Extended periods of hydration do not elicit dehardening to desiccation tolerance in regeneration trials of the moss Syntrichia caninervis

Authors: STARK, LLOYD - University Of Nevada; BRINDA, JOHN - University Of Nevada; MCLETCHIE, NICHOLAS - University Of Kentucky; Oliver, Melvin

Submitted to: International Journal of Plant Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/11/2011
Publication Date: 5/22/2012
Citation: Stark, L.R., Brinda, J.C., McLetchie, N.D., Oliver, M.J. 2012. Extended periods of hydration do not elicit dehardening to desiccation tolerance in regeneration trials of the moss Syntrichia caninervis. International Journal of Plant Science. 173(4):333-343.

Interpretive Summary: Drought is the major contributor to yeild loss in all major crops in the U.S. and worldwide and is a serious threat to global food security. Understanding how plants cope with dehydration of the their cells is a major goal of plant research directed at developing strategies to improve drought tolerance in crops. To approach this issue we have chosen to investigate how plants that are naturally tolerant to the complete dehydration , i.e., desiccation, of their vegetative tissues can survive such a stress. In this study we are investigating the phenomenon that some desiccation tolerant plants actaully lose some of their ability to survive desiccation if they are grown continuously in a fully hydrated state. By understanding the mechanism of the loss in tolerance we hope to get a better understanding of what cellular process are actually necessary for tolerance. We determined that in the desiccation tolerant simple plant, Syntrichia caninervis, a desert species, that the loss of tolerance was only short-lived and that after 12 days of continuous hydration the plants actually became better able to survive a severe dehydration event to the point that they were indistinguishable from control plants that received regular drying treatments. This behavior was not affected by alteration in the levels of atmospheric carbon dioxide the plants were exposed to. These results suggest that it is the ability of plants to repair dehydration induced cellular damage that is critical for desiccation tolerance in these plants and that energy production is not a major limiting factor for the tolerance mechanisms. These insights are important for our search for new strategies to deliver improved drought tolerance in crops.

Technical Abstract: Dehardening (deacclimation) to water stress is seldom studied in plants, and yet is an integral phase of desiccation tolerance. Most bryophytes are desiccation tolerant (DT), and yet even fully DT species lose a significant portion of their ability to withstand desiccation if dehardened. Shoots of the DT moss Syntrichia caninervis were harvested and dehardened after exposure to field CO2 concentrations of 380 ppm (ambient) and 550 ppm (elevated) for 9 yr at the FACE (Free-Air CO2 Enrichment) facility in the Mojave Desert (NV, USA). The dehardening period consists of three phases, with the first phase (3-7 d) the most dehardened (least able to recover from a rapid-drying event). As the hydrating period extends in time through the second (12-18 d) and third (21-27 d) phases, shoots become hardened to a rapid-dry event, approaching control levels. Response variables following this pattern include tissue burning (chlorosis), time to protonemal emergence, apical meristem growth resumption, total regenerative shoot production, and total protonemal area. Dehydrin levels, however, did not increase over time during the 27 d experimental hydrating period, indicating that at least quantitatively, dehydrin proteins cannot account for the constitutive hardening to DT observed in this species. Longer hydration times and elevated CO2 decreased the probability and lengthened the time to fungal attack of regenerating moss tissue. These findings represent the first experimental sequence on dehardening in bryophytes lasting longer than a few days, and run counter to assumptions that field shoots hydrated longer than four consecutive days are entirely dehardened. We propose that what has been construed as a dehardening phase for desiccation tolerance is in reality a recovery phase during which plants are somewhat dehardened, and following recovery from desiccation damage the plants enter a constitutive protective phase. This sequence of events allows the moss to harden to desiccation tolerance without relying on external cues to hardening such as partial dehydration of tissues. Elevated CO2 increased sex expression, but had no effect on aboveground shoot length, biomass, or green zone (recent shoot growth interval) length.