According to the prophetic word of both Testaments, the Messiah had to come from:

• “The seed of the woman”: Gen. 3.15; Mat 1.20; Luke 1.34-35; Gal 4.1-7;
• “The seed of Abraham”: Mat. 1.1; Gal. 3.15-18;
• “The seed of David”: 2 Sam 7; Rom. 1.3;

Resulting in a new Davidic King for the coming age: Jer. 30:9; Ezek. 34:23-24; 37:24-25; Hos 3:5; Mic 5:2.

But, how can the Messiah be both “the root” and “the descendant” of his father David (Rev 22.16; cp. 5.5)?

Word Biblical Commentary, vol. 52, part 1:

“The term riza, lit., ‘root’, means ‘shoot’ (Fekkes, Isaiah, 152) or, less metaphorically ‘descendant’ (Louw-Nida, 10.33), similar to the use of shoresh (KB³, 1530-32)…The Greek phrase e riza riza kai genos is [synonymous], which has been translated here simply ‘descendant’ (see Bratcher-Hatton, Revelation, 332) since riza (lit., ‘root’) has the figurative meaning ‘descendant, offspring’ here (Louw-Nida, 10.33), and genos is very similar in meaning (Louw-Nida, 10.32). kai [is explanatory]: e.g., ‘the shoot, that is, the descendant’.”

Theological Dictionary of the NT:

“Is. 11.10 also lies behind the title ‘root of David’ in Rev 5.5; 22.16 in the sense of ‘shoot out of David’.” 

Expositor’s Greek NT, Rev 22.16:

“[The dawn] was already a messianic symbol [Jer. 23:5; Zech. 3:8; 6:12] to denote the messianic branch or stem, this double usage explains the imagery here.”

NOTE: Early Church Fathers connect it to Ps 110.3: “before the dawn/morning star, I have begotten you.” Tertullian, Justin, et al., cite the LXX.