Mechanical conflicts emerging at the cylindrical and spherical plant organs

Abstract

Because plant cells are glued together via their cell walls, plant cell growth continuously generates mechanical stress. Plant stems are pressurized cylinders, where the epidermis is under tension and the remaining inner components under compression and prescribes maximal tensile stress direction along the circumference. In plant stems, the epidermis is the main tension-load bearing layer that drives organ growth by restricting radial cell growth via oriented cellulose deposition guided by cortical microtubules that respond to stress. Accumulating evidence has shown the importance of biomechanical vision to disentangle plant development. In parallel to mechanical forces that contribute to plant organ morphogenesis, organ integrity also relies on mechanical regulation, as hinted by the longitudinal cracks emerging in the inflorescence stem of Arabidopsis clavata3 de-etiolated3 mutant. Failure to tolerate high levels of mechanical stress, generated from uncoordinated cell growth of inner and outer cell layers, lead to tissue breakage. In this review, we summarize our recent findings on how organ mechanical integrity is maintained in Arabidopsis inflorescence stems. Then, in the second part we instead focus on the cracks found on fleshy fruits. The development of fleshy fruits in most species is characterized by shorter cell division phase versus continuous cell expansion until ripening, which is reminiscent of stem cracking, where increasing tensile stress is thought to exceed the limit of epidermal strength. Although cylindrical stems and spherical fruits display completely different morphologies, both represent ideal systems to decipher how mechanical integrity is sustained during plant growth.