Diversification of seed arrangement induced by ovule rotation and septum formation in Leguminosae

The plants of Leguminosae, which comprises 731 genera, bear monocarpellary flowers. The fruit, known as a pod, generally contains a row of seeds, and the internal structure appears to be simple. In Leguminosae, the seeds are generally arranged longitudinally along the suture of the pod but, in a few genera, namely, Cassia, Indigofera, and Senna, they are arranged transversely. Transverse seed arrangement facilitates the accommodation of a considerable number of seeds and has been considered to be established by rotation of seeds during development. We studied the development both these patterns of seed arrangement, i.e., longitudinal and transverse, in Senna and Indigofera in order to elucidate the structural changes occurring in ovaries (or pods) and determine the forces responsible for rotation of ovules (or seeds) by dissecting ovaries and pods at different developmental stages. Our results showed that at an early developmental stage the ovules were obliquely oriented or transversely arranged; later, the direction of their growth was restricted by the ovary walls or neighboring ovules, and therefore the obliquely oriented ovules rotated inward or outward. Thus, it was clarified that the processes involved in the formation of the internal structure of pods are regulated in a complex manner.