ABSTRACT
Many plant species exhibit chirality or handedness, that is, some morphological peculiarities may be expressed in either of two ways, which mirror each other. Among such traits, one may list asymmetry of leaf blade, clock- or counterclockwise spiral arrangement of leaves along a shoot as well as spiral arrangement or perianth in bud, deflection of a style to the right or to the left (enantiostyly). Here we review the existing data on molecular mechanisms underlying handedness in plants. Almost all these data were acquired through studies on Arabidopsis thaliana, a cruciferous plant, which normally lacks chiral features. These studies uncovered a set of mutations, which cause anomalous spiral twisting of stems, roots, leaf petioles, or floral parts. Generally, three interacting molecular sources of such transformations occur: (i) (i), (ii)or (a), (b) etc. alterations in cytoskeleton content, structure, or associated proteins; (ii) impairing in cell wall biosynthesis; and (iii) changes in auxin polar transport. However, none of these mechanisms has been shown to be responsible for the switch between right and left forms of naturally occurring handedness. Data on inheritance of such handedness are scattered. In some plants, like Medicago truncatula, a single Mendelian gene defines right- and left-handed pod coiling. Some plants are monomorphic with respect to handedness (e.g., all right), but many species exhibit a 1:1 ratio either in population or within a single plant body, evidencing for non-heritable nature of chirality. Some external factors like X-ray irradiation, gravitation, or nutritional regime may influence the choice between two chiral forms.
