Flooding of the apoplast is a key factor in the development of hyperhydricity

Abstract

The physiological disorder hyperhydricity occurs frequently in tissue culture and causes several morphological abnormalities such as thick, brittle, curled, and translucent leaves. It is well known that hyperhydric shoots are characterized by a high water content, but how this is related to the abnormalities is not clear. It was observed that water accumulated extensively in the apoplast of leaves of hyperhydric Arabidopsis seedlings and flooded apoplastic air spaces almost completely. In hyperhydric Arabidopsis seedlings, the volume of apoplastic air was reduced from 85% of the apoplast to only 15%. Similar results were obtained with hyperhydric shoots of statice. The elevated expression of hypoxia-responsive genes in hyperhydric seedlings showed that the water saturation of the apoplast decreased oxygen supply. This demonstrates a reduced gas exchange between the symplast and its surroundings, which will consequently lead to the accumulation of gases in the symplast, for example ethylene and methyl jasmonate. The impairment of gas exchange probably brings about the symptoms of hyperhydricity. Interestingly, stomatal aperture was reduced in hyperhydric plants, a previously reported response to injection of water into the apoplast. Closure of the stomata and the accumulation of water in the apoplast may be the reasons why seedlings with a low level of hyperhydricity showed improved acclimatization after planting into soil.

Trichloroacetate, an inhibitor of wax biosynthesis, prevents the development of hyperhydricity in Arabidopsis seedlings

“…In the present paper, we examine whether the symptoms of HH are reduced by stimulating transpiration through inhibiting wax synthesis and whether apoplastic air and water volumes show the relative amounts characteristic for nonhyperhydric plants as reported by Van den Dries et al (2013).…”

“…Immediately after centrifugation, the leaves were reweighed. The presence of symplastic contamination in the apoplastic water was previously assessed by a malate dehydrogenase assay and found to be negligible (Van den Dries et al 2013). The apoplastic water volume (V water ) in µl g −1 FW was calculated using the formula:…”

“…The swelling was supposedly caused by reduced wall pressure brought about by a reduction in the levels of cellulose and lignin (George 1996). More recently, though, it has been observed that in hyperhydric tissues the extra water replaces the air in the apoplast (Gribble et al 1998; Van den Dries et al 2013). The apoplast consists of the cell wall continuum and the intercellular spaces (for a review on the terminology, see Canny 1995).…”

“…Because of the flooding of the apoplast, gas exchange by cells becomes severely inhibited and as a result the cells suffer from anaerobiosis and accumulation of gases like ethylene (Gribble et al 2003; Van den Dries et al 2013). The poor gas exchange leads to the symptoms of HH.…”

“…The poor gas exchange leads to the symptoms of HH. In Arabidopsis seedlings, severe HH leads to dying off 3 weeks after the first visible symptoms ( Van den Dries et al 2013). Evidently, the excess of water is brought about by too much entering and/or too little removal of water.…”