Abstract
The capability of the soil bacterium Agrobacterium rhizogenes to reprogram plant development and induce adventitious hairy roots relies on the expression of a few root-inducing genes (rol A, B, C and D), which can be transferred from large virulence plasmids into the genome of susceptible plant cells. Contrary to rolA, B and C, which are present in all the virulent strains of A. rhizogenes and control hairy root formation by affecting auxin and cytokinin signalling, rolD appeared non-essential and not associated with plant hormones. Its role remained elusive until it was discovered that it codes for a proline synthesis enzyme. The finding that, in addition to its role in protein synthesis and stress adaptation, proline is also involved in hairy roots induction, disclosed a novel role for this amino acid in plant development. Indeed, from this initial finding, proline was shown to be critically involved in a number of developmental processes, such as floral transition, embryo development, pollen fertility and root elongation. In this review, we present a historical survey on the rol genes focusing on the role of rolD and proline in plant development.
Highlights
The capability of the soil bacterium Agrobacterium rhizogenes to reprogram plant development and induce adventitious hairy roots relies on the expression of a few root-inducing genes, which can be transferred from large virulence plasmids into the genome of susceptible plant cells
Despite the same pattern of expression in shoot apical meristems and embryos [57,58], p5cs2 but not p5cs1 the high sequence similarity shared by P5CS1 and P5CS2 and both genes share the same pattern of expression in shoot apical meristems and embryos [57,58], p5cs2 but not p5cs1 mutants, are embryo lethal suggesting a specific role of P5CS2 in embryogenesis
Other hormone-independent mechanisms are known to modulate root growth, such as the superoxide/hydrogen peroxide ratio reported by Tsukagoshi et al (2010) [90] but the case of proline is quite surprising because the accumulation of this amino acid in the root is under strict abscisic acid (ABA) control under stress conditions [91]
Summary
Formerly known as Agrobacterium rhizogenes [1,2,3,4,5] is the etiological agent of the hairy root disease, consisting of abundant root proliferation at the site of bacterial infection. Since SPS normally inhibits the formation of lateral shoots by altering cytokinin balance, the proliferation of axillary branches of Arabidopis transgenic for rolD may be accounted for by a (secondary) effect of rolD on SPS expression It is not clear how SPS downregulation can affect the cytokinin/auxin ratio, since the synthesis of indole glucosinolates in Arabidopsis proceeds from the transformation of tryptophane to indole-3-acetaldoxime catalysed by CYP79B2 and CYP79B3 [33], while CYP79F1 is involved in the biosynthesis of aliphatic glucosinolates [32,34]. In A. tumefaciens OCD is encoded by genes localized in the non-transferred part of the Ti plasmid [7,37] to be used for opine catabolism, while in A. rhizogenes, OCD has become part of the T-DNA and it is expressed only in the plant cells. Since rolD is only present in the TL-DNA of the agropine-type Ri plasmids, its expression seems not strictly required for hairy root elongation. Attributed to OCD may be accounted for, or contributed to, a perturbed ammonium homeostasis
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