and colleagues have been investigating signalling in tall fescue, Festuca arundinacea, to see what turns rhizome development off and on.
To do this, the team gathered plants from the turfgrass breeding farm at Rutgers University. They selected plants with the same number of tillers, with rhizomes removed and planted them in boxes filled with fritted clay to see how drought stress and re-watering affected them. The plants were given a couple of weeks to settle into their new homes and then the experiment began.
The first group were the control group. They were well-watered as usual. The second group had irrigation withheld for a week, before rewatering began. The team measured the numbers of rhizomes, tillers and roots, as well as the length of individual rhizomes on 30 plants from each group after the drought and rewatering phases. They also examined hormone and sugar content of the plants to see what was happening inside them, and found the internal chemistry differed depending on what phase of the experiment the plant was in.
“Drought-induced inhibition of rhizome initiation could be mainly associated with ABA accumulation (NCED) and signaling (PYLs, PP2Cs, SnRK2s and ABFs), energy metabolism related genes/proteins (galactinol synthase, raffinose synthase, glutamate 5-kinase), and stress response proteins (dehydrin proteins, HSP17, LEA proteins). Drought stress inhibited rhizome elongation via the regulation of GA4, antioxidants (ASA/DHA/GSH/GSSG), and energy metabolism, as well as protein modification/stress response proteins (ASA, MDHAR, GR, cellulose synthase, chaperon, and HSP70 proteins). Rhizome regeneration or initiation in response to re-watering involved IAA and lipid metabolism genes (phospholipase A2, lipoxygenase, allene oxide synthase), and proteins involved in secondary cell-wall development (GDSL esterase/lipase and trans-cinnamate 4-monooxygenase) and nitrogen remobilization and cycling (asparagine glutamine-hydrolyzing synthetase),” write Ma and colleagues.
The results could help explain some of the factors affecting grassland survival in drought, say the botanists. “Rhizome survival during drought and regrowth upon re-watering are critically important for maintaining stands and productivity of rhizomatous plant species in areas with sporadic rainfall events or lack of water for irrigation. This is because rhizomes as underground stems are able to regenerate new daughter plants from rhizome nodes containing meristematic tissues, which is a unique distinct feature from other parts of a plant, such as leaves, roots, tillers or lateral branches… Through taking an integrative analysis of both transcriptomic and proteomic profiles, this study identified major genes and proteins that could be involved in the drought-inhibition of rhizome growth and the initiation or regeneration of new rhizomes and rhizome elongation in response to re-watering in perennial grass species.”