Showing papers by "Martin H. Spalding published in 2020"

LCI1, a Chlamydomonas reinhardtii plasma membrane protein, functions in active CO 2 uptake under low CO 2 .

Abstract: In response to high CO2 environmental variability, green algae, such as Chlamydomonas reinhardtii, have evolved multiple physiological states dictated by external CO2 concentration. Genetic and physiological studies demonstrated that at least three CO2 physiological states, a high CO2 (0.5-5% CO2 ), a low CO2 (0.03-0.4% CO2 ) and a very low CO2 (< 0.02% CO2 ) state, exist in Chlamydomonas. To acclimate in the low and very low CO2 states, Chlamydomonas induces a sophisticated strategy known as a CO2 -concentrating mechanism (CCM) that enables proliferation and survival in these unfavorable CO2 environments. Active uptake of Ci from the environment is a fundamental aspect in the Chlamydomonas CCM, and consists of CO2 and HCO3 - uptake systems that play distinct roles in low and very low CO2 acclimation states. LCI1, a putative plasma membrane Ci transporter, has been linked through conditional overexpression to active Ci uptake. However, both the role of LCI1 in various CO2 acclimation states and the species of Ci , HCO3 - or CO2 , that LCI1 transports remain obscure. Here we report the impact of an LCI1 loss-of-function mutant on growth and photosynthesis in different genetic backgrounds at multiple pH values. These studies show that LCI1 appears to be associated with active CO2 uptake in low CO2 , especially above air-level CO2 , and that any LCI1 role in very low CO2 is minimal.

Structure and function of LCI1: a plasma membrane CO 2 channel in the Chlamydomonas CO 2 concentrating mechanism.

Abstract: Microalgae and cyanobacteria contribute roughly half of the global photosynthetic carbon assimilation. Faced with limited access to CO2 in aquatic environments, which can vary daily or hourly, these microorganisms have evolved use of an efficient CO2 concentrating mechanism (CCM) to accumulate high internal concentrations of inorganic carbon (Ci ) to maintain photosynthetic performance. For eukaryotic algae, a combination of molecular, genetic and physiological studies using the model organism Chlamydomonas reinhardtii, have revealed the function and molecular characteristics of many CCM components, including active Ci uptake systems. Fundamental to eukaryotic Ci uptake systems are Ci transporters/channels located in membranes of various cell compartments, which together facilitate the movement of Ci from the environment into the chloroplast, where primary CO2 assimilation occurs. Two putative plasma membrane Ci transporters, HLA3 and LCI1, are reportedly involved in active Ci uptake. Based on previous studies, HLA3 clearly plays a meaningful role in HCO3 - transport, but the function of LCI1 has not yet been thoroughly investigated so remains somewhat obscure. Here we report a crystal structure of the full-length LCI1 membrane protein to reveal LCI1 structural characteristics, as well as in vivo physiological studies in an LCI1 loss-of-function mutant to reveal the Ci species preference for LCI1. Together, these new studies demonstrate LCI1 plays an important role in active CO2 uptake and that LCI1 likely functions as a plasma membrane CO2 channel, possibly a gated channel.

Arabidopsis plants expressing only the redox-regulated Rca-α isoform have constrained photosynthesis and plant growth

Abstract: Rubisco activase (Rca) facilitates the release of sugar-phosphate inhibitors from the active sites of Rubisco and thereby plays a central role in initiating and sustaining Rubisco activation In Arabidopsis, alternative splicing of a single Rca gene results in two Rca isoforms, Rca-α and Rca-β Redox modulation of Rca-α regulates the function of Rca-α and Rca-β acting together to control Rubisco activation Although Arabidopsis Rca-α alone less effectively activates Rubisco in vitro, it is not known how CO2 assimilation and plant growth are impacted Here, we show that two independent transgenic Arabidopsis lines expressing Rca-α in the absence of Rca-β ('Rca-α only' lines) grew more slowly in various light conditions, especially under low light or fluctuating light intensity, and in a short day photoperiod compared to wildtype Photosynthetic induction was slower in the Rca-α only lines, and they maintained a lower rate of CO2 assimilation during both photoperiod types Our findings suggest Rca oligomers composed of Rca-α only are less effective in initiating and sustaining the activation of Rubisco than when Rca-β is also present Currently there are no examples of any plant species that naturally express Rca-α only but numerous examples of species expressing Rca-β only That Rca-α exists in most plant species, including many C3 and C4 food and bioenergy crops, implies its presence is adaptive under some circumstances