Control of plant organ growth linked to cell division orientation in inner tissues

There is a dominant view that plants are shaped by the ability of their outermost cell layers to resist or yield to pressure from internal tissues. During stem development, outer tissues originate from the tunica layers of the shoot meristem, while inner tissues are produced by the rib zone (RZ), named after its distinctive pattern of transverse cell divisions. Despite over a century of studies, the interplay between inner and outer tissues in shaping the stem remains unclear. Here, we show that mutations in a subfamily of IQ domain (IQD) genes disrupt the orientation of cell divisions in the RZ and increase stem diameter in Arabidopsis. Measurements of cell geometry, growth of marked cell clones, tissue-specific expression, and subcellular localization all support the idea that these IQD proteins reduce longitudinal cell divisions in the RZ, limiting the build-up of vertical cell files in inner tissues and the concomitant radial growth of the stem. Thus, the characteristic orientation of cell divisions in the RZ is important for shaping the stem, and the genetic control of organ shape can be exerted in inner plant tissues.