Plants adjust their water usage based on environmental conditions by recognizing and responding to different stimuli with the help of their guard cells.
Plants manage their water intake through adjustable openings known as stomata, which consist of pairs of guard cells. These cells open the stomata when there is adequate water available and sufficient light for capturing carbon dioxide during photosynthesis. Conversely, in low light conditions or when water is scarce, the stomata close.
SLAC/SLAH-type anion channels in the guard cells play a key role in regulating stomatal behavior. This finding has been established by a team led by Professor Rainer Hedrich, a biophysicist at Julius-Maximilians-Universität (JMU) Würzburg in Bavaria, Germany.
These anion channels are triggered by calcium signals, which occur due to environmental factors like water or nutrient deficiency, soil salinization, or attacks by pathogens. The calcium signals can take various forms depending on the triggering stimulus, often referred to as calcium signatures. A common signature observed is the calcium transient, characterized by a swift and temporary spike in calcium levels within the cells.
Calcium Transient Follows an All-or-Nothing Rule
How much information does a calcium transient convey? To explore this, Hedrich’s team implemented an optogenetic approach using specially designed model plants equipped with light-activated calcium channels: Light pulses were used to induce calcium signals in the guard cells, allowing for analysis of the cellular reactions.
‘We were quite surprised to find that light pulses lasting 0.1, 1, and 10 seconds produced nearly identical calcium transients,’ notes Shouguang Huang, the lead author of a study published in the journal Current Biology. The calcium concentration in the guard cells increased for 30 seconds post-light stimulus and then gradually decreased over the next 30 seconds.
‘We suspect that this all-or-nothing phenomenon occurs because the calcium that enters the cells from the exterior prompts the release of additional calcium from internal storage, thereby amplifying the signal,’ explains Rainer Hedrich. The researchers confirmed their hypothesis when inhibiting calcium storage in the endoplasmic reticulum led to a failure in producing both the calcium transient and the subsequent reaction.
Anion Current Follows Calcium Signal with a Delay
‘We were surprised to discover that, alongside the calcium signal, we also detected the subsequent response in the guard cells, which involved the swelling of the anion current,’ declares Shouguang Huang. Similar to the calcium transients, varying light pulse durations triggered anion currents that were of comparable shape and intensity. These currents lagged behind the calcium signal, only swelling after the calcium concentration in the cytosol exceeded a specific threshold.
After the calcium transient concluded, the anion current could still be detected for an additional 30 seconds. This delayed response is linked to the characteristics of the enzymes that process the calcium signal, which in turn activate or deactivate the anion channels, as explained by Rainer Hedrich. This indicates that a calcium influx lasting just 0.1 seconds is amplified inside the cell, prompting a response that lasts over a hundred times longer.
Guard Cells Have a Counting Ability
How many calcium transients are needed for plants to close their stomata? To investigate this, the researchers exposed guard cells to a 0.1-second light pulse every half-minute and observed the stomata. After the first pulse, the width of the pore decreased by 10 percent; after three pulses, it decreased by 30 percent; after six pulses, by 80 percent; and after 12 or more pulses, by a complete 100 percent closure.
‘This indicates that guard cells are capable of processing six consecutive calcium transients and translating them into stomatal movement. Essentially, guard cells can count up to six,’ states Rainer Hedrich. ‘Notably, increasing the stimulation frequency did not speed up stoma closure, while decreasing it delayed the movement.’
Looking Ahead — Future Research Questions
What are the next steps in this research? ‘Currently, we are investigating which part of the stimulus-response process is influenced by the frequency of calcium transients and how it affects the speed of the response. We are also exploring how guard cells interpret calcium signals and translate them into enzymatic activation of their anion channels in a number-dependent manner,’ says the JMU biophysicist. Additionally, it will need to be determined how long guard cells retain information about the specific calcium signals they encounter.
