Global Compression Reorients Cortical Microtubules in Arabidopsis Hypocotyl Epidermis and Promotes Growth

Plants are able to sense external mechanical stress, such as those due to gravity or obstacles, and alter their growth accordingly [1-8]. Like animals [9, 10], plants can also sense internal mechanical stress that plays a role in regulating their development [11-19]. The internal mechanical stresses also known as tissue stress can result from geometry, cell type, or differential growth [19-21]. In a number of tissues, microtubules have been observed to align with mechanical stress predicted from their geometry. In the unidirectionally growing hypocotyl, the predicted tissue stresses do not reflect its cylindrical geometry. The epidermal layer experiences and resists the tensile stress coming from the expansion of the inner layers [22, 23]; this is known as the epidermal-growth-control hypothesis. Here, we use our recently developed automated confocal micro-extensometer (ACME) [24] to apply relative compressive or tensile stresses to the intact Arabidopsis hypocotyls while monitoring growth and microtubule orientation in the different layers. A finite element model revealed that under relative tension, the pattern of tissue stresses was similar to that in the intact growing hypocotyl, while when relative compression was applied, the pattern of tissue stresses was overcome and the maximum stress direction in the epidermis changed to reflect what one would predict based on the geometry of the hypocotyl. Consistent with this, the microtubules in the epidermis changed orientation under relative compression. Once the direction of stress in the epidermis was altered, the growth of the organ increased.