A novel subtilisin-like protease gene from Arabidopsis thaliana is expressed at sites of lateral root emergence.

Differential screening of a cDNA library for mRNA species that specifically accumulate during auxin-induced lateral root formation in Arabidopsis thaliana led to the isolation of the AIR3 cDNA clone. The corresponding single copy gene consists of 10 exons which encode a protein that possesses all the characteristics of subtilisin-like proteases. The promoter of the AIR3 gene was fused to the gusA (beta-glucuronidase) reporter gene and introduced into Arabidopsis. Expression was almost completely restricted to the outer layers of the parental root at sites of lateral root emergence and could be observed even before protrusion of the newly formed root tip. In the presence of external auxin, GUS activity was visible throughout the parts of the root that are competent for lateral root formation. By digesting structural proteins in the extracellular matrix of cells located above sites of lateral root formation, AIR3 might weaken cell-to-cell connections and thus facilitate lateral root emergence.

Isolation and characterization of cDNA clones corresponding with mRNAs that accumulate during auxin-induced lateral root formation.

Lateral root formation in root cultures of Arabidopsis thaliana can be initiated by exogenous addition of auxin. In order to find cDNA clones of which the corresponding mRNAs accumulate during this process, a cDNA library was constructed from root cultures treated with the active auxin 1-naphthaleneacetic acid (1-NAA). Differential screening of this library with cDNA probes derived from mRNA populations isolated from root cultures treated with 1-NAA and the inactive analogue 2-naphthaleneacetic acid (2-NAA) led to the isolation of four cDNA clones, designated AIR1, AIR3, AIR9 and AIR12. Accumulation of the mRNAs starts between 4 and 8 h and continues till at least 24 h after addition of an active auxin. Sequence analysis revealed that AIR1 encodes a protein that is related to a large family of proteins that consist of a proline-rich or glycine-rich N-terminus and a hydrophobic, possibly membrane spanning C-terminus. The putative function of these proteins is coupling of the cell wall to the plasma membrane. Surprisingly, AIR1 lacks the proline-rich or glycine-rich N-terminus which is thought to be important for interaction with the cell wall. AIR3 encodes a subtilisin-like serine protease which is believed to be active outside the plant cell. Although AIR9 and AIR12 do not show any significant homology to sequences in the database, they are also predicted to function outside the cell. Our screening thus indicates that a variety of genes encoding extracellular proteins are activated during auxin-induced lateral root formation.