Conserved genetic determinant of motor organ identity in Medicago truncatula and related legumes

Abstract

Plants exhibit various kinds of movements that have fascinated scientists and the public for centuries. Physiological studies in plants with the so-called motor organ or pulvinus suggest that cells at opposite sides of the pulvinus mediate leaf or leaflet movements by swelling and shrinking. How motor organ identity is determined is unknown. Using a genetic approach, we isolated a mutant designated elongated petiolule1 ( elp1 ) from Medicago truncatula that fails to fold its leaflets in the dark due to loss of motor organs. Map-based cloning indicated that ELP1 encodes a putative plant-specific LOB domain transcription factor. RNA in situ analysis revealed that ELP1 is expressed in primordial cells that give rise to the motor organ. Ectopic expression of ELP1 resulted in dwarf plants with petioles and rachises reduced in length, and the epidermal cells gained characteristics of motor organ epidermal cells. By identifying ELP1 orthologs from other legume species, namely pea ( Pisum sativum ) and Lotus japonicus , we show that this motor organ identity is regulated by a conserved molecular mechanism.

Genetic basis of the “sleeping leaves” revealed

“…In the absence of these genes, the small, isodiametric, epidermal pulvinus cells with a highly convoluted surface are replaced by much larger and elongated petiole-like epidermal cells (2). This change in the cell type and expansion pattern may explain the elongated petiolule phenotype of the elp1/apu/slp mutants.…”

“…Some of them, like LOB in Arabidopsis, have been implicated in the establishment of the frontiers between meristem and lateral organs (10). Similarly, ELP1 is expressed very early on in the basal region of the leaflet, the region that will later differentiate into the pulvinus, in a fashion reminiscent of a frontier gene (2). Overexpression of ELP1, like overexpression of LOB, leads to dwarf plants (10), suggesting that one common function of these genes would be to control cell expansion.…”

“…From this, the APU and SLP genes could be identified as ELP1 orthologs bearing mutations in the respective pea and L. japonicus mutants. Therefore, the work of Chen et al (2) reveals that the formation of the pulvinus in legumes is likely to be regulated by a conserved genetic network orchestrated by the ELP1/APU/SLP1 genes. Identification of these genes also provides a straightforward way to test whether pulvinus formation in more distantly related species is controlled by the same genetic determinants.…”

“…However, nothing was known about the development of this organ, probably because Arabidopsis lacks an equivalent structure. The report of Chen et al (2) starts to fill this void through the identification of the genetic factor that determinates pulvinus formation in legumes. This collaborative work between teams of three different continents working on three different legume models nicely illustrates what can be achieved when the use of large, well-established collections of mutants meets the most advanced plant molecular genetic approaches.…”

“…He also described a specialized organ, called a joint, cushion, or pulvinus responsible for such movement (1). In PNAS, Chen et al (2) identify the genetic determinant for the formation of these pulvini in three legumes: Pisum sativum (pea), Medicago truncatula (barrel medic), and Lotus japonicus.…”