Stopping with division
A leaf consists out of multiple cell types. Not only is the inside, but the outside also has multiple cell types. Like trichomes and stomata. In addition, there are the so-called pavement cells, these cells cover the majority of the surface of the leaf. Under the microscope these pavement cells look like good fitting puzzle pieces. But up to now it was unknown what is causing these cells to develop into those puzzle pieces. Now Belgian and British scientist discovered that SMR1, a cell division brake, causes the precursors to develop into pavement cells.
Pavement cells share their precursor with stomata. After an asymmetrical division the smaller cell develops further into a stomata. While the bigger cell can either undergo another round of asymmetric division or develop into a pavement cell. Unknown is what is influencing this decision.
Researchers used a cell division mutant to study this, plants that did not have any SMR1. The researchers noticed that the leaves of these SMR1 deficient plants had more pavement cells, and on average less stomata cells. But when the researchers looked closer, they noticed that the pavement cells actually divided more often. In addition to symmetrical divisions, the researchers also noticed asymmetrical pavement cell divisions. From those divisions the smaller cell developed into stomata.
By making more SMR1 the plant can put a break on cell divisions
One way to make the switch from an undifferentiated dividing cell into a differentiated non-dividing cell is by breaking up cell cycle specific protein complexes. Something that earlier research showed that SMR1 does. Therefore, the researchers studied what happens with the pavement cells when these cells could not form certain cell cycle specific protein complexes. In plants missing both cell cycle complex protein CYC2A and SMR1 less pavement cell divisions were observed. Pavement precursors stop with dividing because SMR1 breaks up the CYC2A cell cycle protein complexes.
For plants to be able to regulate the number of stomata in a leaf is important. In a dry environment, a plant prefers to lose less water, and therefore prefers to have less stomata. To analyse the influence of the amount of SMR1 on this, the researchers analysed plants without or with more SMR1. The leaves of plants without SMR1 wilted in too dry environments. No difficulties were seen for the leaves of plants with more than normal SMR1.
By making more SMR1 the plant can put a break on cell divisions. Regulating the number of stomata in its leaves. Making sure that in dry circumstances the pavement cell precursors stop divisions early. Allowing less stomata to develop, and to keep more water inside the plant.
Literature
Dubois, M., Achon, I., Brench, R.A. et al. (2023) SIAMESE-RELATED1 imposes differentiation of stomatal lineage ground cells into pavement cells. Nat. Plants9, 1143–1153. https://doi.org/10.1038/s41477-023-01452-7
Plant en Zo 