Abscisic acid induced a negative geotropic response in dark-incubated Chlamydomonas reinhardtii

Abstract

The phytohormone abscisic acid (ABA) plays a role in stresses that alter plant water status and may also regulate root gravitropism and hydrotropism. ABA also exists in the aquatic algal progenitors of land plants, but other than its involvement in stress responses, its physiological role in these microorganisms remains elusive. We show that exogenous ABA significantly altered the HCO 3 − uptake of Chamydomonas reinhardtii in a light-intensity-dependent manner. In high light ABA enhanced HCO 3 − uptake, while under low light uptake was diminished. In the dark, ABA induced a negative geotropic movement of the algae to an extent dependent on the time of sampling during the light/dark cycle. The algae also showed a differential, light-dependent directional taxis response to a fixed ABA source, moving horizontally towards the source in the light and away in the dark. We conclude that light and ABA signal competitively in order for algae to position themselves in the water column to minimise photo-oxidative stress and optimise photosynthetic efficiency. We suggest that the development of this response mechanism in motile algae may have been an important step in the evolution of terrestrial plants and that its retention therein strongly implicates ABA in the regulation of their relevant tropisms.

Systems-wide analysis revealed shared and unique responses to moderate and acute high temperatures in the green alga Chlamydomonas reinhardtii

“…Previous reports showed that heat treated Arabidopsis and rice leaves had increased ABA contents and exogenous ABA application improved heat tolerance in WT rice plants 43 , 44 . ABA is reported to be involved in tolerance to oxidative stress 45 , HCO3 − uptake 46 , and stress acclimation in Chlamydomonas 47 . TM6 displays similar expression patterns in both 35°C and 40°C treatments, with increased expression during early heat and reduced expression during early recovery.…”

Systems-wide Analysis Revealed Shared and Unique Responses to Moderate and Acute High Temperatures in the Green Alga Chlamydomonas reinhardtii

“…It has been reported that heat treated Arabidopsis and rice leaves had increased ABA contents and exogenous ABA application improved heat tolerance in WT rice plants (Balfagón et al, 2019, 1;Li et al, 2020a). ABA is reported to be involved in tolerance to oxidative stress (Yoshida et al, 2003), HCO3 − uptake (Al-Hijab et al, 2019), and stress acclimation in Chlamydomonas (Colina et al, 2019) After 2-4 h recovery from 40°C, cells started to replicate DNA between 2-and 4-h of recovery at 25 o C, resulting in reduced 1C but increased 2C and 4C cell populations (Figure 4Q). DNA replication continued until 8-h of recovery, resulting in the accumulation of high-ploidy level cells, ranging from 1C to 16C (Figure 4S).…”

“…It has been reported that heat treated Arabidopsis and rice leaves had increased ABA contents and exogenous ABA application improved heat tolerance in WT rice plants (Balfagón et al, 2019, 1;Li et al, 2020a). ABA is reported to be involved in tolerance to oxidative stress (Yoshida et al, 2003), HCO3 − uptake (Al-Hijab et al, 2019), and stress acclimation in Chlamydomonas (Colina et al, 2019). TM6 displays similar expression patterns in both 35 o C and 40 o C treatments, with increased expression during early heat and reduced expression during early recovery.…”

On the biosynthesis and evolution of apocarotenoid plant growth regulators

“…However, our knowledge about the biological function, biosynthesis, and possible signaling pathways in lower photosynthetic and non-photosynthetic organisms is quite limited. In lower photosynthetic organisms, several studies connect ABA to abiotic stress [48,50]. It was also shown that exogenous ABA modulates Chamydomonas reinhardtii HCO3 − uptake and that ABA concentration determines the position of this aquatic algae in the water column depending on light intensities, which could be one of the original functions of this hormone in regulating tropisms [50].…”