Friday Seminar by Mr. Vishal Vashisth

AKT Lab/ Friday/ July 26/ 2019/ 3,30 pm/  Chloroplast retrograde signal PAP: A modulator of ABA signalling and drought stress.
Category: Research
Posted by: bedineel


Chloroplast retrograde signal PAP: A modulator of ABA signalling and drought stress


Vishal Vashisth

Chloroplast is the site of oxygenic photosynthesis in plants and housing the biosynthesis of various important metabolites including amino acids, nucleotides, fatty acids, phytohormones, and sulfur assimilation. Effective communication between the chloroplast and the nucleus is necessary to coordinate cellular metabolism and growth. A variety of chloroplast retrograde signals have been reported and some of these mutants have acquired some form of stress tolerance. The metabolite 3’-phosphoadenosine-5’-phosphate (PAP), a by-product of secondary sulfur metabolism, is a stress-induced chloroplast retrograde signal in plants. SAL1 mutants (fry1/alx8/fou8) accumulate many-fold more PAP demonstrating that PAP is a primary in vivo substrate of SAL1 enzyme. SAL1 regulates its levels by dephosphorylating PAP to AMP and Pi. PAP is maintained at very low levels by the SAL1 phosphatase during vegetative growth of Arabidopsis and accumulates during drought and high light stress. SAL1 phosphatase is inactivated by oxidative stresses and decrease in SAL1 activity in the chloroplast enables PAP to accumulate. PAP can move from chloroplast to the nucleus via cytosol and it inhibits exoribonuclease (XRNs) activity as revealed by the accumulation of XRN substrates in sal1 mutants. Inhibition of XRN activity result in the alternation of many stress-related gene expression, including abscisic acid (ABA)-responsive ones, thus regulating drought stress signaling and responses. SAL1-PAP retrograde pathway also interacts with abscisic acid signaling to regulate stomatal closure in Arabidopsis. PAP restore ABA sensitivity in ABA insensitive mutants such as abi1 and ost1, thus prime an alternative pathway that restores ABA-responsive gene expression, stomatal closure and drought tolerance in ABA biosynthetic mutants. Thus, PAP exhibits many secondary messenger attributes and this SAL1-PAP-XRN retrograde pathway facilitates green plant adaptation to land.

Suggested readings:

  • Estavillo, Gonzalo M., et al. "Evidence for a SAL1-PAP chloroplast retrograde pathway that functions in drought and high light signaling in Arabidopsis." The Plant Cell 23.11 (2011): 3992-4012.
  • Chan, Kai Xun, et al. "Sensing and signaling of oxidative stress in chloroplasts by inactivation of the SAL1 phosphoadenosine phosphatase." Proceedings of the National Academy of Sciences 113.31 (2016): E4567-E4576.
  • Pornsiriwong, Wannarat, et al. "A chloroplast retrograde signal, 3’-phosphoadenosine 5’-phosphate, acts as a secondary messenger in abscisic acid signaling in stomatal closure and germination." Elife 6 (2017): e23361.