Light-induced stabilization of ACS contributes to hypocotyl elongation during the dark-to-light transition in Arabidopsis seedlings.

Hypocotyl growth during seedling emergence is a crucial developmental transition influenced by light and phytohormones such as ethylene. Ethylene and light antagonistically control hypocotyl growth in either continuous light or darkness. However, how ethylene and light regulate hypocotyl growth, including seedling emergence, during the dark-to-light transition remains elusive. Here, we show that ethylene and light cooperatively stimulate a transient increase in hypocotyl growth during the dark-to-light transition via the light-mediated stabilization of 1-aminocyclopropane-1-carboxylic acid (ACC) synthases (ACSs), the rate-limiting enzymes in ethylene biosynthesis. We found that, in contrast to the known inhibitory role of light in hypocotyl growth, light treatment transiently increases hypocotyl growth in wild-type etiolated seedlings. Moreover, ACC, the direct precursor of ethylene, accentuates the effects of light on hypocotyl elongation during the dark-to-light transition. We determined that light leads to the transient elongation of hypocotyls by stabilizing the ACS5 protein during the dark-to-light transition. Furthermore, biochemical analysis of an ACS5 mutant protein bearing an alteration in the C-terminus indicated that light stabilizes ACS5 by inhibiting the degradation mechanism that acts through the C-terminus of ACS5. Our study reveals that plants regulate hypocotyl elongation during seedling establishment by coordinating light-induced ethylene biosynthesis at the post-translational level. Moreover, the stimulatory role of light on hypocotyl growth during the dark-to-light transition provides additional insights into the known inhibitory role of light in hypocotyl development.

Alternative electron transport mediated by flavodiiron proteins is operational in organisms from cyanobacteria up to gymnosperms.

Photo-reduction of O2 to water mediated by flavodiiron proteins (FDPs) represents a safety valve for the photosynthetic electron transport chain in fluctuating light. So far, the FDP-mediated O2 photo-reduction has been evidenced only in cyanobacteria and the moss Physcomitrella; however, a recent phylogenetic analysis of transcriptomes of photosynthetic organisms has also revealed the presence of FDP genes in several nonflowering plant groups. What remains to be clarified is whether the FDP-dependent O2 photo-reduction is actually operational in these organisms. We have established a simple method for the monitoring of FDP-mediated O2 photo-reduction, based on the measurement of redox kinetics of P700 (the electron donor of photosystem I) upon dark-to-light transition. The O2 photo-reduction is manifested as a fast re-oxidation of P700. The validity of the method was verified by experiments with transgenic organisms, namely FDP knock-out mutants of Synechocystis and Physcomitrella and transgenic Arabidopsis plants expressing FDPs from Physcomitrella. We observed the fast P700 re-oxidation in representatives of all green plant groups excluding angiosperms. Our results provide strong evidence that the FDP-mediated O2 photo-reduction is functional in all nonflowering green plant groups. This finding suggests a major change in the strategy of photosynthetic regulation during the evolution of angiosperms.

Diurnal accumulation of K+-dependent L-asparaginase in leaf of common bean (Phaseolus vulgaris L.).

L-Asparaginase (EC 3.5.1.1) activity has been previously reported to fluctuate with the photoperiod in young pea leaves, with higher activity in the light. The present research sought to investigate this phenomenon in developing leaves of common bean (Phaseolus vulgaris L.). There are two genes coding for K+-dependent asparaginase in this species. Expression of PvASPG1 predominates over PvASPG2 in all tissues. The catalytic efficiency of recombinant PvASPG2 was approximately 2-fold lower than that of PvASPG1. Polyclonal antibodies were raised against a specific peptide present in PvASPG1 to use in immunoblotting. In developing seed, asparaginase protein levels in the seed coat stayed constant, whereas levels in cotyledon were lower and progressively declined. In young leaf, asparagine protein levels showed diurnal variation, increasing at the end of the dark period and slowly decreasing during the light period. This was paralleled by changes in activity levels in leaf extracts. These changes accompanied a transient increase in free asparagine concentration at the beginning of the light period. The present results demonstrated that K+-dependent asparaginase activity reaches a maximum level at the transition from dark to light, anticipating dawn, in young leaves of common bean.