Amphivasal vascular bundles and other unique anatomical features of Desmodium (now Pleurolobus gangeticus) leaf.
During a pharmacognostic study of Desmodium gangeticum (now Pleurolobus gangeticus),
it is found that the leaves possess some unusual features like amphivasal vascular bundles in mid-rib, a colorless region of large rectangular cells between palisade and spongy layers, two types of trichomes and one type of gland.
1. Amphivasal vascular bundles
Amphivasal or 'leptocentric vascular bundles
are characterised by xylem surrounding the phloem tissue on all sides.
In Desmodium, the leaf mid-rib is found to possess five large vascular bundles forming a semi-circle on the lower side and, just below the ridge, three small bundles forming a triangle with two large towards the lower side and one small on the upper side (Fig A).
All these three vascular bundles, seen on upper side, are amphivasal in nature where phloem encircled by xylem (Fig B). Such bundles are seen in monocots like Dracaena (after secondary growth) and many monocots like Cyperus.
The significance amphivasal capsular bundle is obscure.
But in a study by Zhong and co-workers (1991), it is noted that in avb1 mutant of Arabidopsis, the collateral vascular bundles are transformed to amphivasal vascular bundles in the stems. avb1 mutation results in auxin flow all around the phloem in a vascular bundle, instead of the localized auxin flow at the opposite side of the phloem seen in wild-type bundles.
The auxin flow around the phloem may thus induce xylem differentiation all around the phloem, forming amphivasal bundles seen in the avb1 mutant. It will be interesting to test this hypothesis by using auxin transport markers.
Therefore it is concluded that the evolution of amphivasal bundles could result from dominant mutation of the AVB1 gene, or at least mutation of the AVB1 gene. could result in amphivasal bundles.
Once the AVB1 gene is cloned, it will be interesting to test whether expression of the Arabidopsis AVB1 gene in the monocots with amphivasal vascular bundles would be sufficient to convert the amphivasal bundles into collateral bundles.
In another study Zhong and Ye (2004) found out that In addition to an alteration in vascular patterning, the avb1 mutation also caused dramatic changes in fiber cell wall thickening and organ polarity, including aberrant formation and proliferation of cauline leaves and branches, production of trumpet-shaped leaves with reversed adaxial-abaxial identity, ectopic growth of carpel-like structures on the outer surface of carpels, and fasciation of inflorescence. Ectopic overexpression of the avb1 mutant cDNA not only phenocopied most of the avb1 mutant phenotypes but also led to additional novel phenotypes such as formation of leaves with extremely narrow blades and ectopic production of branches in the axil of siliques.
2. Colorless layer parenchyma between palisade and spongy
A layer of long colorless rectangular cells seen between palisade and spongy, is another novelty seen in this leaf (C).
Lamina is dorsiventral with two layered palisade on the upper side and spongy tissue on the lower side separated by one layer of long rectangular colorless cells.
These colorless cells are different from spongy tissue which are isodiametric and green due to chloroplasts.
3. Two types of hairs and a gland
Leaves are hairy with two types of trichomes;
1) short hooked trichomes seen on the upper side (D) and
2) long straight trichomes on the lower side, more concentrated towards the petiole and veins (E). Both trichomes are 2-celled with a short basal cell and a distal second cell. The long trichomes are more common with a straight thick walled distal cell.
They have a basal cell which is thick walled having a length of 33 mμ and a breadth of 15 mμ and a very long terminal cell reaching a length of 459 mμ and breadth of 17 mμ. Short trichome also have a basal cell similar to the longer one, but the 2nd cell is short, having a length of about 75 mμ and a characteristic hooked tip.
In addition multicellular glands (F)having a single cell stalk and a club shaped head containing mostly biseriate cells are also noted. Each gland is about 30 mμ in length and 14 mμ in breadth and contains resinous material.
References
Daniel, M. et al (2022), An in-depth study on various parts of Pleurolobus gangeticus(l.) J.ST._ HIL. (Desmodium gangeticum dc), a component of the famous “dasamoola”, WJPPS, 11(11), 1126-1142
Zhong, R. et al (1999) Transformation of the Collateral Vascular Bundles into Amphivasal Vascular Bundles in an Arabidopsis Mutant Plant Physio.1999 May; 120(1): 53–64.
Zhong, R . and Z.Ye (2004), Amphivasal vascular bundle 1, a Gain-of-Function Mutation of the IFL1/REV Gene, Is Associated with Alterations in the Polarity of Leaves, Stems and Carpels, Plant and Cell Physiology, 45(4), 369–385,
Mammen Daniel
